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Respiratory behaviour and quality attributes of fresh apple slices in modified atmosphere systems Dhanawansa, Ullusu Hewage 1992

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RESPIRATORY BEHAVIOUR AND QUALITY ATTRIBUTES OF FRESH APPLE SLICES IN MODIFIED ATMOSPHERE SYSTEMS by ULLUSU HEWAGE DHANAWANSA B.Sc., University of Peradeniya, Sri Lanka, 1978  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Food Science) We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA August 1992  © Ulusu Hewage Dhanawansa  In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  (Signature)  Department of The University of British Columbia Vancouver, Canada  Date  DE-6 (2/88)  re YY-3. LAccri^( cH3  II  ABSTRACT Ascorbic acid (5%) - treated, peeled, cored and sliced apples in modified atmospheres were studied in a two part investigation. Firstly, respiratory behaviour of slices of New Zealand Granny Smith and British Columbia grown Newtown apples at different temperatures (1, 5, 10 Sz. 15°C) was studied in sealed impermeable chambers with 21.5% 02 + 78.5% N2 as the initial gas mixture. Secondly, the respiratory behaviour and quality changes of sliced Newtown apples of the same stock were studied for six weeks, at 1°C, in sealed flexible polymer film pouches made of high gas barrier (CL804), medium high barrier (6 mil polyethylene. PE) and medium low barrier (4 mil PE) packaging films. The same initial gas mixture was used. Regardless of the storage temperature and the variety, CO2 levels for apple slices in respiratory chambers increased linearly with time (r > 0.99). 02 consumption curves for apple slices of both varieties were biphasic with a linear saturated region followed by a logarithmic unsaturated region. However, at 1°C, the 02 consumption curve for Newtown apple slices followed only a logarithmic relationship. The crossover point between the linear and logarithmic portions of the biphasic 07 consumption curve was recognized as the transition 02 level. Depending on the temperature, transition 02 levels for apple slices ranged from 0.81 to 4.5% 09 for Granny Smith and 2.2 to 6% 02 for Newtown apples. Both apple varieties had a 09 consumption rate of 1.45 ml 02/kg.hr at 1°C. A Q10 of 4.2 and a 010 of 5 for the temperature range of 1 to 15°C was calculated for Granny Smith and Newtown apples, respectively. At aerobic 09 levels, respiratory quotient ( RQ) values were less than unity for both varieties. Activation energy for the Arrhenius relationship for the respiratory activity of Granny Smith and Newtown apple slices, at 1 to 15°C, were -64.15 kJ/mole and -71.76 kJ mole, respectively. At the end of storage, apple slices in pouches made up of each different kind of packaging film established low 02 and high CO9 concentrations which reflected the gas permeability of the particular film. Final package headspace equilibrium 07 levels were  III  approximately 0.28%, 0.96% and 1.32% for HB, MHB and MLB films, respectively. CO2 levels at the end of the six week storage period were 31%, 13.5% and 7.3% for the same films. Changes of pouch headspace 02 and CO9 concentrations during the study were significantly different for package film types (p < 0.001). storage time (p < 0.001) and for the package film and storage time interaction (p < 0.001). pH and percent soluble solids of apple slices changed significantly for different packaging films (p < 0.001 for both properties), for storage time (p < 0.001 for both properties) and for the storage and package film interaction (p < 0.001 k, 0.002. respectively). Both properties initially decreased and then began to increase during storage. Percent titratable acidity (as malic acid) decreased as storage time proceeded (p < 0.001) and changed significantly for packaging films (p < 0.001) and for the storage and packaging film interaction (p < 0.001). The two lowest permeable films showed the largest decrease in acidity. The soluble solids/titratable acidity ratio increased until the third week then began to decrease during rest of the storage (p < 0.001). Regardless of the package film type, surface browning of apple puree exposed to air was rapid. Hunter L. a and ID values measured at. different time intervals (0, 30, 60, 90 Sz 120 min) changed significantly but not with a definite relationship. The surface discolouration of MAP apple slices in storage was determined mostly by the cultivar. All five texture measurements, bioyield point force (p = 0.001), deformation to bioyield point (p < 0.001), firmness measured as the ratio of bioyield point force to deformation (p < 0.001), rupture point force (p < 0.001) and the ratio of rupture point force to deformation (p = 0.001) measured with Instron plunger test, decreased significantly during storage. Deformation to bioyield point was statistically significant for the three different packaging films (p = 0.005) and for the packaging film and storage time interaction (p = 0.032). Rupture point force was different. for three different packaging films (p = 0.010) and was statistically significant for the packaging film and storage time interaction (p < 0.001). Deformation to rupture point did not show a significant (p > 0.05) change. Rupture  IV  point force to deformation ratio was different for three packaging films (p = 0.007) and was statistically significant for the packaging film and storage interaction (p < 0.001). Instron texture profile analysis parameters fracturability, Hardness-1 and hardness-2 were significantly different for packaging films (p = 0.010) and decreased in storage (p > 0.001). hardness-1 (p > 0.009) and hardness-2 (p > 0.024) also decreased during storage. Anaerobic microatmospheres resulted in overall higher texture values apple slices than those packed in aerobic microatmospheres. Regardless of the film type, sensory quality of apple slices deteriorated during storage. Appearance (whiteness) was affected most. Following the fifth week of storage, apple slices from the two lower permeability film type (HB and MHB) pouches developed slime and off-flavours while apple slices from MLB film pouches developed a moldy smell. Apple slices showed no weight losses in storage. Ascorbic acid (5%) dipping for 3 min prior to packaging preserved surface colour of sliced Granny Smith apples. Even with a longer dipping time (8 to 10 min) surface colour of sliced Newtown apples became unstable when exposed to air. The cultivar and the 02 permeability of the packaging film were most important factors determining the quality of apple slices in pouches. Therefore, it is proposed, low browning apple cultivars and the high barrier packaging materials which provide less than the optimal 02 requirement which result partial anaerobiosis should be used for MAP apple slices.  V  TABLE OF CONTENTS ABSTRACT TABLE OF CONTENTS^  V  LIST OF TABLES^  IX  LIST OF FIGURES^  XII  ACKNOWLEDGEMENTS^  XIV  1. INTRODUCTION 2. LITERATURE REVIEW^  4  2.1 Package System^  4  2.1.1 Gas Permeability Characteristics of Polymeric films^  4  2.1.1.1 Factors Affecting Permeability of Polymeric Films ^ 4 2.1.1.2 Interaction Effects of Permeates and the film on Permeability^6 2.1.1.3 The Effect of Temperature on Permeability 2.1.1.4 Effect of Respiration Rate of Enclosed Commodities on Film Permeability^8 2.1.1.5 Factors Influncing The Selection of Packaging Materials For Modified Atmosphere Packaging (MAP) of Produce^ 9 2.1.2 Commodity Characteristics in Relation to MAP ^  9  2.1.2.1 Commodity Respiration^ 9 2.1.2.2 The Respiratory Quotient. ^ 11 2.1.2.3 The Critical Oxygen Concentration ^ 12 2.1.2.4 Effect of Surface to Volume Ratio^ 12 2.1.2.5 The Effect. of Temperature on Respiration Rate ^ 13 2.1.2.6 The Effect of Resistance to Gas Diffusion on Respiration ^15 2.1.3 IN.,licroatmosphere^  16  2.1.3.1 The Initial and Final Equilibrium Gas Composition^ 16 9.1.3.2 Fruit to Gas Volume of the Package^ 17 2.1.3.3 Effect. of Microatmosphere on Respiration ^ 18 2.1.3.4 Microatmosphere Effects on C., H4 Biosynthesis and C9H4 Effects ^20 99 2.1.3.5 Effect of Microatmosphere on Enzyme Activity^ ^ 2.1.3.5.1 Polyphenol Oxidase ^ 23 2.1.3.5.2 Respiratory Enzymes 24 97 2.2 The Measurement of Respiration ^ 2.2.1 The Flow Through Gas System^ 2.2.2 The Sealed Impermeable Respiration Chamber System^ 2.2.3 The Gas Permeable Package System ^  97 98 28  VI  2.3 Respiratory Behaviour in Sealed Impermeable Chambers^  28  2.4 Apple Quality^  30  2.4.1 Chemical Properties^  30  2.4.1.1 Soluble Solids^ 2.4.1.2 Organic Acids^  31 31  2.4.2 Textural Properties^  34  2.4.2.1 Firmness^ 2.4.2.2 Texture Profile Analysis^  35 37  2.4.3 Apple Surface Colour^  38  2.4.3.1 The Enzymic Browning of Cut Apples^ 2.4.3.2 Inhibition of Enzymic Browning with Ascorbic Acid ^  38 39  2.4.4 Sensory Quality^  40  2.4.4.1 Apple Aroma^ 2.4.4.2 Apple Taste^ 2.4.4.3 Apple Sensory Texture^  40 41 42  3. MATERIALS AND METHODS^  44  3.1 Materials^  44  3.2 Preparation of Peeled, Cored and Sliced Apples^  44  3.3 Sealed Impermeable Respiratory Chamber Studies^  46  3.4 Packaging in Plastic Pouches^  47  3.5 Fruit. Weight Loss During Storage of the Pouches ^  48  3.6 Headspace Gas Analysis^  48  3.7 Analysis of Chemical Properties^  50  3.7.1 Apple Juice Extraction^  50  3.7.2.1 Determination of pH^ 3.7.9.9 Percent Soluble Solids^ 3.7.9.3 Percent .Titratable Acidity ( DTA)^ 3.7.2.4 Soluble Solids Titratable Acidity Ratio ^  50 51 51 51  3.8 Analysis of Physical Properties^  51  3.8.1 Surface Colour Measurement of Fresh Apple Slices^  51  3.8.1.1 The Measurement of Browning of Apple Puree in Air ^  52  VII  ^ 3.8.2 Texture Measurement ^ 3.8.2.1 Firmness ^ 3.8.2.2 Texture Profile Analysis ^ 3.9 Sensory Evaluation ^ 3.10 Statistical Analysis  57  4. RESULTS AND DISCUSSION ^  58  52 53 53 55  4.1 Respiration Studies in Sealed Gas - Impermeable Chambers ^58 4.1.1 Granny Smith Apples^  58  4.1.1.1 Sensory Quality Attributes of Granny Smith Apple Slices After the Respiratory Study^  67  4.1.2 Newtown Apples^  70  4.1.2.1 Sensory Quality Attributes of Newtown Apple Slices After the Respiration Study^  80  4.2 Newtown Apple Slices in Flexible Film Pouches^  82  4.2.1 Respiratory Behaviour of Newtown Apple Slices in Plastic Pouches ^82 4.2.2 Fruit Weight Loss During Storage^ 88 4.2.3 Chemical Analysis^  88  4.2.3.1 Percent Titratable Acidity ( TA)^ 4.2.3.2 pH Values^ 4.2.3.3 Percent Soluble Solids^ 4.2.3.4 Soluble Solid/ Titratable Acidity Ratio^  88 88 92 93  4.2.4 Physical Properties^ 4.2.4.1 Enzymic Browning of Unstored Apple Slices and of Puree From Stored MAP Slices in Air^  93 93  4.2.5 Texture Measurement^  103  4.2.5.1 Instron Plunger Test^  103  4.2.5.1.1 Bioyield Point Force^ 103 4.2..5.1.2 Rupture Point Force^ 103 4.2.5.1.3 Deformation to Bioyield Point and Deformation to Rupture Point ^107 4.2.5.1.4 Firmness (Ratio of Bioyield Point force to Deformation)^107 4.2..5.1.5 Rupture Point Force to Deformation Ratio ^ 113 4.2.5.2 Instron Texture Profile Analysis (TPA)^  113  4.2.6 Sensory Evaluation^  120  VIII  5. CONCLUSION^  135  6. BIBILIOGRAPHY^ ^ 7. APPENDIX  138 150  IX  LIST OF TABLES 2.1 Carbon dioxide Inhibition of the polyphenol oxidase (PPO) activity^25 3.1 Gas transmission rates for N9, 09, and CO9 and the moisture vapour transmission rate for the polymeric packaging films ^  45  4.1 Respiratory parameters for Granny Smith apple slices at various storage temperature^  64  4.2 Ethylene detection time and initial concentration in the headspace of respiration chambers containing Granny Smith apple slices at various storage temperature^  68  4.3 Sensory evaluation of Granny Smith apple slices after completion of the respiration studies at various storage temperatures^  69  4.4 Respiratory parameters for Newtown apple slices at various storage temperatures^  75  4.5 Sensory evaluation of Newtown apple slices after completion of the respiration studies at various storage temperatures^  81  4.6 Analyses of variance for factors influncing the headspace CO2 and 09 concentrations in pouches containing Newtown apple slices at 1°C ^83 4.7 Changes in the headspace 02 concentrations (%) for MAP Newtown apple slices stored at 1°C^  85  4.8 Changes in the headspace CO2 concentrations (%) for MAP - Newtown apple slices stored at 1°C^  86  4.9 Changes in percent titratable acidity levels (g malic acid/ 100 g apple tissue) of MAP Newtown apple slices during storage at 1°C^89 4.10 Analyses of variance for chemical properties of MAP Newtown apple slices during storage at 1°C^  90  4.11 Changes of pH values of apple juice supernatent from MAP Newtown apple slices during storage at 1°C^  91  4.12 Changes of percent soluble solids (% sucrose) of MAP Newtown apple slices during storage at 1°C^  94  4.13 Changes of soluble solids/ titratable acidity ratio of MAP Newtown apple slices during storage at 1°C^  95  4.14 Hunter L, a and b values of fresh unstored apple slices held for various storage times at 25°C in air^  97  4.15 Hunter L values for puree from MAP Newtown apple slices at various storage times at 1°C^  99  X  4.16 Hunter a values for puree from MAP Newtown apple slices at various  storage times at 1°C^  100  4.17 Hunter b values for puree from MAP Newtown apple slices at various storage times at 1°C^  101  4.18 Statistical significance probability values for Hunter L, a and b values of MAP Newtown apple slices stored for periods up to 6 weeks at 1°C^  102  4.19 Changes of Instron plunger test - bioyield point force (Newtons) of MAP Newtown apple slices during storage at 1°C ^  104  4.20 Analyses of variance of Instron - plunger test texture measurements of MAP Newtown apple slices in storage at 10C ^  105  4.21 Changes of Instron plunger test - rupture point force (Newtons) of MAP Newtown apple slices during storage at 1°C ^  108  4.22 Changes of Instron plunger test - deformation (mm) to bioy-ield point of MAP Newtown apple slices during storage at 1°C ^  109  4.23 Changes of Instron plunger test - deformation (mm) to rupture point of MAP Newtown apple slices during storage at 1°C ^  110  4.24 Changes of Instron plunger test - firmness (ratio of bioyield point force to deformation, N/mm) of MAP Newtown apple slices during storage at 1°C^  112  4.25 Changes of Instron plunger test - ratio of to rupture point force to deformation (N/mm) of MAP Newtown apple slices during storage at 1°C^114 4.26 Analyses of variance of Instron texture profile analysis of MAP Newtown apple slices during storage at 1°C^  115  4.97 Changes of TPA - fracturability (Newtons) of MAP Newtown apple slices during storage at 1°C^  116  4.28 Changes of TPA - hardness-1 (Newtons) of MAP Newtown apple slices during storage at 1°C^'^  118  4.29 Changes of TPA - hardness-2 (Newtons) of MAP Newtown apple slices during storage at 1°C^  119  4.30 Sensory panel ratings for apple odour (in mouth) for MAP Newtown apple slices in storage at 1°C^  122  4.31 Sensory panel ratings for apple sweetness for MAP Newtown apple slices in storage at 1°C^  124  4.32 Sensory panel ratings for apple sourness for MAP Newtown apple slices in storage at 1°C^  125  XI  4.33 Sensory panel ratings for sweet.. sour balance for MAP Newtown apple ^ slices in storage at 1°C  126  4.34 Sensory panel ratings for firmness at finger tips of MAP Newtown apple ^ slices in storage at 1°C  127  4.35 Sensory panel ratings for shear force required by teeth in mouth for MAP ^ Newtown apple slices in storage at 1°C  128  4.36 Analysis of variance of sensory panel ratings of crispness of MAP ^ Newtown apple slices in storage at 1°C  130  4.37 Sensory panel ratings for crispness (sound^ created in mouth) of MAP Newtown apple slices in storage at 1°C  131  4.38 Sensory panel ratings for ^ appearance (whiteness) of MAP apple Newtown slices in storage at 1°C  132  4.39 Sensory panel ratings for off-flavour of MAP Newtown apple slices in ^ storage at 1°C  133  4.40 Sensory panel ratings for overall sensory acceptability for MAP ^ Newtown apple slices in storage at 1°C  134  XII  LIST OF FIGURES 2.1 Regulation of ethylene biosynthesis in apple tissue^  21  3.1 Sample gas chromatogram for the standard gas mixture identification ^49 3.2 Force - deformation curves for materials with and without bioyield point ^54 3.3 A generalized Texture Profile Analysis curve obtained from the Instron Universal Testing Machine^  56  4.1 Changes in headspace CO2 and 02 levels in sealed chambers with Granny Smith apple slices at 1°C^  59  4.2 Changes in headsp ace CO2 and 02 levels in sealed chambers with Granny Smith apple slices at 5°C^  60  4.3 Changes in headspace CO9 and 02 levels in sealed chambers with Granny Smith apple slices at 10t5C^  61  4.4 Changes in headspace CO9 and 02 levels in sealed chambers with Granny Smith apple slices at 15.°C^  62  4.5 The effect of headspace 02 concentration on respiratory quotient values for Granny Smith Apple slices stored at 1°C^  65  4.6 Arrhenius relationship for Granny Smith apple slices ^  66  4.7 Changes in headspace CO2 and 02 levels in sealed chambers with Newtown apple slices at 1°C^  71  4.8 Changes in headspace CO2 and 02 levels in sealed chambers with Newtown apple slices at 5°C^  72  4.9 Changes in headspace CO9 and 02 levels in sealed chambers with Newtown apple slices at. 10°C^  73  4.10 Changes in headspace CO2 and 02 levels in sealed chambers with Newtown apple slices at 15°C^  74  4.11 Arrhenius relationship for Newtown apple slices ^  Ii  4.12 Effect of headspace 09 concentration on the respiration rate of Newtown apple slices stored at 1°C 4.13 The effect of headspace 09 concentration on RQ of Newtown apple slices ^ stored at 1°C  79  4.14 Changes in headspace CO9 and 09 levels in sealed pouches containing ^ Newtown apple slices during storage at 1°C  84  4.15 Changes of soluble solids and SS TA ratio for MAP Newtown apple slices ^ during storage at 1°C  96  XIII  4.16 A profile of force-deformation curves of MAP Newtown apple slices in HB (week 1) and MLB (week 5) pouches (with Lotus - 123)^ 111 4.17 TPA - force-deformation curves of MAP Newtown apple slices (with Lotus - 123)^  121  XIV  ACKNOWLEDGEMENTS I wish to express my sincere gratitude to my supervisor, Dr. W.D. Powrie of the Department of Food Science, for his invaluable advice, guidance and his patience throughout this M.Sc program. I would also like to thank the other members of the Research Supervisory Committee, Dr. T. D. Durance, Dr. B. J. Skura of the Department of Food Science and Dr. C. H. Wu of Pacific Asia Technologies, Inc. for their assistance and helpful suggestions and advice throughtout this study. The assistance given by Mr. S. Yee and Mrs. V. Skura is gratefully acknowledged. Panelists Dr. W. D. Powrie, Mrs. G.L.R. Dhanawansa, Tiek-mein Thai, J. Masuhara and Dr. G. Adegoke for their participation in the sensory evaluation part of the study are appreciated. My wife, Ranjani, for her help achieving this goal, if not because of her this would have not been possible. Yehen, my son, for the time he missed with me, are sincerely acknowledged. Pacific Asia Technologies Inc., which provided finanacial support is also gratefully appreciated.  XV  This thesis is dedicated to my farther, U. H. Daniel Silva and to my mother D. Rathnaweera.  1  1. INTRODUCTION Apples (Malus domestica Borkh.) are the major fruit crop grown in Canada (Agriculture Canada, 1991) and in British Columbia (BC Ministry of Agriculture, Fisheries &T. Food, 1990). In 1989, apple production in Canada averaged 516,000 metric tons (Agriculture Canada, 1991), while in the same year, apple production in British Columbia alone accounted for approximately 89% (on a weight basis) of total provincial fruit crop production (BC Ministry of Agriculture, Fisheries Sz. Food, 1990). The apple is a flavourful and nutritious fruit (Lee & Mattick, 1989) which is consumed in the processed form in such products as apple juice, apple sauce, apple butter, mincemeat, apple jelly and canned apple slices and in the fresh form as a whole commodity or as a sliced product within salad and desserts (Way & McLellan, 1989). Refrigerated fresh apple slices because of their convenience, their year round availability and their firm texture have been used extensively for pies and other bakery products (Ponting & Jackson, 1972). Unlike frozen apple slices, refrigerated fresh apple slices possess tissue with high turgidity and cellular adhesion. However, fresh apple slices must be treated with a reducing agent and/or an acid solution dip prior to refrigeration to prevent enzymic browning on the surfaces (Ponting et al., 1972). The demand for minimally processed fruits and vegetables by the food service sector and the retail market is increasing because of the convenience, nutritional value and high quality of the products (Shewfelt, 1987). A Canadian survey showed that fresh commodities are considered by consumers to be more nutritious and flavourful than canned products (Powrie Skura, 1991). Dougherty (1990) estimated that by the year 1995. more than 50% of the food dollars spent in grocery stores in the USA will be for fresh, ready-to-eat products. Modified atmosphere packaging (MAP) has been applied in extending the storage life of minimally processed fruits and vegetables (Huxsoll k. Bolin, 1989; Kader et al., 1989; Myers. 1989: O'Beirne, 1990; Powrie Skura, 1991; Wang, 1990). Modified atmosphere packaging (MAP) is a process involving the introduction of a food in the whole or cut form  2  into a package system whereupon air around the product is displaced with a specific gas mixture prior to sealing. The packaged product is generally cooled to between 0 and 10°C soon after sealing. The initial gas mixtures used in modified atmosphere packaging of fresh fruits and vegetables generally contain 02, CO2, N2 and may contain rare gases such as argon (Povvrie & Skura, 1991). During storage of the produce, 09 is consumed in the respiration system with the ultimate production of CO2. Low 09 and relatively high CO2 concentrations in the package microatmosphere and low storage temperatures bring about a reduction of enzymic activities in the respiration and ripening processes and an inhibition of microbial growth in fruits and vegetables (Kader et al., 1989; Labuza & Breene, 1989; O'Beirne, 1990; Powrie & Skura, 1991; Prince, 1989; Wang, 1990). For respiring plant tissues, the 09 content in the microatmosphere within the package must not drop below a certain critical concentration; otherwise, the aerobic respiration reverts to anaerobic respiration resulting off-flavour development. Therefore, for each commodity, a packaging material with an adequate 02 permeability must be selected to ensure aerobic respiration (Powrie Skura, 1991). Flexible polymer plastic films with a wide variety of gas permeabilities have become available commercially. Thus, modified atmosphere packaging of plant produce can be designed on the basis of a respiration-permeation equilibrium of respiratory gases within the package (Deily & Rizvi, 1981; Hayakawa et al., 1975; Henig & Gilbert, 1975; Jurin & Karel, 1963; Kader et al., 1989; Labuza & Breene, 1989; Powrie & Skura, 1991; Prince, 1989; Wang, 1990). Optimization of packaging parameters by modelling the package behaviour had been used to design packages for fresh whole or minimally processed commodities (Deily k Rizvi, 1981; Hayakawa et al., 1975; Henig & Gilbert, 1975: Kader et al., 1989; Labuza & Breene, 1989; Powrie Skura, 1991: Prince, 1989). However, for cut fruits and vegetables with the potential for enzymic browning, high gas barrier packaging films with low 02 transmission have to be selected in order to reduce browning (O'Beirne, 1990; Powrie, 1992).  3  Powrie et al. (1990b) reported that cut fruit pieces could be held for several months under MAP conditions if the packaging material had high gas barrier properties so that a high CO2 content and a very low 07 content in the microatmosphere could be maintained. The MAP method of Powrie et al. (1990b) is based on maintaining uniform distribution of in situ CO2 in fruit tissue within the closed package system. Under the conditions of this  method, microbial growth was inhibited, respiration and ripening rates were markedly reduced, chilling injury was eliminated and anaerobic off-flavour development was retarded. The objectives of this research study were: 1. to investigate the influence of storage temperature (1, .5, 10 & 15°C) on the respiration rate and quality attributes of peeled, cored and sliced New Zealand Granny Smith and BC Newtown apples and; 2. to investigate the influence of three types of flexible polymer packaging materials (high gas barrier, medium high gas barrier and medium low gas barrier) on the quality attributes of peeled, cored and sliced BC Newtown apples, stored under MAP conditions over a six week period at 1°C, with a gas mixture of 21.5% 02 and 78.5% N2 as the initial gas flush.  4  2. LITERATURE REVIEW 2.1  PACKAGE SYSTEM  Many factors must be considered in designing a package system for increasing the storage life of fresh horticultural produce in a container with a modified atmosphere (Jurin Sz: Karel, 1963; Paine & Paine, 1983). These factors can be divided into three major categories: 1. gas permeability characteristics of the packaging polymer plastic films; 2. characteristics of the fresh commodity to be packaged; 3. gas composition in the microatmosphere (Chinnan, 1989; Deily & Rizvi, 1981; Henig & Gilbert, 1975; Jurin & Karel, 1963; Kader, 1986; Kader et al., 1989; Labuza & Breene, 1989; Powrie & Skura, 1991; Powrie et al., 1990b; Prince, 1989; Wang, 1990). 2.1.1  GAS PERMEABILITY CHARACTERISTICS OF POLYMERIC FILM  In selecting packaging materials for a package system, gas permeability characteristics must be known since a specific equilibrium gas composition in the microatmosphere is essential for optimizing the storage life of the selected fresh produce. Generally, low 02 and high CO2 contents in the microatmosphere of a package system can prolong the storability of fruits and vegetables by reducing the rate of aerobic respiration, decreasing the rate of ripening and quality deteriorative metabolic reactions and retarding microbial growth (Jurin Karel, 1963; Powrie & Skura, 1991; Tolle, 1962). 2.1.1.1  FACTORS AFFECTING PERMEABILITY OF POLYMERIC FILMS  Gas or vapour (I-170 and volatile compounds) permeability through a nonporous flexible polymer plastic film is an activated diffusion phenomenon (Chao k Rizvi, 1988) governed by the rate of solubilization and diffusion of the respective gas or the vapour inside the film (DeLassus et al., 1988; Pascat, 1986). According to Pascat (1986), when there is no physico-chemical interaction between the permeate and the polymer. the gas or the vapour permeability through a film can be expressed as:  5 Pm = D.S Pm = Permeability coefficient D = Diffusion coefficient of the permeate S = Solubility coefficient of the permeate At a given temperature, gas or vapour permeability across a polymeric film is a function of both the polymeric film characteristics and the permeate properties and their interaction effects. All of these factors are collectively called permeability characteristics (Kader et al., 1989; Pascat, 1986). These characteristics can be affected by the free volume of the package, air velocity around the package and effectiveness of the seal of the package (Deily & Rizvi, 1981; Kader et al., 1989). Structural irregularities in polymeric films may prevent the precise measurement of film permeability. Variation in thickness, minute fissures, creases and pinholes at random locations in polymeirc film can cause erratic results from film sample to film sample (Tolle, 1962). When there is no physico-chemical interaction between the packaging material and the permeates at a given temperature, the quantity of permeate (Q) diffusing through a particular packaging material is proportional to the film area (A), time (t), pressure or concentration difference (d ) and inversely proportional to the film thickness (L). Thus, the equation can be written as (Delassus et al., 1988; Pascat, 1986): Q = Pm.A.t.dp.L-1 Permeability coefficient (Pm) is a characteristic of the polymeric film/permeate couple. Therefore, when there is interaction, instead of Pm, Q/At (rate of permeate transfer) or Pm/L (Permeability per unit thickness) is used (Pascat, 1986). Landrock and Proctor (1952) reported that the relationships between Pm and 1/1., (L = film thickness) for polymeric films were straight lines with each different film having a  6  specific slope and intercept. Pascat (1986) expressed the relationship between Q/At and L as follows: Q/At = f (1/ )(I) Y is a coefficient varying between 0.8 and 1.2. Therefore, 1 mm thick film will have a transfer coefficient (Q/At) of between 0.4 and 0.6 times that of a 0.5 mm thick film of the same polymer (Pascat, 1986). Powrie et al. (1990a) reported that the gas permeation values of polyethylene films with thicknesses of 1.5 mil and 0.8 mil were very close. Likewise the gas permeation values through polyethylene films with thickness ranging from 1 to 20 mil did not differ significantly (Meyer et al., 1957). Lack of circulation affects gas permeation through polymeric films. When there is circulation of gases, permeability coefficients of He, 02, CO2 and N2 are the same whether these gases existed in pure or mixed state (Meyer et al., 1957). Under noncirculating conditions, CO2 in a 02 or a N2 mixture had a reduced permeability compared to the permeability of pure CO2. Permeability of 02 in a CO2 mixture (noncirculating) has been found to have twice the normal permeability through polyethylene film (Halek, 1987; Meyer et al., 1957). 2.1.1.2  INTERACTION EFFECTS OF PERMEATES AND THE FILM ON PERMEABILITY  Permeability of some polymeric films is affected by relative humidity of the microatmosphere (Delassus et al., 1988; Halek, 1987; Kader et al., 1989; Meyer et al., 1957). Hydrophilic films, such as nylon, polyvinylalcohol (PV0H), ethylene vinyl alcohol (EVOH), become plasticized in the presence of water. This plasticizing (changes in film structure caused by interacting permeates), in turn, increases the permeability to gases (Delassus et al., 1988; Halek, 1987; Meyer et al., 1957). Dry EVOH and PV0H are regarded as very good 09  barriers but when moistened. these films become highly permeable to 09 (Delassus et al.,  1988; Halek, 1987). Water vapour does not affect permeability of gases through hydrophobic  7  films such as polyethylene, and gases do not affect water vapour permeability through those films (Meyer et al.. 1957). Plasticizing of polymeric films by organic permeates increases permeability to gases. The absorption of limonene by 4 mil LDPE increased 02 permeability of the film with the increase being proportional to the limonine concentration in the microatrnosphere (Sadler Braddock, 1990). Dry EVOH is an excellent barrier for apple aroma compounds. However, moistened EVOH and low density polyethylene (LDPE) are poor barriers for apple aroma compounds particularly for trans-2-hexenal (DeLassus et al., 1988). DeLassus et al. (1988) reported a series of experiments that involved the measurement of permeation of single and mulfiple aroma components through LDPE at 28°C. With a mixture of three apple aroma compounds, trans-2-hexenal, hexanal and ethyl-2-methyl butyrate, permeation through the film increased for first two compounds while the permeability of the last compound decreased. 2.1.1.3  THE EFFECT OF TEMPERATURE ON PERMEABILITY  The relationship between the permeability coefficient (Pm), the solubility coefficient (S), the diffusion coefficient (D) and the transfer coefficient (Q/At) for gases for a given packaging film with temperature follows the Arrhenius equation. For noninteracting gases and vapours, solubility and diffusion through a flexible polymer film increase with temperature (Pascat, 1986). Solubility also depends on the nature of the permeate. The activation energy of solubility (Es) for gases is positive and relatively high. However. solubility of vapours decreases when the temperature increases and Es is negative. At storage temperatures above 0°C, activation energy for permeability (Er)) remains positive and the Pm increases by approximately 30 to 50% for each 5°C variation in temperature (Pascat. 1986).  8  Interaction with the film reduces the solubility of permeates in the film. Diffusion of such permeates is concentration dependant. Therefore. permeability of organic volatiles which interact with the film and moisture vapour transfer through hydrophilic films decreases with decreasing temperature (Pascat, 1986). For many packaging materials such as polypropylene (PP), polyethylene (PE), polyethylene terapthalate (PET) and PET/PE composite, the slope of Arrhenius relationship varies within a narrow range of temperature between 0 and -12°C. Permeability, diffusion and solubility show pronounced variations when the temperature reaches this temperature range (Pascat, 1986). Therefore, Arrhenius relationship can be applied only below and above this range of temperatures. This narrow range represents the glass transition temperature of the packaging material (Delassus et al., 1988; Pascat, 1986). Meyer et al. (1957) found that as the temperature rose from 0 to 30°C, 02 permeability of polyethylene films increased about five-fold. Eustace (1989) reported that, at 0°C, the 02 permeability values for flexible film laminates used for vacuum packaging of meat were about 10% of their 02 permeability values at 25°C. 2.1.1.4  EFFECT OF RESPIRATION RATE OF ENCLOSED COMMODITIES ON FILM PERMEABILITY  Tolle (1962) examined over 59 methods that have been used in measuring film permeability and none of the methods examined duplicated the exact gas dynamics of the permeability-respiration phenomenon taking place in a package of living produce. The methods did not include a variation in the microatmospheric gas composition over storage time as would occur in the microatmosphere of packaged produce. Changes in the gas composition in the microatmosphere of packaged produce were reflected in the changes in respiration rate. Further, with apples in boxes lined with polyethylene film, equilibrium permeability of the film could be attained only after 7 to 30 days of storage at refrigerated temperatures. Tolle (1962) doubted that equilibrium permeability could ever be obtained for produce on retail display.  9  Ballantyne (1987) found that brussels sprouts had very high variations in the rates of respiration from lot-to-lot within the same season. The seasonal differences of rates of respiration of the sprouts were as large as 50%. The increase of rate of respiration as the season progressed made it difficult. to select one or two suitable packaging materials for successful packaging. 2.1.1.5  FACTORS INFLUENCING THE SELECTION OF PACKAGING MATERIALS FOR MODIFIED ATMOSPHERE PACKAGING (MAP) OF PRODUCE  Kader et al. (1989) indicated that the following factors were important for selecting plastic films for MAP of fresh produce: 1. specific permeabilities for the different atmospheric gases; 2. good transparency and gloss; 3. light weight; 4. high tear strength; 5. low temperature heat sealability; 6. nontoxicity; 7. nonreactant with produce; 8. good thermal and ozone resistance; 9. good weatherability; 10. commercial suitability; 11. ease of handling and 12. ease of printing for labelling purposes. 2.1.2  COMMODITY CHARACTERISTICS IN RELATION TO MAP  The major commodity characteristics are the respiration rate, diffusion resistance to gases through the tissues, (Kader et al., 1989; Povvrie et al., 1990b), susceptibility to microbial spoilage (Paine Sz Paine, 1983), metabolic pattern and surface area to volume ratio (Jurin & Karel, 1963; Powrie et. al., 1990b). 2.1.2.1  COMMODITY RESPIRATION  Some of the factors influencing the rate of respiration of a commodity are: 09, CO2 and C2H4 concentrations in the microatmosphere: storage time; storage temperature; maturity stage; cultivar; physical condition; commodity weight (Chinnan, 1989; Jurin Karel, 1963; Kader et al., 1989; Powrie et al., 1990a); water loss: individual biological variation (Kader et al., 1989; Paine Sz. Paine; 1983: Powrie et al.. 1990b); calcium; light and growth hormones (Kader et al., 1989)..  10  Respiration by plant tissues, under aerobic conditions, involves breakdown of organic acids, lipids, carbohydrates and proteins via many enzymic reactions which include B - fatty acid oxidation, glycolysis, Krebs cycle and oxidative phosphory-lation with the production of ATP as cell energy and the release of CO2, H20 and heat (Kays, 1991; Powrie & Skura, 1991; Wills et al., 1990; Zagory & Kader, 1988). Enzymes of the oxidative pentose phosphate pathway also break down sugars to CO2. But reducing power (NADPH2) produced by this pathway cannot be used directly to produce ATP since the oxidative phosphorylation is NAD specific. Therefore, reducing power in NADPH2 is used in the electron transfer chain via the reduction of oxaloacetate (OAA) to malate (Kays, 1991). A modified atmosphere (MA) around a commodity in a package reduces the rate of respiration leading to reduced depletion of respiratory substrates and 02 and reduced CO2 evolution (Zagory & Kader, 1988). The control of respiratory activity of produce conserves the food reserves which, in turn, prolongs tissue survival and the potential shelf life (Massey, 1989; Zagory Kader, 1988). Mature, unripe fruits maintain relatively steady, low to moderate respiratory rates (Hulme & Rhodes, 1970) but as each commodity undergoes its natural process of ripening and senescence, the rate of respiration increases. Apples at 5°C are fruits having low respiratory rates with CO2 evolution of 5 to 10 mg CO2/kg/hr (Kader et al., 1989). As climacteric fruits, apples show a dramatic rise in respiration rate during postharvest ripening (Zagory & Kader, 1988). However, under modified atmospheres, apples do not reach a climacteric (Goodenough & Wright, 1981; Zemlianukhin Ivanov, 1978). The 07 concentration at which the rate of respiration begins to decrease varies with the apple cultivar (Fidler et al., 1973; Solomos. 1985). Solomos (1985) reported that, for the five apple cultivars studied, the microatmospheric 02 concentration at which the rate of CO2 evolution began to decrease ranged from 4.5 to 10%. These differences were mainly due to variations in the diffusivity of 02 as well as to the rate of 02 consumption. Rizvi (1981)  11  observed that the effects of low 02 content and refrigerated temperature had synergistic effects during a study of MAP of Crispin apples. 2.1.2.2.  THE RESPIRATORY QUOTIENT  Respiratory quotient (RQ), described as the ratio of amount of CO2 liberated to amount of 02 consumed, can vary depending on the substrate utilized and storage conditions. RQ of produce can vary from 0.7 to 1.5 (Wills et al., 1990; Zagory &7, Kader, 1988). When hexoses are the major initial metabolite, RQ is 1, but for organic acids and fatty acids the RQ is above 1 and below 1, respectively. Thus, RQ may be used to predict the chemical nature of the respiratory metabolism. Malic acid and sugars are major respiratory substrates in apples. Because of this, at adequate 02 levels, RQ for apples varies from 1.0 to 1.33 (Burton, 1982). Under anaerobic conditions, when there is no influence of microatmospheric CO2, CO2 output decreases to one-third of the CO2 output under aerobic conditions, simultaneously with an increase in ethanol output (Zemlianukhin Ivanov, 1978). Carbon dioxide - fixation inside plant tissues, such as pyruvic acid condensation with CO2, affects RQ (Zemlianukhin Ivanov, 1978). Microatmosphere inside the MA packages may affect RQ (Hayakawa et al., 1975; Henig Sz Gilbert, 1975; Kader et al., 1989). Powrie et al. (1990a) and Beveridge Day (1991) showed that, in sealed gas impermeable respiratory chambers, sweet cherries, cvs. Van, Stella and Lambert, had RQ values far below one (RQ << 1) at the beginning of storage. RQ values increased once the critical 02 concentration was exceeded. Increased solubility and increased CO2 partial pressure in sealed impermeable respiratory chambers could affect RQ (Powrie et al., 1990a). Zemlianukhin and Ivanov (1978) reported that, for apples in the absence of CO2, 2% 09 did not affect RQ. In the presence of high CO2 concentrations and with 13 to 16% 09, in the microatmosphere. a fall in RQ was evident and RQ dropped further as the 02 concentration decreased.  12  RQ may be useful in detecting the onset of physiological injury of a commodity in conventional air storage. If there is substantial departure of the RQ from the normal value of a particular commodity, abnormal respiration due to anaerobiosis or chilling injury would be indicated (Wollin et al., 1985). 2.1.2.3  THE CRITICAL OXYGEN CONCENTRATION  The critical 02 concentration of the commodity marks the 02 level where RQ starts to go up steadily due to anaerobic respiration (Wills et al., 1990), from a relatively low level (around 1). Anaerobic respiration of apples starts when the 02 concentration in the microatmosphere is about 5%. (Kader, 1988). Knee (1980) reported that critical 02 level for apple is 2%. However, there is evidence that the critical 02 concentration of a commodity changes during storage. Initially, stored Granny Smith apples can tolerate zero 02 level (Wollin et al., 1985). The response of the apples to an anaerobic microatmosphere is slow and takes about two weeks before a slight amount of ethanol is produced. After several months in storage, the apples require an elevated 02 level to avoid low 09 stress disorders (Wollin et al., 1985). The critical 02 concentration at which partial anaerobiosis is established varies with the apple cultivar (Fidler et al., 1973). These differences could be either metabolic or physical in nature or both (Solomos, 1985). 2.1.2.4  EFFECT OF SURFACE TO VOLUME RATIO  Fick's first law of diffusion (Chao k Rizvi, 1988; Solomos, 1987) states that the amount of gas diffused through the surface of a material is proportional to the surface area. For many plant organs, surface/volume ratio is an useful indicator of the relative level of concentration of gases diffusing into tissues. With leaves having a high surface/ volume ratio, microatmospheric gases have less distance to diffuse to the center (Kader et al., 1989).  13  Tolle (1971) reported that with an increase in surface area of produce, the rate of respiration increased. Slicing or cutting of fresh produce increases the respiration rate by increasing the surface area for respiratory gas diffusion. Cutting a head of broccoli into individual florets increased respiration by 50%. The respiration of peeled and sliced 3 mm square strips of carrots increased 6 to 7 times (Beveridge b.; Moyls, 1988). Slicing increased the rate of respiration for bananas (Palmer & McGlasson, 1969), strawberries, pears (Rosen Kader, 1989), and for mushrooms (Ballantyne, 1987). 2.1.2.5  THE EFFECT OF TEMPERATURE ON RESPIRATION RATE  The respiration rate for produce follows the Arrhenius equation (Beveridge & Day, 1991; Goodenough Wright, 1981; Murata, 1990; Powrie et al., 1990a; Tolle, 1962; Zagory Kader, 1988). For commodities susceptible to chilling injury, the activation energy (Ea) increases dramatically upon the drop of temperature to injurious levels. A break in an Arrhenius plot at a particular temperature means that the activation energy (Ea) change is caused by chilling injury (Goodenough Wright, 1981; Murata, 1990). This break has been used to determine the chilling sensitive critical temperatures for commodities in storage (Murata, 1990). With chilling sensitive species at low temperatures, the Ea rises markedly for mitochondrial oxidation reactions. Since mitochondrial reactions proceed very slowly compared to glycolytic reactions, toxic inhibitory products such as ethanol, acetaldehyde and keto acids begin to accumulate causing tissue damage (Simon, 1981). Many tropical products such as the avocado, mango and papaya are chilling sensitive and have to be stored above 13°C. On the other hand, nonchilling sensitive commodities such as many cultivars of apples. broccoli and pears can be stored at temperatures as low as -1°C ( Zagory Kader. 1988). Some of the apple cultivars that develop chilling injury when stored at near 0°C are McIntosh, Cox's Orange Pippin and Yellow Newtown (Bramlage &Mein 1990).  14  Research has shown that low temperature storage caused physiological disorders in Cox's Orange Pippin and Idared apples. These symptoms in Idared apples were followed by an increased rate of respiration (Blanpied, 1990). Accumulation of oxaloacetic acid (OAA) and successive inhibition of succinate and alpha-ketoglutarate oxidation has been observed in low temperature stored apples (Fidler et al., 1973). Phase transitions were observed in isolated mitochondria from apples stored at low temperatures (McGlasson & Raison, 1973). The rates of respiration and other enzymic processes are dependant on the storage temperature of the produce. A 10°C drop in temperature can bring about a 2 to 7 fold decrease in the respiration rate (Powrie Skura, 1991). Therefore, Q10 phenomenon explains why cold storage of apples is effective in increasing life expectancy of apples (Massey, 1989). Plant organs respond to low temperature storage by dramatically reducing the rate of respiration (Rees et al., 1988). Rees et al. (1988) found that the reduction of respiration rate of potato tubers, between 2 and 8°C, was accompanied with an activity loss of four key enzymes regulating glycolysis. These four enzymes, ATP: phosphofructokinase (EC 2.7.1.1; PFK-ATP), PP: phosphofructokinase (EC 2.7.1.190; PFK-PP), glyceraldehyde - PO4 dehydrogenase (EC 1.2.1.12) and pyruvate kinase (EC 27.1.40) were identified as cold labile. Low temperature storage of potatoes caused dissociation of phosphofructokinase-ATP into subunits. Such cold lability was not observed with enzymes of the oxidative pentose pathway and sucrose synthase. Low temperature storage of apples increases their susceptibility to low 02 injury, possibly because of the low 02 level raising the temperature threshold for chilling injury. Golden Delicious and Jonathan apples are such chilling sensitive varieties which develop skin browning (disease called soft scald) at 0°C with a microatmosphere having 2 to 2.5% 09. Low 02 injury (1 to 2% 02) of Granny Smith and Delicious apples has been inhibited by 1 to 2% CO2 (Blanpied. 1990).  15 2.1.2.6  THE EFFECT OF RESISTANCE TO GAS DIFFUSION ON RESPIRATION  Gas diffusion within a fruit is affected by respiration rate, maturity stage, commodity weight and volume, pathways and barriers for diffusion, properties of diffusing gases, concentration gradient across barriers and the fruit temperature (Kader et al., 1989; Rajapakse et al., 1990). The barriers against diffusion of gases in apples involve the skin, the intercellular space, cell wall and cell membrane (Solomos, 1985, 1987). Cell size and fruit weight were negatively correlated with the percentage of intercellular space area for Golden Delicious apples (Kruger & Stosser, 1989). In pome fruits, the distribution of 02 and CO9 is not uniform throughout the tissue and a gradient exists from the periphery to the center (Goodenough Sz Wright, 1981; Rajapakse et al., 1990; Solomos, 1987; Zemlianukhin Ivanov, 1978). During storage of apples, the total amount of tissue gases and the CO2 content increased, whereas the amount of 02 decreased. The surface layers of the apples possessed 13 to 19% 02 and 2.5 to 7.0% CO2. The innermost portion of the central part of the fruit and intercellular spaces had 1 to 09 and 25 to 30% C09. Hypoxia, and even anaerobic conditions, have been found to exist in the central tissue of fruits during storage (Zemlianukhin & Ivanov, 1978). Apple tissue possesses about 20 to 30% intercellular space. As the fruit ages, the volume fraction of intercellular space increases (Khan & Vincent, 1990; Powrie & Skura, 1991: Zemliaukhin & Ivanov, 1978). Gas - filled intercellular channels in fruits and vegetables may be partially filled with liquid so that intercellular air system is not continuous (Burg, 1990). The gas diffusion rate changes with the introduction of liquid into the intercellular spaces of apple tissue (Burg, 1990; Khan & Vincent, 1990; Zemlianukhin & Ivanov. 1978). The rate of 09 diffusion through apple flesh varied with the cultivar and were broadly consistent with the intercellular space volume. Significant. flesh resistance to gas diffusion has been reported for apples (Rajapakse et al., 1990; Solomos, 1987), potatoes (Banks & Kays, 1988), avocados (Ben Yehoshua et al., 1963; Burg & Burg, 1965).  16  Therefore, the critical 02 concentration of apples should be based on the flesh resistance to gas diffusion (Rajapakse et al., 1990). Dangyang et al. (1991) reported that low 07 MA storage of Granny Smith and Yellow Newtown apples, at 10°C or below, increased resistance to CO2 diffusion. The modes of gas diffusion may be different for different cultivars of a fruit ( Zemlianukhin Ivanov, 1978). 2.1.3  MICROATMOSPHERE  The microatmosphere in MA package systems with horticultural products are composed of various levels of 02, CO2, C2H4 and N2. Gas composition at the permeationrespiration equilibrium is referred to as the final equilibrium gas composition (Kader et al., 1989; Powrie et al., 1990b). 2.1.3.1  THE INITIAL AND FINAL EQUILIBRIUM GAS COMPOSITION  The gas composition of the microatmosphere within a sealed package system at a specific storage time depends upon the characteristics of the commodity, permeability characteristics of the packaging material and the initial gas composition (Beveridge & Moyls, 1988). With passive atmospheric modification in MAP technology, the commodity is enclosed within a package system with air as the input gas (Kader et al., 1989; O'Beirne, 1990). A modified atmosphere for a stored packaged product is established through respiration. Strawberries (cv. Domini) in a package system consisting of a low gas barrier film at 5°C were held in satisfactory condition for 7 to 8 days when air was the input gas (O'Beirne, 1990). According to O'Beirne (1990), Crispin apples were stored for 6 months at 3°C in a package system with air as the input gas without appreciable quality deterioration. With active atmosphere modification in MAP technology, the input gas in a package system is a gas mixture with a composition different to that of air (Kader et al., 1989;  17  O'Beirne, 1990). Such a gas mixture reduces the time to reach equilibrium gas composition in the microatmosphere (Zagory Kader, 1988). Reduction of the pre-equilibrium period lengthens the shelf life of produce considerably (Tolle. 1971). Air separation technology has been used to reduce the cost of production of gases to be used in MA gas mixtures (O'Beirne, 1990). Flushing of MAP package systems with a specific gas mixture has been used with fresh produce having low respiration rates and particularly where enzymatic browning must be controlled (O'Beirne, 1990). According to O'Beirne (1990), MA packaged shredded iceburg lettuce, without a specific gas mixture flushing, resulted in leaf browning because of the delay in the establishment of a low 02 concentration. Flushing the lettuce with a gas mixture of 5% 02 and 5% CO2 delayed the developemnt of browning and doubled the storage life to 14 days at 5°C (O'Beirne, 1990). Ballantyne (1987) reported that a MA package of brussels sprouts should contain about 3 to 5% 02 to allow for variation in fill weight and film thickness as well as some temperature abuse without resulting in offflavours in the product. When the permeability of flexible polymer packaging films are not sufficient to maintain a suitable equilibrium gas composition for the high respiring commodity, microperforated films (Geeson et al., 1988: Hewitt, 1984; O'Beirne, 1990; Powrie & Skura, 1991) and microporous breathable membranes, which can be attached to a window of the packaging material to increase the gas permeability, are used. Further, gas (CO2, 02 and C2H4) absorbents and adsorbents have been used to establish the desired modified atmosphere (Powrie Skura, 1991). 2.1.3.2  FRUIT TO GAS VOLUME OF THE PACKAGE  Powrie et al. (1990b) considered the ratio of commodity volume to package free volume as an important MA packaging parameter for fresh horticultural commodities and used ratios varying form 0.2:1 to about 3:1. Package gas volume does not affect final  18  equilibrium gas compositions, but it affects the rate of permeation and the time required to establish equilibrium atmospheres (Deily & Rizvi, 1981; Henig & Gilbert, 1975). 2.1.3.3  EFFECT OF MICROATMOSPHERE ON RESPIRATION  The concentrations of 02 and CO2 in gas mixtures and duration of exposure of commodities to gas mixtures are important factors in the success of MAP technology. The maintainance of near optimum 02 and CO2 levels which do not exceed tolerance limits of the commodities is essential. Fresh fruits and vegetables can tolerate low 02 and high CO2 levels for short time periods. Low 02 and high CO2 effects on the inhibition of respiration and inducement of anaerobiosis are additive (Kader et al., 1989). Subjecting the commodity to stress levels of respiratory gases at a given temperature and time period may result in various physiological disorders, development of off-flavours, and increased susceptibility to decay (Fidler et al., 1973; Kader et al., 1989; Labuza & Breene, 1989; Powrie Skura, 1991; Prince, 1989; Wang, 1990; Yang & Chinnan, 1988). Even 02 levels around the critical 02 concentration (2 to 3%), because of the biological variation of samples, may lead to some anaerobiosis (Wills et al., 1990). High CO2 levels reduce certain physiological disorders of whole apples, but levels above 13 to 20% can cause CO2 injury due to accumulation of metabolites (e.g. acetaldehyde) from anaerobic respiration (Fidler et al., 1973; Knee, 1980). In CA storage, some apple cultivars are extremely intolerant to CO2 levels above 2 to 3%. Some others can tolerate 5 to 6% CO2, whereas others are not injured by 8 to 10% CO2 which is the highest used in the industry. Apple fruit tissue is more sensitive to CO2 injury when the temperature and 02 concentration are low (Fidler et al., 1973). In CA storage, whole apples tolerate low 02 (2%) levels for considerable periods of time at about 2 to 3°C (Fidler et al., 1973). But very low 02 levels (below 1%) for extended storage periods may accelerate ethanol production in apples. Low 02 may inhibit some  19  physiological disorders in apples while ethanol production induced by anaerobiosis may cause some specific disorders (Fidler et al., 1973). Ethanol has been found in CA stored apple fruits with the level being determined by the 02 content in the microatmosphere. Ethanol disappears with the subsequent increase of 09  level (Fidler et al., 1973; Wollin et al., 1985). Most apple cultivars, when intermittently  exposed to ethanol levels above 40 mg/100 g fruit, developed skin and internal disorders as well as off-flavours. Apples with internal ethanol levels in excess of 120 mg/100 g fruit possessed low 02 injuries (Lidster et al., 1985). Determinaton of the ethanol content of apples has been used as an index to identify susceptibility of apple fruits to low 09 injury (Fidler et al., 1973). High CO2 and low 02 levels in the microatmosphere can reduce the rate of respiration of apples (Fidler et al., 1973; Knee, 1980). The respiration rate of apples stored in a microatmosphere containing 2% 02 decreased but it took 1 to 4 days to become apparent (Knee, 1980). Burton (1982) suggested that respiration of MA apple fruit is under the control of some other 02 requiring enzymic process. Since C2H4 affects respiration, there is a possibility that apple respiration is mediated indirectly via the depression of C2H4 production (Banks et al., 1984). A study of CA storage (3% CO2 + 1% 02) of different apple cultivars at 3.5°C for a few harvest years showed no disorders related to respiratory metabolism (Bohling Sz Hansen, 1984). 02 levels below 1% further reduced respiration of Golden Delicious at 3.5°C but the same 02 levels, below 1% did not significantly reduce respiration of other apple cultivars, even when the temperature was dropped to 1°C. Since the solubility of 02 in the cell sap is temperature dependant. the major limiting factor for respiration rate change could be storage temperature (Bohling & Hanson, 1984). For McIntosh apples respiring in a closed system, the rate of respiration decreased with the decreasing 07 concentration; the decrease was most marked at 09 levels below 8% Zemlianukhin Ivanov. 1978). The RQ remained relatively constant down to an 02 level  20  of approximately 3.57, after which the RO increased. The respiration rate of commercially mature apples was decreased by 5 to 10% CO2 in the microatmosphere, but the respiration rate of newly harvested fruit was not affected (Zemlianukhin Ivanov, 1978). The rates of CO2 production by unripe Cox's Orange Pippin apples were similar for 2% 02 and air microatmospheres over the first 4 days at 3°C (Knee, 1980). After this period, the CO9 production rate in air increased but the rate in 2% 02 remained stable for 40 days. The transfer of apples from air at 3°C to 2% 02 resulted in a decline of CO2 production and 02 uptake after 24 hr. However, the reciprocal transfer did not lead to an increase in CO2 production until 4 days had elapsed. Production of CO2 was about 40% lower in 2% 02 than in air and 02 uptake about 30% lower, but the apparent difference in RQ was of borderline significance at 5% level (Knee, 1980). 2.1.3.4  MICROATMOSPHERE EFFECTS ON C2H4 BIOSYNTHESIS AND C2H4 EFFECTS  In plant tissues, including the apple, methionine has been shown to be the biological precursor for C2H4 synthesis (Fig. 2.1). Methionine reacts with ATP to form Sadenosylmethionine (SAM) which is converted enzymically to 1-aminocyclopropane 1carboxylic acid (ACC) by the enzyme ACC sy-nthase (Yang, 1981). C2H4 synthesis can be effectively inhibited by interfering with ACC synthase activity. ACC is broken down enzymically to C9H4 and, during the process, CO2 is released. The formation of ACC is the rate limiting step in C2H4 biosynthesis (Yang, 1981). This reaction and the resynthesis of methionine from 5-methylthioadenosine are 09 dependent. reactions in the C2H4 biosynthetic pathway (Yang & Hoffman, 1984). The conversion of ACC to C2H4 is catalysed by the ethylene forming enzyme (EFE) which is proposed to be located mostly in the tonoplast and to a certain extent in the plasma membrane. Membrane integrity is a key factor required for an active enzyme. ACC is also converted to C9H4 by oxidants and free radicles (VanDer Straeten Van 'Montagu, 1991).  ^  CH -S -CH -CH -CH-COO2 2 3 I^.4. NH 3  (MET  )  Ribose  ATP  -Accept PPi + CH -S -CH2 -CH2 - CH -0003 , I+ Ado^. NH 3  CH -S- Ribose  Ade  Fruit ripening I AA 2+ Ca - cylok inin Physical wounding Chilling injury  HC I^_  ^H  2  C^COO (ACC)  Ripening), En*.  Anaerobiosis Uncouplers Co  Wounding  2+  Temp. > 35° c CH2 =CH2  Free radical scavengers  Fig. 2.1 Regulation of ethylene biosynthesis in apple_tissue_ id: this reaction is normally suppressed and is the rate-limiting step in the pathway: ( N.): induction of synthesis of the enzyme: ( e: ): inhibition of the reaction. Met. Ado and Ade stand for methionine. adenosine, and adenine, respctively.  (modified from Yang, 1981)  22  Low 02 levels in the microatmosphere slow down ACC accumulation, the increase in ACC synthase activity (Bufler Streif, 1986) and C9H4 production (Bufler Streif, 1986; Dangyang et al., 1991; Fidler et al., 1973; Knee, 1980; Powrie Skura, 1991). The time of onset of C9H4 production has been found to be inversely related, and the maximum rate of production was directly related, to the microatmospheric 02 level (Knee, 1980). Banks et al. (1984) reported that 02 concentrations in the range of 0.7% and above in the microatmosphere affected the rate of C9H4 production of apple slices. Maximum C2H4 production was in air and no further stimulation resulted from exposure to higher 02 concentrations. Km02 (Km = Michaelis constant; substrate concentration giving half maximal activity of the enzyme) of C2H4 production by the apple tissue occured at 02 concentrations between 1.7 and 1.2%. Results showed that a change in the accumulation of ACC could be observed only when the 09 concentration was below 10%. Km02 for the conversion of ACC to C2H4 in peel tissue of ripe apples was found to be about 1.3% (Bufler Streif, 1986). High CO2 levels in the microatmosphere may increase, decrease or may not affect C'2H4 production in fruits; the influence of CO2 on C2H4 synthesis is concentration dependant (Powrie Skura, 1991). High CO2 reduced C2H4 production in apples, pears, avocados, tomatoes and sweet peppers (Wang, 1990). Inhibitory effects of CO2 with low 02 levels on apples were additive (Fidler et al., 1973: Knee, 1980). Increasing CO2 (3 to 6%) concentrations increased the effectiveness of low 02 (Bufler Streif, 1986). The competitive inhibition of C2H4 effects on plant organs by CO2 is 02 dependant (Povvrie Skura, 1991; Wang, 1990) and the receptor site has very high 02 affinity as does cytochrome oxidase (Wang, 1990). 2.1.3.5  EFFECTS OF MICROATMOSPHERE ON ENZYME ACTIVITY  Senescence proceeds with no concomitant increase in respiration. The retarding effects of low 02 concentrations on fruit ripening involve processes other than respiration  23  (Goodenough k Wright, 1981; Knee, 1980). The extension of postharvest shelf life of fresh commodities in microatmospheres with low concentrations of 09 cannot be explained by effects on either the cyanide sensitive or cyanide resistant respiratory pathways alone. Focus has been directed to other oxidative events with a lower affinity for 02 (Goodenough Wright, 1981; Knee, 1980). Low 02 concentrations are significant when 02 toxicity has a profound effect on the quality of fruits. Chlorophyll degradation and the oxidation of membrane fatty acids leading to the production of some of the flavour volatile compounds are processes undertaken by 09 free radicals during ripening (Knee & Sharples, 1981). Low 02 levels in the microatmosphere of a fruit inhibit the ripening process and flavour production. Apple softening is also related to 02 free radicals. Degradation of cell wall polysaccharides is mediated by 02 free radicals (Knee Sz Sharples, 1981). Besides respiratory enzymes, glycolate oxidase, ascorbate oxidase, polyphenol oxidase, lipoxidase, amino acid oxidase, oxygenase functions of peroxidase, cleavage of 1aminocyclopropane 1-carboxylic acid (ACC) to C2H4, and the oxidation of C9H4 are some of the other potential sites of 02 use (Knee, 1980). The reduction of respiration rate at 09 levels above 2% is not due to reduced cy-tochrome oxidase activity, but can be due to reduced activity of low affinity oxidases (Goodenough k Wright, 1981; Kader, 1986; Knee, 1980; Powrie^Skura, 1991). For example, in cultivated mushrooms, an increase in 09 uptake is followed by an increase in polyphenol oxidase (PPO) activity (Goodenough Wright, 1981). 2.1.3.5.1  POLYPHENOL OXIDASE  Polyphenol oxidase (EC 1.14.1.1; monophenol, dihydroxy-phenylalanine: 02: oxidoreductase) is a copper containing enzyme and is mainly responsible for enzymic browning of injured plant tissues. Two polyphenol oxidases. namely tyrosinase (EC 1.14.18.1: monophenol monooxygenase) and diphenol oxidase (EC 1.10.3.2; diphenol  24  oxidoreductase) have been found. In apples, polyphenolic substrates are located in the vacuole while PPO is present in chloroplasts. Apple PPO, in the form of isozymes, have molecular weights ranging from about 1100 to about 46,000. These enzymes have maximum activity at about pH 5.0 (Mayer Harel, 1981). PPO has low affinity for 09. -Various isozy-mes of apple PPO had Km02 values ranging from 4.6 to 0.63 mmol. 1 (VamosVigyazo, 1981). The inhibition of enzymic browning of fruits and vegetables can be brought about by reducing the activity of phenolases (Buesher & Henderson, 1977; Murr & Morris, 1974) and by lowering the 02 level in a microatmosphere to 5% or below (Burton, 1982). Varying levels of CO9 (Table 2.1) and 02 in the microatmosphere inhibit PPO activity (VamosVigyazo, 1981). The effects of elevated CO2 levels on enzymic browning depend on the plant species (Buesher k Henderson, 1977; Siriphanich Kader, 1986). In mushrooms, complete inhibition of o-diphenolase activity with high CO2 levels was never achieved. Except for 0% 02, other 09 levels did not inhibit mushroom o-diphenolase activity (1\ilurr Morris, 1974). 2.1.3.5.2  RESPIRATORY ENZYMES  Respiratory enzymes are located in the cytosol and mitochondria of plant cells. Many types of reactions such as phosphorylation, dehydrogenation, decarboxylation and oxidation are catalyzed by respiratory enzymes. Some of these enzyme reactions may be impeded by the introduction of a MA around the commodity (Powrie Skura. 1991). Goodenough and Wright (1981) reported that for tomatoes stored at 12°C in 5% 09 5% C09, hexokinase (EC 2.7.1.1; glucose --> glu-6-PO4) activity decreased during the eight-week storage period. Further, phosphoglucomutase (EC 2.7.5.1; glu-1-PO4 --> glu-6PO4) activity increased while glu-6-PO4 isomerase (EC 5.3.1.9: glu-6-PO4 --> fruc-6-PO4) decreased slightly during the storage of tomatoes. High CO2 reduced the activities of glycolytic enzymes namely ATP: phosphofructokinase and PPi: phospho-fructokinase, increased the concentrations of fructose-6-PO4, fructose-2,6-bisphosphate and  25  Table 2.1^Carbon dioxide inhibition of the polyphenol oxidase (PPO) activity Name of the^Inhibitor conc.^Extent of PPO^References Commodity^of the gas(es)^inhibition Mushrooms^CO2/CO (80:20)^68% inhibition^1 Red Delicious^30% CO2^70% inhibition^1 apples Mushrooms^> 0% 02^no inhibiton^2 Mushrooms^> 25% CO2^50% activity^2 Snap beans^> 30% CO2^inhibited PPO^3 & reduced phenol content Lettuce^15% CO2^inhibited PPO^4 8.7_, reduced phenol content 1 Vamos-Vigyazo, 1981. 2 Murr^Morris, 1974. 3 Buesher & Henderson, 1977. 4 Siriphanich^Kader, 1985.  26  reduced fructose-1,6-bisphosphate concentration (Kerbal et al.. 1988). According to Goodenough and Wright (1981), 6-phosphogluconate deliydrogenase (EC 1.1.1.44; 6-phosphogluconate --> D-ribulose-5-PO4) activity and glu-6-PO4dehydrogenase (EC 1.1.1.49; glu-6-PO4 --> 6-phospho gluconolactone) activity decreased during an eight week period of modified atmosphere storage of tomatoes. Again this would have led to reduced glucose utilization, lowered production of reducing power and ribose units for nucleic acid synthesis. Malate dehydrogenase (EC 1.1.1.37; malate --> OAA), isocitrate dehydrogenase (EC 1.1.1.42; isocitrate --> ketoglutarate) and malic enzyme (EC 1.1.1.40; malate --> pyruvate) were highly active after eight weeks of storage at 12°C in modified atmospheres. NADH dehydrogenase (EC 1.6.99.3) and NADPH - dehydrogenase (EC 1.6.99.1) are most active in tomatoes at the mature green stage but after eight weeks of storage the tomatoes had low dehydrogenase activities (Goodenough & Wright, 1981). High microatmospheric CO2 levels (5 to 20%) around produce can influence the activity of enzymes in the respiration process and uncouple oxidative phosphorylation. High CO2 inhibit.s succinic dehydrogenase and aldehyde dehydrogenase resulting in the  accumulation of succinic acid and acetaldehyde in apples ( Fidler et al., 1973; Kader, 1986; Knee, 1973 Monning, 1983), lettuce, pears, carrots, peaches, cherries and apricots (Powrie & Skura, 1991). High CO2 levels in the microatmosphere of apples have been found to stimulate malate oxidation and suppress the oxidation of citrate. isocitrate, alphaketoglutarate, succinate, fumarate, and pyruvate (Knee, 1973; Shipway & Bramlage, 1973). Such changes in the oxidation reactions have been attributed to structural and conformational changes in the mitochondria (Shipway & Bramlage, 1973). According to Kader (1986), CO2 caused fragmentation and changes in shape of the mitochondria in Bartlett pears as well as a decrease in the size of the mit.ochondria. High CO7 levels in the microatmosphere of apples inhibited glycolysis and succinic dehydrogenase activity hut not. malic dehydrogenase activity (Monning, 1983). At. 3°/c 02 in t.he microat.mosphere, t.he cit.ric  27  acid cycle is influenced at sites where pyruvate is converted to citrate and where 2-oxyglutarate is converted to succinate (McGlasson Sz- Wills, 1972). Cytochrome oxidase (EC 1.9.3.1; ferrocytochrome c: 02 oxidoreductases) is the terminal oxidase of the mitochondrial respiratory chain and, has a very high affinity for 07. The Km02 value for 02 at 25°C is about 0.1 uM., which is equivalent to 0.01% 02 in the gas phase. Even lower Km values were measured in vitro (Knee, 1980). According to Goodenough and Wright (1981), Km09 values for potato cytochrome oxidase range from 3 x 10-6M to 7 x 10-8M. The reaction of 02 with cytochrome oxidase involves 02 intermediates such as superoxide radical (02-) and singlet 02 (02.). Cytochrome oxidase itself is a superoxide scavenger, which acts as a built in defence mechanism, so it remains unaffected. The enzyme also has catalase, peroxidase and superoxide dismutase activities. It acts as both an electron donor and an electron acceptor (Naqui & Chance, 1986). At low temperatures, when microatmosphere 02 concentrations are low, Km02 value for apple fruit cytochrome oxidase rises to a higher value (Goodenough Sz Wright, 1981). 2.2  THE MEASUREMENT OF RESPIRATION  2.2.1  THE FLOW THROUGH GAS SYSTEM  The flow-through system for respiration measurement involves: 1. the continuous passage of a gas mixture into an airtight chamber through an inlet port to occupy the void space around a commodity held at a specific temperature; 2. the continuous passage of gas out of the chamber through the exit port. The concentrations of 02 and CO9 in the outlet gas are monitored continuously. This flow through gas method is suitable for produce with high respiration rates. Thorough mixing of gases in the chamber. low void space in the chamber and low flow rates may be used to minimize errors. Humidified input gas should be used to prevent water loss from the produce tissue (Lee, 1987).  28 2.2.2  THE SEALED IMPERMEABLE RESPIRATION CHAMBER SYSTEM  The sealed impermeable chamber system consists of an airtight chamber containing produce at a constant temperature and a specific input gas mixture in the void space. A port for gas sampling is generally on the top of the chamber. This system is suitable for produce with low respiration rates and large surface to volume ratios. With this method, it is assumed that the rate of respiration is a function of the gas composition of the surrounding microatmosphere and the pressure is constant. The first assumption becomes true when the lag phase is shorter and the respiration rate is lower. The second assumption is true when the respiratory quotient of the commodity is one (Lee, 1987). Beveridge and Day (1991), Deily and Rizvy (1981), Hayakawa et al. (1975), Henig and Gilbert (1975), Jurin and Karel (1963), Powrie et al. (1990a) and Tucker and Laties (1985) used the sealed impermeable container system for their respiration studies. 2.2.3  THE GAS PERMEABLE PACKAGE SYSTEM  This system consists of a closed package system made with plastic film having known gas permeabilities. This system is suitable for commodities with low and moderate respiration rates. Gases diffusing into and out of the package reduce the effect of "lag time found in the sealed impermeable chamber system method (Lee, 1987). 2.3  RESPIRATORY BEHAVIOUR IN SEALED IMPERMEABLE CHAMBERS  Oxygen consumption curves (gradual decrease of 02 concentration in the headspace of a produce containing sealed impermeable chamber with time) for commodities respiring in closed respiratory systems follow three patterns. When CO2 is allowed to accumulate inside the microatmosphere, headspace 09 concentration initially drops linearly, followed by an exponential depletion of 02 as the level reaches values near zero. This type of 02 depletion curve is called a biphasic curve. The linear part of the curve is described by zero order kinetics while the exponential part of the curve is described by first order kinetics (Beveridge  29  Moyls, 1988; Deily & Rizvi, 1981: Powrie et al., 1990a). Biphasic types of 02 consumption curves have been reported for potato tubers (Chevillotte, 1973; Mapson & Burton, 1962), preclimacteric banana (Hayakavva et al., 1975), tomatoes and peaches (Deily & Rizvi, 1981), avocado (Tucker Laties, 1985) as well as strawberry, cv. Ranier, sweet cherries and broccoli (Powrie et al., 1990a). A second pattern was noted when the headspace CO2 was removed by absorbants. Under these conditions 09 consumption curves of preclimacteric banana (Hayakawa et al., 1975), tomatoes and peaches (Deily & Rizvi, 1981) were found to have only two linear segments. The third pattern is a curve representing the exponential rate of 02 depletion (Powrie et al., 1990a; Solomos 1985; Tucker & Laties, 1985). The rate of 02 uptake followed zero order kinetics until the microatmospheric 09 concentration decreased to about 0.4% for 1.5 mm thick sweet potato slices, in air, within a closed system (Solomos, 1985). Tucker and Laties (1985) also reported that when headspace 09 concentrations were decreasing relatively rapidly, 02 consumption curves for avocado were monophasic exponential curves. If 02 concentrations decreased slowly, 02 isotherms were biphasic. The 02 concentration, at which a break in the 02 consumption curve was visually observed, has been named the "transition 02 level" by Powrie (personal communication) and is synonymous with the the transformation 02 level of the "hockey stick" respiratory pattern (biphasic 02 consumption curve) described by Beveridge and Moyls (1988). The 09 consumption curve below the transition 09 level represents the region where anaerobic respiration starts and the beginning of off-flavour development (Beveridge k Day, 1989). The transition 09 level has been reported to be 3% for strawberries. cv . Ranier and 4.5 to 6.5% 09 for Van. Stella and Lambert cherries (Powrie et al.. 1990a). Many researchers have attempted to explain the biphasic nature of the 02 consumption curves for various commodities. The possible implications of two terminal respiratory oxidases, one with low 09 affinity and the other. cytochrome oxidase, with high  30  07  affinity has been suggested (Solomos, 1985; Tucker .Sz: Laties, 1985). Solomos (1985) and  Beveridge and Moyls (1988) favoured this hypothesis. Chevillotte (1973) proposed that the bipliasic mode of changes in the 09 consumption rate in response to the decrease in the microatmospheric 07 level was caused by the restriction of activity of cytochrome oxidase by the progressively lower diffusion rate of 02. Contrary to this, Beveridge and Moyls (1988), based on the apparent insensitivity of transformation 02 level to temperature, suggested that the biphasic nature of the 02 consumption curve was due to unsaturation of respiratory sites and not related to 02 diffusion rate or 02 solubility in the tissue. Tucker and Laties (1985) studied cyanide sensitive and cyanide resistant. respiration of avocado fruits in a closed circulating system and found no evidence for supporting any appreciable role of cyanide resistant respiratory oxidase in avocado fruit respiration. Therefore, they supported the idea of influence of rate of 02 diffusion into the tissue on the biphasic nature of the 09 consumption curve. 2.4^APPLE QUALITY The quality of an apple depends on its physical and visual characteristics and chemical composition. All the treatments received by the fruit from immature stage to the commercially mature stage affect the quality attributes (Watada & Abbott, 1982). The quality of fresh fruits are generally considered to be superior to those of canned and frozen fruits (Powrie, personal communication). 2.4.1  CHEMICAL PROPERTIES  The chemical composition of fruit depends on the variety. season, agricultural practices. soil, climatic conditions, geographical area, tree. location of the fruit in the tree, methods of harvesting, state of maturity, handling and storage of the fruit (Hulme Rhodes, 1970; Lee^Mattick, 1989; Smock k Neubert, 1950; Ulrich. 1970).  31  2.4.1.1  SOLUBLE SOLIDS  Edible apple flesh contains about 84% water. The major portion of total carbohydrates (about 15%) consists of sugars. For many North American varieties the total carbohydrate content ranges from 6.6 to 15.9% (Lee k Mattick, 1989: Smock Neubert, 1950). The major sugars are fructose, glucose, sucrose, sorbitol (Noro et al., 1988; Smock & Neubert, 1950; Ulrich, 1970) and trace amounts of arabinose and xylose (Hulme 8z: Rhodes, 1970). The proportions of these sugars in apple tissue are dependent upon the variety and maturity. The distribution of sugars in the various parts of the fruit is variable (Smock & Neubert, 1950). Soluble solids in an apple are mostly sugars but also include acids and volatiles (Smock & Neubert, 1950). The soluble solids content of Golden Delicious apples was directly related to the dessert quality (Watada Abbott, 1982), and to the taste but not to the maturity of apples (Lau, 1984). Soluble solids, sweetness and flavour were inversely related to the tartness of Delicious apples (Watada & Abbott, 1982), Low 09 levels in the microatmosphere of apples decreased the rate of sugar decomposition (Smith et al., 1985; Lee, 1983) and retarded the conversion of protopectin to soluble pectin (Wang, 1990). Research has indicated that California Granny Smith (at 0 or 10°C) and yellow Newtown apples (at 5 or 10°C) stored in 0.25 or 0.02% 09, during a 5 week storage period, maintained average soluble solids values of 11.9% and 13.4%, respectively (Dangyang et al., 1991). 2.4.1.2  ORGANIC ACIDS  Organic acids are one of the most important constituents from the standpoint of eating and cooking quality of apples (Lee k Mattick, 1989: Ulrich, 1970: Weichmann, 1986). The major organic acid in apples is malic (Lee^Mattick, 1989; Smock k- Neubert, 1950; Ulrich, 1970) although citric, and quinic acids are also found in moderate quantities (Lee 1\ilattick, 1989: Noro et al., 1988). Quinic acid is the predominant acid in immature apples  32  (Lee, 1983). Besides the Krebs cycle acids, small amounts of glycolic. lactic, glyceric, glyox-ylic, oxalic, tartaric. citramalic. galacturonic, caffeic, chlorogenic, p-coumarilquinic and ascorbic acids have been found in apples (Lee & Mattick, 1989; Smock & Neubert, 1950; Ulrich, 1970). The total acidity of apples is related to climatic conditions and the length of the growing season (Lee & Mattick, 1989). Malic acid is present in apple tissue at levels between about 3 and 19 milliequivalents per 100 g fresh weight and constitutes 80 to 90% of the total acidity (Ulrich, 1970). The total acidity (% malic acid) of many apple varieties has been reported to range from 0.22 to 0.85% (Lee & Mattick, 1989). The determination of total titratable acidity of apple tissue involves neutralizatiOn of all available acidic groups of organic acids including phenols and amino acids in the soluble phase (Ulrich, 1970). As the apple fruit matures, the percent titratable acidity decreases while pH increases (Smock & Neubert, 1950; Ulrich, 1970). The variation in both the total acidity as percent malic acid and pH among apple cultivars is appreciable but only small differences exist within cultivars. Bramley Seedling and York Imperial apples had total acidity levels as high as 1.07% (Lee & Mattick, 1989). Based on a three year study (1979 to 1981), the average pH for apple juice from a large number of cultivars from many areas of the United States was estimated to be 3.69 (Lee & Mattick, 1989). The reported pH values of apple juice ranged from 3.18 to 6.54 (Lee Mattick, 1989; Smock Neubert, 1950; Ulrich, 1970; Way & McLellan, 1989). In another study of 33 varieties, the pH of apple juice ranged from 3.03 to 5.40 and the total titratable acidity ranged from 0.026 to 1.45%. In the same study, the pH and titratable acidity of the apple juice were highly correlated (r = 0.9265) (Smock and Neubert, 1950). Low 09 and high CO9 levels in microatmospheres have been found to retard the decomposition of organic acids in the apple (Bohling & Hansen, 1984; Hewitt & Thompson, 1988: Smith et al., 1985; Wang, 1990: Weichmann, 1986). Low CO2 levels reduce the degradation of organic acids while high CO2 concentrations enhance it (Kader, 1986; Kader  33  et al., 1989; Weichmann, 1986). Knee (19M) reported that acid losses in apples increased with 5 or 8% CO9 combined with 21 or 3% 09 in the microatmosphere. Favourable CA conditions for retaining the rating quality of apples during storage were much better in retaining organic acids (Lau, 1983a, 1983b; Lau & Looney, 1982; Lau et al., 1983). Even under favourable CA conditions, C9H4 levels of above 500 ppm in the microatmosphere surrounding apples resulted in increased organic acid losses (Liu, 1978). Apples stored under CA conditions had high retention levels for malic acid (Hewitt k, Thompson, 1988; Lidster et al., 1983; Murata & Minamide, 1970; Singh et al., 1972; Smith et al., 1985). The retention of organic acids in apples stored under CA or MA storage may be attributed to dark CO2 - fixation into malic acid and the inhibition of respiratory metabolism with the consequence of ldwer degradation rate of malic acid. However, other organic acids such as citric, succinic, oxaloacetic, pyruvic, alpha-ketoglutaric, shikimic, quinic and citramalic acids are also affected by the lowered rate of degradation (Wang, 1990; Weichmann, 1986). Organic acid concentration may increase or decrease in apples in low temperature storage and under different 02 concentrations in the microatmosphere (Metlitskii et al., 1983). Acid loss in apples was accelerated when low temperature breakdown started; when the CO2 level was raised or when the CO2 level further decreased (Fidler & North, 1967). Carbon dioxide concentrations above 1% cause cell acidification (Kader, 1986). High CO2 levels brought about a drop in the cytoplasmic and vacuolar pH by 0.4 and 0.1 units, respectively, in lettuce (Siriphanich & Kader, 1986). Regardless of the effects of CO2, anaerobiosis also induces cytoplasmic acidification (Kurkjian Guern, 1989). Cell pH regulation is somehow related to malate synthesis. The alkalinization of the cytoplasm induces malate synthesis and accumulation. Similarly, acidification induces increased degradation of malic acid (Kurkdjian Guern, 1989: Ulrich, 1970). Two enzymes are reponsible for maintaining malic acid levels in the cell. NADP - dependent malic enzyme carries out both oxidative decarboxylation and fixation of CO2 into n-ialic acid. NAD -  34  dependant malate dehydrogenase converts malate to oxaloacetic acid (Ulrich, 1970; Wang. 1990). Other enzymes also can affect cell organic acid content. PEP - carboxykinase reversibly and PEP - carboxylase irreversibly catalyse the condensation of CO2 into oxaloacetic acid. Catalysis of the carboxylation reaction by PEP - carboxykinase also produces ATP (Ulrich, 1970). 2.4.2  TEXTURAL PROPERTIES  Apple fruit tissue is composed mostly of parenchymatous tissue and a large percentage (about 25%) of intercellular spaces. Parenchyma cells are thin walled and polygonal or isodiametric in shape. The elastic cell wall, made up of interwoven cellulose microfibrils, is the covering for the thin cytoplasm which overlies the large vacuole. The cementing material or the middle lamella is composed of pectic substances or esterified polygalaturonic acid. The adhesive strength of the pectic substances determines the degree of cell separation when the apple tissue is mechanically stressed or whether the cell fractures (Hamann, 1983; Mohsenin, 1986). The middle lamella of immature apple tissues is composed of water insoluble protopectin. Protopectin is converted to water-soluble pectin during ripening of the apple. This change brings about a decrease in hardness and an increase in mealiness of apples (Hamann, 1983; Mohsenin, 1986). Freshly harvested apples possess swollen (turgid) cells which have a high hydrostatic pressure (turgor pressure) on their cell walls. The combined effects of cell turgidity and cell wall elasticity determine the tissue viscoelastic properties (Mohsenin, 1986). As cell turgidity of apple tissue increases, axial cell compression strength decreases and tensile strength increases (Hamann, 1983). Shear stress is the most significant failure parameter. Critical shear stress was highly influenced by the axial strain rate. Limiting shear was dependant on both the strain rate and the compressive stress for apple flesh (Miles  35  Rehkugler, 1973). Apples with high turgor and firm apples, when subjected to uniaxial compression, fail along shear planes (l\flohsenin, 1986). Tijskens (1979) used plunger and plate compression to study texture of Golden Delicious apples in storage. Plate compression was found to be the most suitable texture measurement method. The breaking force and, to a lesser extent, the shear slope were useful texture parameters. Tijskens (1979) observed that the greatest decrease in apple firmness and hardness occurred long before the quality of the apples reached the lower limit of sensory acceptability. Rodriguez et al. (1990), using transmission electron microscopy studied microscopic changes of Granny Smith apple tissues subjected separately to stresses of impact and compression. The two different stresses caused intensive but different types of cellular injuries including breakage of tonoplast and plasmalemma. The retardation of apple tissue softening, by low 02 microatmospheres at low temperatures, accompany decreased formation of soluble polyuronides (Knee, 1980) and increased concentrations of polyamine compounds, putrescine, spermidine and spermine, which inhibit middle lamella degradation (Kramer et al., 1989). 2.4.2.1  FIRMNESS  Texture is one of the most important quality attributes of fresh apples (Vickers, 1988). Apple fruit firmness is an important indication of apple texture. Apple tissue loses firmness during postharvest storage (Fidler et al., 1973). Fruit size, location of measurement and cultural practices are equally responsible for the variation in firmness of apples (Worthington k, Yeatman, 1968). Compression tests of intact biological materials are used in determining mechanical properties significant in quality evaluation. In contrast to single measurements in subjective methods, the production of complete force deformation curves allow the determination of a number of compressive properties in one operation (Nlohesenin, 1981).  36  Single probe instruments have been used to produce force deformation curves. A force deformation curve for apples with the use of single probe instruments is characterized by the presence of bioyield point which is defined as the point where an increase in deformation results in a decrease or no change in force (Mohsenin, 1986). For apple fruit, two types of force deformation curves from single probe instrumental measurements are evident: a) an increase in force after the bioyield point was reached with fresh apples, b) a constant force after the bioyield point was reached with apples held for long periods of time in cold storage. Bioy-ield point is the force required for the probe to penetrate into the tissue and cause the cells to burst and the tissue to disrupt. In the punch test, the bioyield point force is proportional to the surface area of the punch and shearing around the perimeter of the punch (Bourne, 1982a; Mohsenin, 1986). The rupture point in single probe instrumental methodology, which represents the rupture of the tissue under an axial load, always follows the bioyield point. Bioyield point is related to a failure in the microstructure while the rupture point is related to a failure in the macrostructure of the tissue. Rupture point is defined by Mohsenin (1986) as the point on the force deformation curve at which the loaded specimen shows a visible or invisible failure in the form of breaks and cracks." The point is detected by a continuous decrease of the load in the force deformation curve (Mohsenin, 1986). Smooth and polished surfaced compression tools, radius and the shape of the penetrating head of the tool being dependant of the specimen, are used to apply pressure to the sample. A plunger, with a known radius without rounded edge, is suitable for soft materials such as fruits and vegetables, particularly for those which exhibit a bioyield point (Mohsenin. 1981). In a compression test, the full diameter of the cylindrical tip of the plunger is brought in contact with the specimen surface, with as little deformation as possible, in order to get a distinct bioy-ield point on the recording chart. The compression is continued through and beyond the rupture point. For larger and softer foods, such as fruits and vegetables, a sample support should be used to ensure that deformation of the entire  37  sample is negligible. A minimum of 20 specimens for each sample is required due to variations of shape, size, age and cellular structure. Test specimens are conditioned to the desired temperature and relative humidity (Mohsenin, 1981). Puncture test values of firmness decrease with a decline in the temperature of the apple tissue. Firmness-temperature relationship was found to be approximately linear for a temperature range of 0 to 45°C (Bourne, 1982b). Low 02 and high CO2 levels in microatmosphere of stored apples promotes the retention of desirable texture (Hewitt &7: Thompson, 1988). The lower the 02 content in the microatmosphere of stored apples, the better the texture retention (Lau, 1983a, 1983b; Lau Looney, 1983; Weichmann, 1986). Apples from microatmospheres with low 02 levels of 0.5 to 1.5% had a much better texture than those stored in 3 to 4% 02 (Lidster et al., 1981; Little et al., 1982). Weichmann (1986) reported that apples from a 3% 02 microatmosphere retained their firmness to a higher degree than apples stored in air. Stored apples in a microatmosphere of 3% CO2 +3% 02 had firmer texture than those in 0.5% CO2 + 3% 02. For Sparten and Melrose apples, 2% CO2 + 1.5 % 07 microatmosphere was better than 5% CO2 + 3% 02 in retaining firmness of apples in storage (Lange & Fica, 1982). According to Lau et al. (1984), short-term (10 day) treatment of apples in a microatmosphere having a high CO2 (17% ) level along with 5% 02 enhanced the retention of tissue firmness. With a CO2 treatment of 13 days and with a 10 to 20% CO2 microatmosphere, apple tissue was firmer than that the treated apples mentioned above. Acid solutions soften while alkaline solutions harden cut apples (Bauernfeind Pinkert, 1970; Pouting et al., 1971). Pouting et al. (1971) reported that ascorbic acid softened cut apples more than sulfurous acid. 2.4.2.2  TEXTURE PROFILE ANALYSIS  Instrumental texture profile analysis (TPA) is used to measure several texture parameters (Bourne, 1982a). With this analysis, a uniform bite-size sample is compressed  38  twice between two horizontal plates to a known percentage compression and the resulting force time curves for the "two bites" are used to calculate textural parameters (Breene, 1975). These instrumental texture parameters have been correlated to sensory textural parameters (Bourne, 1982a). Szczeniak et al. (1963) defined various TPA parameters. The major drop in force during the first compression cycle was defined as fracturability. At this point, the sample tissue cracks. The force at the instant of maximum compression was defined as hardness-1. The same parameter, in the second compression cycle, was defined as hardness-2. Breene (1975) showed that apples had a pronounced fracturability peak but the hardness peak was much higher than the fracturability peak. The unsymmetrical shape of the second compression cycle represents a plastic deformation in the apple tissue (Mohsenin, 1986). Bourne (1986) found that apple tissues with high water activity (Aw = 0.99) produced typical TPA force time curves with hardness values always being higher than fracturability values. 2.4.3  APPLE SURFACE COLOUR  2.4.3.1  THE ENZYMIC BROWNING OF CUT APPLES  When an apple is cut, a brown discolouration of the tissue becomes apparent within few minutes at room temperature (Bauernfeind Pinkert, 1970; La Belle, 1981). The enzymic browning of cut or bruised apple is the result of oxidation of certain phenolic compounds into o-quinones and subsequent polymerization of o-quinones into melanins with the reaction being catalysed by polyphenol oxidase (Bauernfeind Sz Pinkert, 1970; La Belle, 1981; Sapers Sz Hicks, 1989). Phenolic compounds, which contribute to apple astringency and the red colour of skin are present at high concentrations during early development of the fruit (Hulme Rhodes, 1970). Hunter reflectance L values and delta-L values have been used to measure the rate of browning on cut apples (Bauernfeind^Pinkert, 1970; La Belle, 1981). Visual ranking of  39  apple juice for browning by trained panelists revealed that Hunter L values and visual ranking were related (La Belle, 1981). 2.4.3.2  INHIBITION OF ENZYMIC BROWNING WITH ASCORBIC ACID  Generally, apple fruit contains low concentrations of ascorbic acid and has high polyphenol oxidase activity (La Belle, 1981). Ascorbic acid content varies from trace amounts to 34 mg/ 100 g fresh weight of apple tissue depending on the cultivar, growing environment, season, fruit acidity, storage temperature and storage conditions. Ascorbic acid is the most common naturally occurring inhibitor of enzymic browning in fruits and vegetables (Baurnfeind Pinkert, 1970). L-ascorbic acid (AA) and its isomer D-erythorbic acid (also called D-isoascorbic acid) have been used as inhibitors of enzymic browning for decades. L-ascorbic acid is more effective than isoascorbic acid in inhibiting enzymic browning. These compounds can inhibit melanin formation by reducing the o-quinones in the enzymic browning pathways to the original o-diphenols. When the ascorbic acid in the fruit is exhausted, o-quinones are altered and eventually brown pigments called melanins are formed. Browning of cut apples and the oxidation of ascorbic acid show good correlation (Bauernfeind & Pinkert, 1970). Ponting et al. (1972) found that the storage life of apple slices could be prolonged by dipping the slices in a 1% ascorbic acid + 1% CaC12 solution at pH 7 for 3 min. Sapers et. al. (1990) reported that cut apples treated with sodium ascorbate and CaC12 solution inhibited browning for up to 20 days at 4°C. Apple cultivars with low browning potential are preferable for the manufacture of sliced fresh products (Sapers ik Hicks, 1989). Sapers and Douglas (1987) reported that, of the six cultivars tested. Idared and Granny Smith had the least browning potential. With respect to cut fruits, the presence of 02, PPO, peroxidase and ascorbic acid oxidase in tissue can contrbute to the destruction of ascorbic acid. Even in the absence of active tissue enzymes, ascorbic acid oxidation occurs with the rate of reaction being  40  dependent on the temperature and pH of the tissue. Apple varieties show considerable differences in the rate of oxidation of added ascorbic acid (Bauernfeind^Pinkert, 1970). 2.4.4  SENSORY QUALITY  Volatile compounds, acids, sugars and sometimes phenolic compounds are involved in the perception of overall apple flavour (La Belle, 1981). Consumer preference for specific apple cultivars is based on specific sensory attributes such as sweetness, sourness, odour, crispness and juiciness (Watada Abbott, 1982). According to McLellan et al. (1984), the following five sensory attributes that accounted for 72% of the intercultivar variation among McIntosh, Rome, Northern Spy, Cortland, Rhode Island Greening and Golden Delicious apple slices were: 1. colour; 2. sourness and astringency; 3. sweetness and fruitiness; 4. grain type and particle break down size; 5. and firmness and cooked aroma 2.4.4.1  APPLE AROMA  Specific volatile compounds are essential for the odour characterization of the fruit. About 20 to 40 volatiles, out of over 1000 volatiles reported for apples, are known to be responsible for apple aroma (Williams et al., 1977). Some of the odour active volatile compounds exist only at levels of about one to a few picograms/gram apple juice (Cunningham et al., 1986). The production of apple aroma compounds changes dynamically and characteristically during ripening and is influenced by the degree of maturity of the flint. The maximum emanation of certain aroma volatiles coincides with the respiratory climacteric. Some volatile compounds appear during senescence (Brown et al., 1966). Modified atmospheres and refrigerated temperatures affect qualitative and quantitative changes in volatile compound production. Apples stored in CA synthesized less than normal quantities of volatiles and very few specific aroma volatile compounds  41  ( Brackmann, 1989; Lidster et al., 1981, Lidster et al., 1983; Paillard, 1990; Patterson et al.. 1974; Willaert et al., 1983; Yahia et al., 1990). Flavour suppression was severe with increasing duration of CA storage (Lidster et al., 1981, Lidster et al., 1983). Low 02 levels of about 3 to 4% 02 in the microatmosphere did not hamper the production of normal apple aroma (Patterson et al., 1974). The inability of apples to produce aroma volatile compounds in normal quantities is a result of low temperature and the reduced production and action of C2H4 in low 02 and high CO2 concentrations in the microatmosphere (Bangerth Streif, 1987; Smith, 1984; Streif & Bangerth, 1988). Therefore, if the apples had not been in storage for long time periods, the normal aroma volatile synthesis would have occurred when the fruits were held at room temperature in air for several days (Bangerth & Streif, 1987). 2.4.4.2  APPLE TASTE  Watada and Abbott (1982) found that the soluble solids content was correlated to overall flavour of five apple cultivars. Further, soluble solids content, total titratable acidity and headspace volatile compounds were correlated with the sourness of Golden Delicious and York Imperial apples. The total titratable acidity accounted for about 56% of the variation in acidity of the two cultivars. The addition of two more volatile compounds to the variables increased correlation only by 4%. Therefore, on the basis of a small increase of the correlation value and the lack of specificity of volatiles contributing to the acidity, the authors questioned the possible involvement of volatiles in sensory sourness. Sweetness of York Imperial apples was correlated to soluble solids and total titratable acidity (Watada Abbott, 1982). Perring (1989) noted that the taste sensation ratings of the various zones of an apple changed during storage as the proportions of dry matter (mainly sugars), water and acid contents varied. Movement of water within the fruit could influence this balance (Perring, 1989).  42  2.4.4.3  APPLE SENSORY TEXTURE  Sensory texture assessment of a food involves the use of different sensory organs such as the tongue and the teeth as well as the fingers (Peleg, 1980). Szczesniak (1991) defined sensory texture as "the sensory manifestation of the structure of the food and the manner in which this structure reacts to the applied forces, the specific senses involved being vision, kinesthetics and hearing." This definition acknowledges that sensory texture is a multiparameter attribute which involves molecular, microscopic and macroscopic structures (Szczesniak, 1991) and sensory perception involves the senses of feel or pressure, vision and hearing (Peleg, 1980; Szczesniak, 1991). Sensory crispness is regardrd as a multiparameter sensation (Szczeniak, 1988) and the best measurements of sensory crispness are combinations of both acoustical and force deformation measurements (Vickers, 1988). A crisp food is one that is firm and fractures easily (rather than bends) upon deformation with the result that a crunchy/crackly sound is emitted (Szczeniak, 1988). Crispness is an important quality attribute of dessert apples (Ponting et al., 1971). Sensory crispness of apples has been significantly correlated to the hardness of apples and has been expressed by the following regression equation (r = 0.99) (Brennan. 1982). Crispness = -153.2 + 2.45 (Leg) 32.4 (Wf) - 0.5 (Fr) Leg = the equivalent continuous sound level in dB (decibles) Wf = the work done during fracture [Nm (Newtonmeters) x 1031 Fr = the fracture rate in msec-1 (milisecond). Highest correlation of apple crispness was found to be with various sound wave characteristics during biting, with maximum peak force at tissue failure or with work done. Crispness and juiciness of apples have been considered important quality attributes for consumer acceptability (Vickers, 1988). Crispness and juiciness were found to be better for  43  apples stored under microatmospheres with high CO2 and low 02 levels compared to apples stored in air (Hewitt, 1984; Hewitt Sz Thompson, 1988). Watada and Abbott (1982) and Vickers and Bourne (1976) showed that fracturability and hardness4 of apples were correlated with sensory crispness. Moreover, hardness, toughness and crispness of apples have been correlated with the area under the force deformation curve. Apple sensory crispness and shear strain were negatively related suggesting that a high degree of crispness was associated with small deformations to tissue failure (Watada & Abbott, 1982). Sensory firmness (hardness), defined as the force to bring teeth together (bite through food), has been correlated with tissue failure stress (Hamann, 1983). Mealiness increases as apple fruit maturity increases, while intensity of sensory texture attributes decrease (Watada & Abbott, 1982). The movement of water within the fruit affects texture. The increasing availability of water to the outer zones of the apple may indeed affect juiciness and texture quality to a different extent than that in the tissue in central zones. Such movement can bring about an increase in cell turgidity and initiate cell separation with the consequence of increased mealiness ( P erring, 1989).  44  3. MATERIALS AND METHODS 3.1  MATERIALS  Fancy grade (count 113) New Zealand Granny Smith apples were obtained from a local wholesale distributor (Pacific Food Ltd., Vancouver, BC) during the summer 1990. Fancy grade (count 100) Newtown apples grown in BC were obtained from the same wholesale distributor during the last week of November 1990. All apples were stored at 1°C after receipt. Three flexible polymer plastic films with high gas barrier, medium high gas barrier and medium low gas barrier properties, respectively, were selected for the study of behaviour of packaged apple slices. The high gas barrier film was CL 804. The medium high barrier film was 6 mil polyethylene (PE) and the medium low barrier film was 4 mil polyethylene (PE). All the films were obtained from DuPont Canada, Kingston, ON. The high gas barrier film was a laminate consisting of PE/tie/EVOH/tie/PE (Powrie et al., 1990b). Gas and water vapour transmission properties of the polymeric films are presented in Table 3.1. 3.2  PREPARATION OF PEELED, CORED AND SLICED APPLES  Apples with no apparent bruises were selected, washed in a 50 ppm chlorine solution at room temperature and then rinsed with tap water. Apples were peeled and cored with a peel/core machine available in the Dept. of Food Science, LIBC. and were sliced radially into 8 segments (slices) with a fruit slicer. The average weight of the slices was about 19 g. The apple slices were immediately dipped in a 5% L-ascorbic acid (Sigma Chemical Co., St. Louis, MO) solution (pH 2.66) for about 5 min at 20°C (room temperature) and excess solution was removed by gravitational draining in a strainer for 5 to 10 min. Newtown apple slices used for both the respiratory and packaging studies were immersed in the 5% L-ascorbic acid solution for about 8 to 10 min. While excess solution was being removed by gravitational draining, the slices were also exposed to a  45  Table^3.1^Gas^transmission^rates^for^N2,^02,^and^CO2^and^the moisture^vapour transmission rate for the polymeric packaging films' Film type  Transmission rate2 N2  High barrier4 (CL 804)  0.02  02  M.V.T.R.3 CO2  0.1  0.3  0.3  Medium high barrier (6 mil polyethylene)  20.0  73.0  367.0  0.2  Medium low barrier (4 mil polyethylene)  30.0  110.0  550.0  0.3  1Gas transmission rate data were obtained from DuPont Canada, Kingston, Ontario. 2mL/100 in2/ 24 hr/ atm at 23°C. 3Moisture vapour transmission rate. g/100 in2/ 24 hr at 95% relative humidity. 4Arbitrary definitions.  46  air current from a summer cooling fan with the highest rotation velocity to remove excess moisture. One four liter L-ascorbic acid solution was used for each box of apples used in the packaging study. 3.3  SEALED IMPERMEABLE RESPIRATORY CHAMBER STUDIES  Respiration studies on apple slices were carried out in sealed gas impermeable polyvinyl chloride (PVC) respiratory chambers which were described by Beveridge and Day (1991). Sealed PVC respiratory chambers used to replicate a particular treatment had total volumes between 880 and 910 mL. Respiratory behaviour of sliced apples were studied at 1, 5, 10 and 15°C. For each temperature treatment, slices from four replicate batches were analysed. About 340 (±5) g of sliced apples were packed in each 2 mil polyethylene pouch and the top opening was closed by hand without sealing. The pouches were placed in refrigerated incubators (model 3825, Forma Scientific Co., Marieta, OH) held at 1, 5, 10 and 15°C for temperature equilibration. Granny Smith and Newtown apple slices were held at equilibrating temperature for about 10 and 5, hrs, respectively. The internal air temperature of the sample pouches was monitored with a digital electronic thermometer (Micronta Digital Thermometer, Tandy Corporation, Arlington, TX). After temperature equilibration, each replicate of apple slices was transferred to a respiration chamber previously held in the same incubator at the same temperature. The chambers were flushed with a gas mixture of 21.5% 02 and 78.5% N2 and then, immediatly thereafter, the lids were applied and tightened sufficiently to ensure a hermetic seal. The temperatures of the refrigerated incubators and cold rooms were monitored with mercury thermometers periodically (Fisher Scientific Co., Ottawa, ON). The refrigerated incubator temperatures were maintained with an accuracy of + 0.2°C. Headspace gas samples of 0.5 mL were removed from the chambers by insertng the needle of a 1.0 mL syringe (Dynatech Pressure-Lok series A, Precision Sampling Corporation, Baton Rouge, LA) through the septums of the sampling port.  47  When each chamber was opened, the headspace gas volume was determined by filling the chamber containing the apple slices with water and measuring the volume of the water with a 1.0 L measuring cylinder. The sealed respiration chambers with about 340 g of apple slices had headspace volumes of about 475 mL. The ratio of apple slice weight to chamber headspace volume was about 0.7:1. The average surface area for a peeled and cored Newtown apple when cut into 8 slices was about 650 cm2. In calculating the surface area of apples, each apple was considered to be spherical. The cylindrical core removed from each apple had a diameter of 2.4 cm. Therefore, average diameter of peeled apples less the core diameter was 4.34 cm. The sensory quality of slices were evaluated informally by a panel consisting of six persons in the Dept. of Food Science. Details are given in the section 3.9. 3.4  PACKAGING IN PLASTIC POUCHES  About 500 (±5) g of apple slices were placed into each plastic pouch (20 cm x 20 cm or 800 cm2). Prior to sealing, headspace air inside the each pouch was evacuated by a vacuum pump. The pouch was flushed with a gas mixture consisting of 21.5% 02 and 78.5% N2 (Union Carbide Canada Ltd., Vancouver, BC) and then heat sealed. Air evacuation, gas flushing and sealing were carried out with a vacuum sealer unit (PAC, model 1873VS, Packaging Aid Corporation, San Francisco, CA). The seal width was 11411 . Every sealed pouch, following inspection for proper sealing, was transferred immediately into the cold room maintained at 1°C. Pouches were laid on shelves lined with 2" mesh plastic fence material (available in hardware stores) in the cold room in order to facilitate maximum air circulation around the pouches. The temperature of the cold room was monitored with mercury thermometers (Fisher Scientific Co., Ottawa, ON). Sealed pouches, filled with 500 g of apple slices, had an average total volume of 1400 mL as determined by the displacement of water upon pouch submersion. The headspace of a  48  pouch was estimated to be about 900 mL and thus the apple slice volume would be about. 500 mL. The ratio of apple slice volume to package headspace volume was about 1:2. Every seven days, starting at day 0, head space gases (02, CO2, C2H4), pouch weight loss, sensory quality and texture of apple slices were evaluated. Four replicates were used for each packaging treatment. The juice prepared from the slices of each pouch was used to determine pH, titratable acidity and soluble solids. Hunter L. a, b values of puree and fresh intact apple tissue were determined with the use of the Hunter colour difference meter. Detailed descriptions of the methods of measurements used are described elsewhere. 3.5  FRUIT WEIGHT LOSS DURING STORAGE OF THE POUCHES  Each pouch of apple slices was weighed to the nearest 0.5 g prior to cold storage. At each seven day interval, pouches (four pouches per treatment) of apple slices removed from storage for gas analysis were reweighed before further analysis. 3.6  HEADSPACE GAS ANALYSIS  The headspace gas composition (02, CO2, C2H4 and N2) inside the PVC respiration chambers and the sealed pouches were determined with a gas chromatograph (Shimadzu model 14A, Shimadzu Corporation, Kyoto, Japan) equipped with a dual column (stainless steel columns, 1.8 m length and 3.2 mm internal diameter) setup (60/80 mesh, molecular sieve 5a for 02 separation; 80/100 mesh, Porapak-N for CO2 and C2H4 separation, Supelco Inc., Toronto, ON) and flame ionization (FID) and thermal conductivity (TCD) detectors attached to Shimadzu CR501 chromatopac integrater (Shimadzu Corporation, Kyoto, Japan). A column temperature (CITP) of 60°C, an injection temperature (INJT) of 150°C and TCD temperature (DETT) of 150°C were used. The total elution time was about 5 min and absolute retention time was used to identify peaks (Fig. 3.1). Helium carrier gas (rate: 1.75 kg/cm9 pressure for port P; 5.0 kg/cm2 pressure for the port M) and air (rate: 5 ml/min) were used.  49  Ctrs. •■-■ •  Cu  Fig.  CV')  3.1 Sample gas chromatogram for the standard gas mixture identification (Order of peaks; •  CO2' C H^02 and N2 ) 2 4'^-'  50  All of the pure gases (industrial grade) and the gas mixtures were obtained from Union Carbide Canada Ltd Vancouver, BC. The gas chromatograph (GC) was calibrated with 0.5 mL of a gas mixture having a composition of 14.00% CO2, 0.50% C2H4 and 4.49% 02, and the balance being N2. Each sample aliquot of 0.5 mL was withdrawn from a chamber or package with a 1.0 mL syringe (Dynatech Pressure-Lok series A, Precision Sampling Corporation, Baton Rouge, LA). During sampling, the plunger of the syringe was drawn completely to the rear end of the barrel. A drawn sample was isolated from the external atmosphere by turning the syringe nose two and half rounds back away from the barrel. Such manipulations of the syringe were carried out to minimize any sample mixing with external atmospheric air or loss of sample during the time from sample withdrawal to sample injection (Anonymous, 1989). 3.7 3.7.1  ANALYSIS OF CHEMICAL PROPERTIES  APPLE JUICE EXTRACTION  100 grams of apple tissue, blended in a Waring blender for about one min at high speed, was centrifuged at 13.200 x G for 15 min at 20°C. The supernatant was filtered through Whatman No. 4 filter paper (Whatman International Ltd., Maidstone, England). The filtered juice was used in the determination of percent soluble solids, pH and titratable acidity. Analytical determinations were carried out in duplicate. 3.7.2.1  DETERMINATION OF pH  The pH of undiluted crude juice was measured with a pH meter (Fisher Accumet pH meter, model 620. Fisher Scientific Co., Ottawa, ON). The pH meter was calibrated to a pH value of 4.00 with a certified standard buffer solution (Fisher Scientific Co., Fair Lawn, NJ) at 22.5°C. (Ruck, 1976).  51  3.7.2.2 PERCENT SOLUBLE SOLIDS Percent soluble solids (as sucrose) values were obtained with an Abbe digital refractometer (model Mark II, Cambridge Instruments Inc., Buffalo, NY) at about 21°C (Ruck, 1976). 3.7.2.3 PERCENTAGE TITRATABLE ACIDITY (%TA) 10.0 g of undiluted apple juice, diluted to 100 ml with double distilled deionized water, was titrated with 0.1N NaOH to pH 8.1, using a Fisher Accumet pH meter (model 620, Fisher Scientific Co., Ottawa, ON) for pH measurements. The pH meter was calibrated to a pH value of 7.00 with a certified standard buffer solution (Fisher Scientific Co., Fair Lawn, NJ) at 22.5°C. %TA was calculated as the percentage weight of malic acid according to the following formula (Ruck, 1976): %TA  equivalent weight of malic acid x normality of NaOH x titre 10 x weight of the sample  3.7.2.4 SOLUBLE SOLIDS/TITRATABLE ACIDITY RATIO Soluble solids/titratable acidity ratio for each treatment was calculated by dividing the percent soluble solids by corresponding percent titratable acidity value. 3.8  ANALYSIS OF PHYSICAL PROPERTIES  3.8.1 SURFACE COLOUR MEASUREMENT OF FRESH APPLE SLICES Surface colour of ascorbic acid - treated fresh Newtown apple slices was measured as Hunter L, a and b values with a Hunter-Lab spectrocolorimeter (model Labscan 6000, 0°/45° obsever, Hunter Associates Laboratory Inc., Reston, VA) fitted with D-65 illuminant, and 6.35 mm aperture. The optical head had been interfaced with an IBM-XT personal computer.  52  personal computer. The spectrocolorimeter (100 observer) was standardized with white (Standard No. LS -1368.5; X = 79.80, Y = 84.67, Z = 91.23) and black tiles provided. The surface colour in terms of Hunter L, a Sz, b values was measured at 0, 30, 60, 90 and 120 min after exposure to air with the viewing port of the Hunter-Lab spectrocolorimeter in the downward direction. All three surfaces of the apple slice were scanned by holding the apple slice beneath the viewing port with the hand. Viewing port had covered from external light with black 8 mil polypropylene sheet material during measurements. Fifteen slices per replicate were scanned. 3.8.1.1 THE MEASUREMENT OF BROWNING OF APPLE PUREE IN AIR The degree of oxidative browning of ground apple puree was measured as Hunter L, a and b values. Ground apple puree was prepared by macerating apple slices from each replicate with a Kitchen Aid (model K5A, The Hobart MFG. Co., Troy, OH) grinder (model FGA, Kitchen Aid, Inc., St. Joseph, MI). A layer of apple puree (about 7 mm thick) was placed in each plastic petri dish (100 x 15 mm sterilized disposable plastic petri dishes, Fisher Scienctifc Co., Ottawa, ON). Petri plates were covered with the lids except during measurements. The surface colour in terms of Hunter L, a & b values of the puree was measured at 0, 30, 60, 90 and 120 min after maceration (Sapers & Douglas, 1987) with the viewing port of the Hunter-Lab spectrocolorimeter in the downward direction. Five readings per petri plate (at four corners and the middle) for two petri plates per replicate were recorded. 3.8.2  TEXTURE MEASUREMENT  An Instron Universal Testing Machine (model 1122, Instron Corporation, Canton, MA) with a .500 kg load cell attached to a IBM-XT compatible computer with a JCL6000 chromatography data system program (Mandel Scientific Co. Ltd., Guelph, ON) was used  :5 3  for Instron texture profile analysis (TPA) and plunger probe test of apple slices. The JCL6000 computer software recorded 10 data points per sec. These data points were plotted [number of millivolts (mV) against time] continuously. The maximum strength of the signal was 10 mV and the time axis was variable. Data on a 360K floppy diskette were later transformed into force deformation curves with the use of Lotus - 123 program (version 2.1, Lotus Development Corporation, Cambridge, MA). The mV signal was calibrated against a known weight (2 Kg) in order to convert the mV signal to force in Newtons. TPA and firmness measurements of slices at about 22°C of each data set per replicate consisted of 16 measurements (two readings per slice and eight slices per replicate). 3.8.2.1  FIRMNESS  The firmness of apple slices was determined as described by Bourne (1965). A wooden support with a wedge-shaped groove was used to mount each apple slice prior to probe penetration. The top surface of each apple slice was cut in order to make a flat contact surface for the probe. A flat tip probe (8.0 mm in diameter) was lowered perpendicularly into the apple tissue at a rate of 5 cm/min (sensitivity 2 kg and chart speed 20 cm/min) to a depth of 8.0 mm. Force deformation curves (Fig. 3.2) generated for each reading were used to calculate the bioyield point force (N), rupture point force (N), deformations (mm) up to the bioyield point and to the rupture point, the bioyield point force to deformation ratio (N/mm) or the fruit firmness and the rupture point force to deformation ratio (Njrnm) were also calculated. 3.8.2.2  TEXTURE PROFILE ANALYSES  The texture profile analysis of apple slices were carried as described by Bourne (1968). A cork borer with a internal diameter of 16.0 mm was used for the preparation of each apple cylinder which was removed perpendicular to the core. Cylinder pieces with a height of 10 mm were cut with a scalpal blade. Height of the cylinders was checked with a digital dial electronic caliper (Mitutoyo. Japan).  54  RUPTURE  BiOYELD P1 LU CC^ 0 U.  RUPTURE  PI  DPS  DISTANCE Fig. 3.2 Force-distance curves for materials with and without bioyield point. PI = point of inflection, DPI = distance at point of inflection. (modified from Mohsenin, 1981).  DO  For texture profile analysis, a compression plate was lowered onto the top of each apple cylinder at a 5 cm/min crosshead speed and 50 cm/min chart speed. Two force cycles with 80% compression were used. The same number of readings used in determination of firmness were taken. Lotus - 123 program was used to transform TPA data from mV to force (N) values. The initial maximum force (fracturability) and maximum force for the first bite (hardness-1) and for the second bite (hardness-2) were calculated (Fig. 3.3). 3.9  SENSORY EVALUATION  The panelists (25 to 65 years of age, four males and two females, all non smokers) involved in this study gained experience in sensory analysis of apple slices during training sessions when the panelists and the leader discussed the evaluation of quality attributes and sensory methodology. Seven trained panelists from the Department of Food Science, UBC, evaluated apple slices for apple aroma in the mouth, sweetness, sourness, sweetness/sourness balance, firmness with finger tip pressure, crispness (sound created during tissue breaking in mouth), off-flavour and overall acceptability. A list describing off-flavours of apples was provided on the evaluation sheet for the identification of types of off-flavours. A descriptive five-point rating scale (10 cm) was used (Appendix). Each panelist placed a slash at a point for the judged numerical value of an attribute. The dividing line at the midpoint between very acceptable and unacceptable scores was arbitrarily set at a value of 3.0 except for whiteness which was 4.0. Two sessions were conducted in each day of evaluation. The second session (two replicates per session) was conducted 30 min after the first. For each evaluation session, two slices (one slice from each replicate) were provided in white disposable plates coded with 3 digit figures. Water and unsalted crackers for pallet clearing were provided between each sensory evaluation. Evaluation was conducted in sensory panel room lighted with white fluorescence lights (Larmond, 1977).  FIRST BITE  ^SECOND BITE  —DOY,'NSTROKE ^ e -4^UPSTROKE  a-  ^DOWNSTROKE---0. -4-UPSTROKE t-  HARDNESS 1  TIME  Fig. A generalized Texture Profile analysis curve obtained from the Instron Universal testing machine. (modified from Bourne, 1982b)  57 3.10  STATISTICAL ANALYSIS  The statistical design involved a two way - two factor factorial design with 3 packaging films X 7 wk storage periods (includes wk 0) X 4 replicates. Data were analysed for effects of packaging films, storage time, the film type and storage time interaction and replicates. Whenever a set of means were statistically significant, they were compared with Tukey's HSD posthoc test. All the analyses were carried out with Systat (version 5.1, Systat Inc., Evanston, IL) statistical analysis program with a IBM-AT compatible personal computer. However, regression analysis of respiration studies results was carried out with Lotus - 123 spreadsheet software.  58  4. RESULTS AND DISCUSSION 4.1 RESPIRATION STUDIES IN SEALED GAS - IMPERMEABLE CHAMBERS 4.1.1  GRANNY SMITH APPLES  During storage of Granny Smith apple slices, in sealed gas - impermeable respiration chambers held at 1, 5, 10 and 15°C, 02 levels in the microatmospheres progressively decreased to levels below 2%. Rates of 02 depletion were dependent on the storage temperature (Figs. 4.1 to 4.4). The 02 consumption curves in Figs. 4.1 to 4.4 (graphical relationships between chamber 02 concentration and storage time) for Granny Smith apple slices were biphasic with the regions of linear and logarithmic relationships. The linear segments of the 02 consumption curves (r2 ) 0.97) and the corresponding segments of the CO2 accumulation curves (r2 ) 0.98) had very high linear correlations. Logarithmic portions of the 02 consumption curves were short and limited to a few data points. Beveridge Sz Moyls (1988) identified linear and logarithmic segments of a 02 consumption curve for McIntosh apples at 1°C as the 02 - saturated and the 02 - unsaturated regions, respectively. At the end of each 02 consumption curve, a few data points did not change with storage time. These low constant 02 levels may be due to the presence of argon and/or 02. With the gas chromatographic method used in this study for gas analysis, argon coeluted with 02 and thus the proportions of the two gases could not be estimated. Apparently 02 used in industry may contain a trace amount of argon as a contaminant (Verzele et al., 1981). Moreover, the fruit tissue may have contained some argon as part of intercellular space gas. Beveridge and Day (1991) reported that, in their studies in the respiration of sweet cherries in impermeable chambers with air as the input gas, small amounts of 02 (corrected for argon content) existed at the end of the logarithmic curve. The transition 02 level for Granny Smith apple slices increased slightly with storage temperature. For the investigated range of storage temperatures, the transition  24 20  0 25 50 75 100 125 150 175 200 225 250 275 Time (Hours) C O2  02  Fig. 4.1 Changes in headspace 02 and CO2 lev2ls in sealed chambers with Granny Smith apple slices at 1uC . Bars indicate S.E.M.  0  ^  20  ^  40  ^  60^80  ^  100 120 140 160  Time (Hours) 002  02  Fig. 4.2 Changes in headspace 02 and CO2 levtls in sealed chambers  with Granny Smith apple slices at^C.  Bars indicate S.E.M.  30 25  10 5 0  0^20^40^60^BO  100  120  140  Time (Hours) CO2  02  Fig. 4.3 Changes in headspace 02 and CO2 lev2ls in sealed chambers with Granny Smith apple slices at iouc. Bars indicate S.E.M.  160  30 25  10  5  0^10 20 30 40 50 60 70 80 90 Time (Hours) CO2  02  Fig. 4.4 Changes in headspace 02 and CO2 levels in sealed chambers with Granny Smith apple slices at I5ic. Bars indicate S.E.M.  63  02 levels ranged from 0.81% at 1°C to 4.5% at 15°C (Table 4.1). Beveridge and Moyls (1988) using the respiration data published by Jurin and Karel (1963) calculated a transition 02 level of 6.7% for uncut McIntosh apples at 1°C. The rate of respiration (respiration parameter, bo) for 02 levels in the saturated region of the 02 consumption curve for each storage temperature was calculated according to the equation reported by Beveridge and Day (1991) as follows:  Respiration parameter (b0) =  V (Head space volume) X d%02 100 X W (weight of fruit tissue). dt(hr)  Where: bo = mL/kg/hr; V = mL; W = kg As shown in Table 4.1, respiration rate increased with increasing storage temperature for sliced apples. The Qi0 value for Granny Smith apple slices at the selected temperatures between 1 and 15 °C was four. Respiration rates of fruits and vegetables have Q10 values between 2 to 7 (Powrie, personal communication). Respiratory quotients (RQ) for apple slices were calculated by dividing the value of the rate of respiration for the linear portion of the CO2 accumulation curve by the corresponding value of the 02 consumption curve (Beveridge & Day, 1991). RQ values of apple slices decreased as the storage temperature was lowered but the differences were not statistically significant (p > 0.05). RQ values of sweet cherries cvs, lambert, Van and stella studied under similar conditions were significantly different (Beveridge & Day, 1991). As shown in Fig. 4.5, RQ values were not influenced by the chamber 02 concentration until 02 levels dropped below about 1.5% at which point the RQ rose to a value as high as 15. Arrhenius relationship between the rates of respiration and storage temperatures between 1 and 15°C for apple slices (Fig. 4.6) was linear (r2 > 0.99). The average activation energy (Ea) for the storage temperature range of 1 to 15°C was -64.15 kJ per mole. The negative value of Ea and also the straight line without breaks implies that, within the range of storage tempertures,  64  Table 4.1 Respiration parameters for Granny Smith apples slices at various storage temperatures  Temperature ((DC)  Range of transition 02 levels (%)  ResRiration rate! (mL/kg/hr)  Respiratory quotientl  1  1.3^- 2.0  1.45  ± 0.09d  0.59 ± 0.02a  5  0.81 - 2.0  2.25 -I- 0.12c  0.79 ± 0.04a  10  1.0^- 3.0  3.27 ± 0.18b  0.84 + 0.03a  15  3.2^- 4.5  6.04 + 0.15a  0.85 ± 0.01a  1 Mean (n = 4) + standard error of the mean. Values in each column were significantly different (p < 0.01). Different letters represent significantly different means calculated with Duncan's new multiple range test.  16  12  2^4^6^8^10 12 14 16 18 20 CHAuliek OXYGEN CUADINTRATUDIN 00 Rep. 1^• Rep. 2^- Rep. 3 Fig. 4.5 Effect of headspace 02 concentration on RQ of Granny Smith apple slices stored at 1vC.  2.00  1.60  0 n C _1  1.20  0.80  -  -  •  ^  -  0.40  0.00 3.45  1  ^I^  3.50  3.55  I  I^  3.60  3.65  1/TEMPERATURE (1/ K x 10 ) FIG. 4.6 ARRHENIUS RELATIONSHIP FOR GRANNY SMITH APPLE SLICES  3.70  67  there would be no tissue damage by chilling injury (Goodenough & Wright, 1981). Ethylene was detected in the microatmosphere of the apple slices at all storage temperatures. Small peaks for C2H4 were observed in the gas chromatograms of gas samples from chambers with apple slices and initial values of C2114 are reported in Table 4.2. For slices held at 1°C, quantitative values for C2H4 could not be determined because the peak area values were below the minimum for integrator response. As shown in Table 4.2, the time of onset of C2114 production was delayed as the storage temperature decreased. According to Yang (1981), the optimum temperature for the maximum C2H4 production in apples is about 25°C. Knee (1980) reported that inhibition and delaying of the production of C2H4 in apples occurred when 02 level in the microatmosphere was low. Ethylene production in apple slices stored in the chambers would have occurred when 02 levels were relatively high (02 saturation region). 4.1.1.1 SENSORY QUALITY ATTRIBUTES OF GRANNY SMITH APPLE SLICES AFTER THE RESPIRATORY STUDY  Sensory analysis of the apple slices was carried out to assess quality attributes after the chambers were opened at the end of the respiration experimentation. The data are presented in Table 4.3. As shown in Table 4.3, the overall acceptability of each of the samples stored between 1 and 15°C was rated low with values between 2.7 and 3.1. In particular, the apple  flavour and aroma, crispness, force required to shear by teeth and firmness (by the finger tip pressure) were scored consistantly low for all slice samples. An off-flavour wa.s detected in each of the samples and, presumably, anaerobic respiration was responsible for the development of off-flavour compounds. In general, the apple slices remained white with little or no brown spots. At a storage temperature of 15°C, some apple slices had translucent surfaces after the duration of the respiration studies. This translucent appearance may be due to water logging caused by absorption of water during ascorbic acid dipping.  68  Table 4.2 Ethylene detection time and initial concentration in the headspace of respiration chambers containing Granny Smith apples slices at various storage temperatures  Temperature (oc,)  Initial detection time (hrs)  Initial C2H4 level (ppm)  15  10 -^24  2891  10  23 -^34  2231  5  45 - 120  1522  1  'Average for four replicates. 2Average for two replicates.  -  69  Table 4.3 Sensory evaluation' of Granny Smith apple slices after completion of the respiration studies at various storage temperatures  Sensory attribute  Temperature (°C)  1  5^10  15  Apple flavour  2.5 + 1.12  2.3 + 0.8  2.3 + 0.9  2.3 + 1.1  Firmness (by finger tip pressure)  3.3 + 1.0  2.8 + 1.2  3.1 + 0.6  3.3 + 0.8  2.5 + 0.9  1.8 + 0.8  2.0 + 1.0  2.9 + 1.0  Force required to shear by teeth  2.4 + 0.7  2.1 + 0.6  1.8 + 0.6  2.9 + 1.0  Whiteness  4.3-4- 0.7  3.7+ 1.0  4.8 + 0.3  4.5+ 0.5  Apple aroma  1.9 + 0.5  2.6 + 1.0  1.7 -I- 0.7  1.9 + 0.6  Off-flavour  3.9 + 1.4  3.6 + 1.4  3.9 + 1.1  3.6 + 1.1  Overall Acceptability  2.9 + 1.4  2.7 + 0.8  3.1 + 0.9  2.7 + 1.0  Crispness (sound created in mouth)  'Sensory rating of 1 to 5, with 5 being excellent. 2Mean (n = 5) + standard error of the mean.  70  Ponting 8z, Jackson (1972) reported that apple slices vacuum infiltrated with different solutions of anti-browning agents had translucent or waterlogged appearance. 4.1.2  NEWTOWN APPLES  As shown in Figs. 4.7 to 4.10, the 02 consumption curves for Newtown apple slices were biphasic at all storage temperatures except at 1°C. With apple slices at 1°C, a single logarithmic regression curve for 02 consumption was observed (Fig. 4.7). Linear portions of the 02 consumption curves for slices stored at 5, 10 and 15°C had high linear regression coefficients (r2 0.95). The biphasic 02 consumption curves for apple slices indicated that the rate of 02 consumption increased with temperature from 5 to 15°C. Transition 02 levels for Newtown apple slices decreased with increasing storage temperature and ranged from 2.2% 02 at 5°C to 5.9% 02 at 10 and 15°C. Transition 02 levels of Newtown apple slices were slightly higher than those of Granny Smith apple slices (Table 4.4) at corresponding temperatures. Because of the logarithmic nature of the respiratory curve for Newtown apple slices stored at 1°C, a transition 02 level was not apparent. The increase of the transition 02 level for apple slices with increasing storage temperature can be explained in part on the basis of increased respiration rate in the fruit tissue (Chevillotte, 1973; Solomos, 1985; Tucker & Laties, 1985). For an adequate 02 level in the mitochondria to ensure aerobic respiration, a specific minimum concentration gradient between the mitochondrial respiration sites and the microatmosphere must be maintained. With an increase in respiration rate of tissue due to temperature increase a higher specific minimum concentration gradient will be required; thus, the transition 02 level should be higher. Newtown apple slices had RQ values lower than 1 (Table 4.4). These values are comparable with those reported by Beveridge and Day (1991) for cherries and Powrie et al. (1990a) for blueberries, cv. Bluecrop, sweet cherries, cvs. Van, Stella and Lambert and  25  0  20  147,  15  o  10  co  0 25 50 75 100 125 150 175 200 225 250 275 Time (Hours) ^ CO2  ^  02  Fig. 4.7 Changes in headspace 02 and CO2 levels in sealed chambers  with Newtown apple slices at luC.  Bars indicate S.E.M.  25  20 C o le--;^15  (2  +-, C  a) 0 C o 10 0 Cl)  ti5  0  1^I^I^I^I^I^1^  I^I^I  0 25 50 75 100 125 150 175 200 225 250 275 Time (Hours)  C O2  02  Fig. 4.8 Changes in headspace 02 and CO2 levels in sealed chambers with Newtown apple slices at ^C. Bars indicate S.E.M.  25  20 o  +7, a)  15  o  10  •  20  ^  40.  ^  60  ^  80  ^  100  Time (Hours) 002  02  Fig. 4.9 Changes in headspace 02 and CO2 levels in sealed chambers with Newtown apple slices at teC. Bars indicate S.E.M.  45 40 —8 35  30 25 20 15 co 10 5  0 0 10 20 30 40 50 60 70 80 90 100 110 Time (Hours)  ^  CO2  Fig. 4.10 Changes in headspace 02 and 92 levels in sealed chambers with Newtown apple slices at feC.Bars indicate S.E.M.  75  Table 4.4 Respiration parameters for Newtown apples slices at various storage temperatures  Temperature (oc)  Range of transition 02 level (%)  1  Respiration ratel (mL/kg/hr)  Respiratory quotient'  1.45 + 0.17d  0.52 + 0.05a  5  2.9 - 5.9  3.09 + 0.36c  0.60 + 0.04a  10  2.2 - 4.8  5.08 + 0.50b  0.67 + 0.09a  15  2.6 - 4.3  6.97 + 0.21a  0.68 + 0.02a  1 Mean (n = 4) + Standard error of the mean. Values in each column were significantly different (p < 0.01). Different letters represent statistically different means calculated with Duncan's new multiple range test.  76  strawberries, cv. Ranier. RQ values lower than unity for fruits may be attributed, in part, to the decarboxylation of organic acids under high CO 2 levels in the microatmosphere and also by CO 2 - fixation by PEP-carboxykinase and PEP-carboxylase (Ulrich, 1970; Wang, 1990). Fig. 4.11 shows the Arrhenius relationship between the respiration rates and storage temperatures of 1 to 15°C. The activation energy calculated from the slope of the linear regression curve was -71.76 kJ per mole. The negative value of E a suggests that no chilling injury should occur between 1 and 15°C (Goodenough & Wright, 1981). According to Bramlage and Meir (1990), Yellow Newtown apples are sensitive to chilling injury when stored at temperatures near 0°C. Even chilling-sensitive apple cultivars can be stored for periods up to about three months at near chilling-sensitive temperatures without chilling injury. Low 0 2 and high CO 2 in the microatmosphere may suppress or enhance chilling injury symptoms depending on the composition of the microatmosphere (Bramlage Meir, 1990). The effect of headspace 0 2 level on the rate of respiration of Newtown apple slices is presented in Fig. 4.12. The respiration rate decreased markedly until about 13% 0 2 concentration in the microatmosphere was reached and, thereafter, respiration rate dropped slowly as the 02 level decreased to 4% 02. Respiration rate decreased further at 0 2 levels below 4%. The RQ of Newtown apple slices remained constant to an 0 2 level of 1.5% whereupon it rose markedly (Fig. 4.13). The comparision of effects of 02 concentration on respiration rate (Fig. 4.12) and RQ (Fig. 4.13) shows that the headspace 02 concentration required for the lowest rate of respiration of Newtown apple slices without anaerobiosis was between 4 and 1.5% 02 at 1°C. Blanke (1991) reported that the differences between high respiring and slow respiring fruit species were due to the their differences in the structure and function of mitochondria, the operation of cyanide resistant respiratory pathway, accumulation of CO 2 in the  2.00  •  1.60  • 0.80  •  0.40  0.00 ^ 3.45  3.50^3.55^3.60^3.65^3.70 1/TEMPERATURE (1/ K x 10  -3 )  FIG. 4.11 ARRHENIUS RELATIONSHIP FOR NEWTOWN APPLE SLICES  10 8 6 4 2  0^5^10^15^20  ^  25  Chamber headspace OiconcentratIon (%) Fig. 4.12 Effect of headspace 02 concentration on the respiration rate of Newtown apple slices stored at 1°C  15 12 9 6 3  0^5^10^15^20  ^  25  Chamber headspace OponcentratIon (%) Fig. 4.13 Effect of headspace 02 2oncentration on the RQ of Newtown apple slices stored at 1vC  80  intercellular spaces, CO2 diffusion rate within the fruit and CO2-refixation via PEPcarboxylase. Therefore, the differences in the respiratory behaviour of two cultivars of apples can be due to cultivar differences and to the differences in the maturity stage of fruits. De Barsy et al. (1989) studied subcellular changes of Golden Delicious apples during 7 - month storage at 10°C in air. Upon harvesting, parenchyma cells of mature apples were at the start of senescence and cellular membranes, plastids and mitochondria were disappearing. Senescence of apple cells continued as the length of the storage proceeded. At the end of the storage period, only a few mitochondria, fragments of membranes and vesicles in the parenchyma were evident. The researchers hypothesized that apples, harvested before the senescence of parenchyma commenced, could withstand long-term storage. Therefore, high respiratory activity in Newtown apple slices may be attributed to the presence of intact organelles. 4.1.2.1  SENSORY QUALITY ATTRIBUTES OF NEWTON APPLE SLICES AFTER THE RESPIRATORY STUDY  Sensory evaluation data of Newtown apple slices removed from the chambers after respiration studies are presented in Table 4.5. Regardless of the storage temperature, panel ratings of overall acceptability were moderately low (2.9 to 3.5). Sourness of the slices at all storage temperatures was moderately acceptable (3.0 to 3.5) but the sweetness was considered by the panel to have a low rating (2.4 to 2.8). The utilization of sugars as substrate in the respiration processes would account for the low sweetness levels. The apple odour in the mouth for slice samples from all of the chambers was considered by panel members to be low. It is of interest to note that tissue firmness and crispness of all samples, regardless of storage temperature, were rated moderately high. In contrast to Granny Smith apple slices, Newtown apple slices developed surface browning during storage at 5, 10 and 15°C and were rated low in whiteness by the panel.  81  Table 4.5 Sensory evaluation' of Newtown apple slices after completion of the respiration studies at various storage temperatures  Sensory attribute  Temperature (°C) 1  5^10  15  Apple odour in mouth  2.9 + 0.212  2.7 + 0.4  3.0 + 0.3  2.8 + 0.3  Sweetness  2.8 + 1.2  2.8 + 1.3  2.8 + 1.2  2.4 + 1.0  Sourness  3.4 + 1.6  3.5 + 1.6  3.0 + 1.3  3.2 + 1.3  Sweet/sour balance  2.6 + 1.2  2.6 + 1.2  2.9 + 1.3  2.4 + 1.0  Firmness (by finger tip presure)  4.2 + 1.9  3.8 + 1.7  4.3 + 1.4  4.0 + 1.7  Crispness (sound created in mouth)  3.8 + 1.7  3.5 + 1.5  3.8 + 1.5  3.6 ± 1.5  Force required to shear by teeth  3.5 + 1.6  3.3 + 1.5  3.3 + 1.4  3.1 + 1.3  Whiteness  4.1 + 1.8  3.2 + 1.5  2.9 + 1.3  2.2 ± 1.2  Off-flavour  4.4 + 2.0  3.8 + 1.7  4.3 + 1.8  3.4 + 1.5  Overall acceptability  3.5 + 1.6  3.0 + 1.4  3.6 + 1.5  2.9 + 1.2  1Sensory rating of 1 to 5, with 5 being excellent. 2Mean (n = 7) + standard error of the mean.  82 4.2 NEWTOWN APPLE SLICES IN FLEXIBLE FILM POUCHES 4.2.1  RESPIRATORY BEHAVIOUR OF NEWTOWN APPLE SLICES IN PLASTIC POUCHES  Sealed pouches filled with about 500 (±10.0)g of apple slices had an average total volume of 1400 mL as determined by the water displacement method. The headspace of a pouch was estimated to be about 900 mL and the apple slice volume would be about 500 mL. The ratio of apple slice volume to package headspace volume was about 1:2. As shown in Table 4.6, the pouch headspace 02 and CO2 levels were significantly influenced by film type (p < 0.001), storage time (p < 0.001) and film and storage time interaction (p < 0.001). As shown in Table 4.7, for storage times between 2 and 6 weeks, the headspace 02 levels for HB, MHB or MLB film - type pouches were not different appreciably. MLB and HB film - type pouches had the highest and the lowest headspace 02 levels, respectively. Changes in the headspace 02 and CO2 levels in sealed pouches containing Newtown apple slices during storage at 1°C are presented in Fig. 4.14 and Tables 4.7 and 4.8. The headspace 02 levels for the apple slices packed in HB, MHB and MLB film - type pouches dropped from 21.5% at the start of the storage to below 2% over a 2 - week storage period and thereafter, headspace 02 remained at the same low levels. The CO2 levels in the -headspaces of pouches for each film type were markedly different at specific storage times from 2 to 6 weeks. As shown in Fig. 4.14, CO2 levels in the headspace of HB film - type pouches rose steadily in linear fashion from 0 to 5 weeks of storage and reached a level of 35% at the 5th week of storage. At week 6, the CO2 level in the headspace dropped to 31% (Table 4.8). With the more gas permeable films, MHB and MLB, the CO2 levels in the slice - containing pouches increased over a 2 - week storage period to about 9% for the MHB film and around 6% for the MLB film (Table 4.8). With subsequent storage of the apple slices in the pouches, the respective headspace CO2 levels for the MHB and MLB film - type pouches remained relatively constant up to 5 weeks but rose slightly at the sixth week storage period.  83  Table 4.6 Analyses of variance for factors influencing the headspace CO2 and 02 concentrations in pouches containing Newton apple slices at 1°C.  Type of^Source of gas^variation Oxygen  SS  Block 1.23 4.73 Packaging film Storage time 4355.18 P. film x 42.18 storage time 10.39 Error  DF  MS  F-ratio  0.410 2.365 725.863  2.33 13.44 4122.22  0.083 0.001 0.001  3.515 0.176  19.96  0.001  3 2 6  1.563 1709.820 484.585  0.82 893.15 253.15  0.490 0.001 0.001  12 60  144.809 1.914  75.64  0.001  3 2 6 12 59  Carbon dioxide 4.69 Block Packaging film 3419.64 Storage time 2907.51 P. film x 1737.71 storage time 114.86 Error  40.0 36.0 32.0 28.0 24.0 20.0 16.0 12.0 8.0 4.0 0.0 0  ^^ ^ 1 2^3^4^5 7  Storage time (weeks) HB film  - OMB f i lm M17-AfTim  Fig. 4.14 Changes in headspace 02 and CO2 levels in sealed p2uches containing Newtown apple slices during storage at luC  ^ ^  85  Table 4.7 Changes in the headspace 02 concentrations ,(%) for MAP Newtown apple slices during storage at luC Storage time (weeks)  Packaging material' HB  ^  MHB^MLB^P 3  0^21.51 + 0.00 2 a^21.51 + 0.00a^21.51 + 0.00a^ns  ^5^a^a  1^8.08 4 + 0.77b^9.18 + 0.37b^5.02 + 0.60b^0.015  ^a^b^ a  2^0.43 + 0.05c^0.87 + 0.09c^1.45 + 0.37c^0.024 a^ oTio^ b 3^0.29 + 0.03c^0.83 + 0.06c^1.26 + 0.04c^0.001  ^a  ^  c b^ 4^0.25 + 0.01c^0.94 + 0.06c^1.36 + 0.03c^0.000 c a^ b^  5^0.27 + 0.04c^0.98 + 0.03c^1.42 + 0.13c^0.011  ^a^b^  c 6^0.32 + 0.06c^0.96 + 0.04c^1.25 + 0.02c^0.002 a^ b^ c P 3^0.001^0.004^0.001  1 HB = High barrier; MHB = Medium high barrier; MB = Medium low barrier.  2 Mean (n = 4) + standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by the Tukey's test at or below the corresponding probability given beneath while the different lower letters indicate that the means within the same row are significantly different by the Tukey's test at or below the probability given across. 3 The maximum probability among the p - values for pairs of means calculated by the Tukey's test. ns = not significant. 4 Average of three replicates.  ^ ^  86  Table 4.8 Changes in the headspace CO2 concentrations (%) for MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging materiall HB^MHB^ MLB3 P 4  0^0.00 + 0.00 2 a 0.00 + 0.00a^0.00 + 0.00a^a^a^Ct 1^7.29 + 0.27b^5.20 + 0.09b^5.34 + 0.23b^0.001 ^ b^ b ^a 2^15.97 + 0.70c^8.81 + 0.18c^6.00 + 0.24b^0.004 ^a ^ b^ c 3^21.24 + l.09 + 0.25c^5.86 + 0.23b^0.007 a^ b^ c 4^10.08 + 1.35e 10.09 + 0.47c^5.75 + 0.15b^0.013 a^ E.^ c ,  ^  5^34.81  + 1.24e 10.50 + 0.53c^6.16 + 015b^0.009 a^b^ c 6^30.59 + 1.75e 13.46 + 0.98d^7.29 + 0.13c^0.011  ^o. P  3  4  ^  b^  c  0.033^0.004^0.004  1 HB = High barrier; MHB = medium high barrier; MLB = medium  low barrier.  2 Mean (n = 4) + standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability given beneath while the different lower letters indicate that the means within the same row are significantly different by Tukey's test at or below the corresponding probability given across. 3 The maximum probability level among significant p - values  among pairs of means calculated by the Tukey's test.  87  At 23°C, the rate of 02 permeation through the walls of HB film pouches was calculated to be 0.005 mL/800 cm2/hr/atm. At such a low rate of 02 permeation, insufficient 02 in the headspace of the HB film - pouches would lead to anaerobic respiration in the apple slices. Such a situation is apparent from the ongoing rise in the CO2 accumulation curve for the HB film pouch. Fidler et al. (1973), after investigating post-storage quality of a large number of CA - stored apple cultivars, recommended that microatmosphere 02 levels below 1 to 3% and CO2 levels above 10 to 13% should not be used. However, headspace 02 levels in HB and MHB film - type pouches and CO2 levels in HB film - type pouches after the second week exceeded those limits in the present study. Very low 02 and high CO2 levels can cause physiological disorders and off-flavour development (Fidler et al., 1973; Kader, 1986; Kader et al., 1989; Powrie & Skura, 1991; Wang, 1990). Localized anaerobic respiration in apples is natural (Zemliakhunin & Ivanov, 1978) and apples can tolerate anaerobic respiration for about two weeks without detectable quality changes (Wollin et al., 1985). Therefore, if apple slices attained anaerobic respiration during the second week, it would be expected that the shelf life of the slices would be about four weeks. Ultra low 02 levels such 0.5% 02 have been shown to be useful in maintaining apple quality (Lidster et al., 1983; Massey, 1989; Powrie & Skura, 1991). Safe minimum levels of 02 and CO2 requirements for Newtown apples are not known. No C2H4 was detected in the headspaces of the apple slices within the various film type pouches throughout the storage period. Lack of C2H4 production by apple slices can be attributed to the inhibition of C2H4 synthesis by low 02 and high CO2 (Kader et al., 1989; Knee, 1980; Powrie & Skura, 1991) and by the low storage temperature.  88 4.2.2  FRUIT WEIGHT LOSS DURING STORAGE  Pouches drawn every week from storage had about the same weight as their initial weight recorded at the time of sealing. It is apparent that all those film types were capable of inhibiting water loss from apple slices. 4.2.3  CHEMICAL ANALYSIS  4.2.3.1  PERCENT TITRATABLE ACIDITY (%TA)  The effects of storage time and film type on the percent titratable acidity of MAP Newtown apple slices at 1°C are presented in Table 4.9. Regardless of the package film type, the titratable acidity decreased significantly from about 0.58 to 0.48 - 0.51% over the 6 week storage period (p < 0.001). Slices in MLB film - type pouches had the least change in titratable acidity content over time (P < 0.027). Percent titratable acidity (g malic acid/ 100 g of apple flesh) of apple slices was significantly different (Table 4.10) in pouches consisting of three different films (p < 0.001). About 80 to 90% of the titratable acidity of apples consists of malic acid which is the major respiratory substrate (Smock & Neubert, 1950; Ulrich, 1970). The respiratory utilization of malic acid reduces the total organic acid content in apple tissue. The decrease of apple acidity is part of natural ripening process (Lee, 1983). High levels of CO2 in modified atmospheres can cause rapid reduction of organic acids in apples (Kader, 1986; Kader et al., 1989; Siriphanich & Kader, 1986; Weichmann, 1986). The difference in the acidity of apple slices in HB and MHB film pouches which had high CO2 levels and the acidity of apple slices in MLB film pouches which possessed a low CO2 level may be attributed to the differences in the CO2 content in the microatmospheres. 4.2.3.2 pH  VALUES  pH values for the apple juice supernatants of the homogenates from stored apple slices taken from the three different film - type pouches are presented in Table 4.11.  ^  89  Table 4.9 Changes of percent titratable acidity levels (g malic acid /100 g apple t4ssue) of MAP Newtown apple slices during storage at luC. Storage time (weeks)  Packaging material' HB  ^  MHB^MLB^P 3 <  2  0^0.58 + 0.03 a^0.58 + 0.03a^0.58 + 0.03ac^5^a^ a 1^0.54 + 0.02ab^0.57 + 0.02a^0.63 + 0.01a^0.035 a^a^ b 2^0.55 + 0.01ab^0.51 + 0.01ab^0.57 + 0.02ac^0.039 ^aT^b^ a 3^0.50 + 0.01b^0.52 + 0.01ab^0.54 + 0.01bc^0.029 a^ab^b 5^0.47 + 0.01b^0.48 + 0.01b^0.52 + 0.00bc^0.001 ^a^a^ b 6^0.48 + 0.01b^0.48 + 0.01b^0.51 + 0.01bc^ns a^a^ CA 3 0.008^0.009^0.027  P.;  1 HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier.  2 Mean (n = 4) + standard error of the mean. The different upper letters indicate that the means within the same column are siginificantly different by Tukey's test at or below the corresponding probability shown beneath while the different lower letters indicate that the means within the same row are siginificantly different by Tukey's test at or below the corresponding probability shown across. 3 The maximum probability level among significant p - values for  pairs of means calculated by the Tukey's test. ns = not siginificant.  90  Table 4.10 Analyses of variance for chemical properties of MAP Newtown apple slices during storage at 1°C  Attribute Source  ^  SS^DF^MS^F-ratio  Percent titratable acidity Block Packaging film Storage time P. film x Storage time Error  0.503 2.218 9.853  3 2 5  0.168 1.109 1.971  1.49 9.86 17.52  0.228 0.001 0.001  1.363 5.737  10 51  0.136 0.113  1.21  0.306  Block Packaging film Storage time P.^film x Storage time Error  0.005 0.05 0.27  3 2 6  0.002 0.025 0.045  1.48 20.83 36.39  0.229 0.001 0.001  0.06 0.07  12 60  0.005 0.001  4.12  0.001  Percent soluble solids Block Packaging film Storage time P. film x Storage time Error  0.10 6.34 2.81  3 2 6  0.033 3.170 0.468  0.28 6.47 3.91  0.838 0.001 0.002  6.26 7.18  12 60  0.522 0.120  4.36  0.001  Soluble solids /Acidity ratio Block Packaging film Storage time P. film x Storage time Error  0.033 0.098 1.750  3 2 5  0.011 0.049 0.350  0.74 3.33 23.88  0.533 0.044 0.001  0.094 0.747  10 51  0.009 0.015  0.64  0.772  pH  ^  9$  Table 4.11 Changes of pH values of juice supernatent from MAP Newtown apple slices during storage at 10C. Storage time (weeks)  Packaging material' HB  ^  MHB^MLB^p 3 <  0^3.33 + 0.02 2 a^3.33 + 0.02a^3.33 + 0.02a^-  ^w^a^a  1^3.22 + 0.02bd^3.28 + 0.01j^3.27 + 0.03a^ns a.^a^fa 2^3.21 + 0.04120^3.19 + 0.02c^3.19 + 0.02b^ns a^a^a 3^3.16 + 0.01b^3.23 + 0.01bcd 3.16 + 0.02b ^0.002 a^-TO-^a 4^3.17 + 0.02b^3.18 + 0.01c^3.09 + 0.02c^0.033 a^a^b 5^3.23 + 0.02bc^3.24 + 0.02bc^3.13 + 0.01b^0.002 a^a^b 6^3.27 + 0.01acd 3.28 + 0.01ad^3.16 + 0.01b^0.001 ^U^ a^b 3 P <^0.043^0.005^0.044 1 HB = High barrier; MHB = Medium high barrier; MLB = Medium low  barrier.  2 Mean (n = 4) + standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability shown beneath while the different lower letters indicate that the means within the same row are significantly different by Tukey's test at or below the corresponding probability shown across. 3 The maximum probability level among significant p - values for  pairs of means calculated by the Tukey's test. ns = not significant.  92  For each film type, the juice pH from stored MAP apple slices decreased gradually and significantly during the storage period (P = 0.001) from 0 to 4 weeks and then began to rise after the fourth week. The changes in pH of MAP apple slices in pouches of three different packaging films (p < 0.001) were statistically significant (Table 4.10). Changes in pH values were also significant for the storage time and package film interaction (P < 0.001). The pH values for the apple juice from MAP Newtown apple slices were within the low end of the range of pH values (3.03 to 3.69) reported in the literature (Lee & Mattick, 1989; Smock & Neubert, 1950). The pH of juice from the MAP apple slices at all storage periods was well below the safety margin of 4.6 at which level pathogenic organisms including Clostridium botulinum will not grow. Any changes in pH of juice from a plant tissue homogenate reflects the pH value changes in the cytosol and the vacuole of apple parenchyma cell. Anaerobiosis and increased solubility of CO2 under high CO2 levels in the microatmosphere can bring about a decrease in cell pH (Kurkjian & Guern, 1989; Siriphanich & Kader, 1986; Zemliakhunin & Ivanov, 1978). The continuous increase of apple tissue pH during storage can be contributed to the depletion of acids by respiration as ripening proceeds (Lee, 1983; Ulrich, 1970). High CO2 also induces the degradation of malic acid (Kader, 1986; Kader et al., 1989; Siriphanich Sz Kader, 1986; Weichmann, 1986). These pH value changes of apple juice from MAP apple slices during storage may be related to changes in acid content in apple juice (Table 4.9) and to the composition of acids in apple juice. 4.2.3.3  PERCENT SOLUBLE SOLIDS  As indicated in Table 4.10, the percent soluble solids content of MAP apple slices was significantly influenced by three different films (p < 0.001), storage time (p = 0.002) and by the film and storage time interaction (p < 0.001).  93  As shown in Table 4.12, the soluble solids measured as percent sucrose for MAP apple slices in three different film type pouches, stored for periods up to 6 weeks, did not change appreciably. Soluble solids of apples consist of sugars, acids and water - soluble volatiles (Ulrich, 1970). A concentration change of one of the two major components, either acids or sugars, can alter the soluble solids value of a commodity. Table 4.12 shows that MAP Newtown apple slices had soluble solids levels between 11.5 and 13.2%. Levels of soluble solids for North American varieties have been reported to be between 6.6 to 15.9% (Lee & Mattick, 1989; Smock & Neubert, 1950). Tissue sugar levels of apples are altered during storage under modified atmosphere conditions. Apples under aerobic conditions in MA storage accumulate sugars whereas those under anaerobic MA conditions undergo a reduction in sugar content during storage (Fidler  & North, 1967). Modified atmospheres also reduce the accumulation of soluble pectin or polygalacturonic acid in stored apples (Knee, 1980; Wang, 1990). 4.2.3.4 SOLUBLE SOLID/TITRATABLE ACIDITY RATIO As shown in Fig. 4.15, and Table 4.13, soluble solids/titratable acidity ratio for apple slices in pouches of various films increased significantly with the storage time (p < 0.001) up to about the third week. Thereafter, the soluble solids/titratable acidity ratio remained constant up to the sixth week of storage. 4.2.4 PHYSICAL PROPERTIES 4.2.4.1 ENZYMIC BROWNING OF UNSTORED APPLE SLICES AND OF PUREE FROM STORED MAP SLICES IN AIR  Ascorbic acid - treated fresh apple slices preserved acceptable whiteness during a 2-hr  holding period. The Hunter L, a and b reflectance values for apple slices held for various times (0 to 120 min) in air at 25°C are shown in the Table 4.14. Preliminary trials with Granny Smith apple slices showed that ascorbic acid-treated apples retained the whiteness  ^  94  Table 4.12 Changes of percent soluble solids (% sRcrose) of MAP Newtown apple slices during storage at 1 C. Storage time (weeks)  Packaging materiall HB  ^  MHB^MLB^P 3 4  2  0^12.80 + 0.06 a^12.80 + 0.06a^12.80 + 0.06ab a ^a^CA^ 1^12.08 + 0.34a^12.20 + 0.27ab^13.18 + 0.09a a^ a^ T ^+ 0.11b^12.33 + 0.16b 2^12.63 + 0.1l, a a^ b^ ^+ 0.25ac^12.73 + 0.20ab 3^12.28 + 0.l6, ^a^a^a 4^12.65 + 0.26a^11.95 + 0.09bc^12.73 + 0.10ab ^a^6^a 5^11.95 + 0.09b^12.23 + 0.10ab^12.80 + 0.19ab a^ a^ b 6^12.23 + 0.08a^11.95 + 0.21bc^13.23 + 0.20a a^1W^ C 3 0.045^0.033^0.011 P  0.032 0.004 ns 0.040 0.036 0.007  1 HB = High barrier; MHB = Medium high barrier; MLB = Medium low  barrier.  2 Mean (n = 4) + standard error of the mean. The different upper  letters indicate that the means within the same column are significantly different by Tukey test's at -.or below the corresponding probability given beneath while the different lower letters indicate that the means within the same row are significantly different by Tukey's test at or below the corresponding probability given across.  3 The maximum probability level among significant p - values for  pairs of means calculated by Tukey's test. ns = not significant.  ^  95  Table 4.13 Changes of soluble solids/ titratable acidity ratio of MAP Newtown apple slices during storage at 1°C. Storage time (weeks)  Packaging materiall HB  ^  MHB^MLB^P 3 4  0^22.35 + 1.22 2 a^22.35 + 1.22ac^22.35 + 1.22a^U^ u^ a 1^22.43 + 0.66a^21.40 + 0.52a^21.00 + 0.39a^0.027 a^ ab^-6 2^23.08 + 0.57ac^22.10 + 0.31a^21.88 + 0.58ab^0.041 a^ b^ b 3^24.53 + 0.37ac^24.19 + 0.09ab^23.38 + 0.46bc^0.033 ck^ a^ b 5^25.28 + 0.37ac^25.75 + 0.30b^24.50 + 0.32cd^ns a ^a^0.^ 6^25.60 + 0.44bc^25.08 + 0.60bc^25.98 + 0.81d^ns ^u ^ a^ a 3 p 4^0.037^0.033^0.046  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2 Means (n = 4) + standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability given beneath while the lower different letters indicate that the means within the same row are statistically significant at or below the corresponding probability given across. 3 The maximum probability level among significant p - values for  pairs of means calculated by Tukey's test. ns = not siginificant.  30 _ss/_12LI,,-._:==2._::-...,--,,.4 o 25^ ,-1^ _ _____ _w _ - ..-_-v---..--.......^......... ,,_...______.^ 20 Ei  0 15 U)  ro -1 ,-; o m  soluble solids  ^ -.. --^  ..... — — (51  1 0  H 4 0  1  0^1^2  4^5^6 Storage time (weeks)  HE film  MHB film  — -MLB film  Fig. 4.15 Changes of soluble solids and 9/TA ratio for MAP Newtown apple slices during storage at 1vC  97  Table 4.14 Hunter L, a and b values of fresh, unstored Newtown apple slices held for various times at 25°C in air.  Hunter value  Holding time, min 0  30  L  82.61 + 0.371  82.66 + 0.58  79.99 ± 1.83  82.29 + 0.64  81.43 + 0.98  a  -2.26 + 0.33  -1.89 + 0.32  -1.81 + 0.09  -2.01 + 0.05  -2.06 + 0.26  b  24.42 + 0.51  23.92 + 0.76  23.64 + 0.31  24.33 + 0.50  24.39 + 0.53  60  90  120  'Mean + standard error of the mean. Mean of 15 measurements for each of four replicates.  98  when exposed to air even after two weeks in refrigerator storage. However, ascorbic acidtreated MAP Newtown apple slices started to discolour between 2 to 4 hr after slicing. It is obvious that slices should be packaged under MA conditions as soon as possible after ascorbic acid treatment. Apple slices from MA pouches stored at 1°C for various periods up to 6 weeks were ground to a puree to assess the rate of enzymic browning. The Hunter L, a and b values for apple puree samples (Tables 4.15, 4.16 and 4.17) changed significantly with different holding time intervals after grinding. As shown in Table 4.18, Hunter L values were significantly different for each storage time at 60 to 120 min holding times (p < 0.001). Moreover, the packaging film type and storage time interactions for 0, 90 and 120 mm (p = 0.006, 0.041 & 0.030, respectively) were statistically significant. Hunter a values were statistically significant (Table 4.18) for 30 min holding time (p = 0.031), for packaging films, for all of the storage times (p < 0.001) and for the film type and storage time interaction for 0 min (p = 0.006). Hunter b values were significantly different (Table 4.18) for storage times (p < 0.001) except for 0 and 90 mm holding times and for the film type and storage time interaction for different time intervals of 0, 30, 60 and 90 mm (p = 0.002, 0.003, 0.029 and 0.029, respectively). Regardless of the treatment of apple slices, browning of apple puree was rapid during the first few minutes after grinding and was fastest during the first 30 min of holding time (Tables 4.15 and 4.16). As the holding time proceeded, the rate of browning was much slower. As shown in Table 4.15, the Hunter L values of purees from stored apples were lower than those values of unstored apples. In Table 4.17, it should be noted that the Hunter b values for puree from 0 storage time apple slices did not change appreciably over the 120 min holding time regardless of the film type. However, the Hunter b values for puree from stored slices decreased with a holding time of 120 min. These results show that complete inhibition of enzymic browning of puree from MAP apple slices could not be achieved with ascorbic acid treatment and modified atmosphere packaging.  99  Table 4.15 Hunter L valuesl for puree from MAP Newtown apple slices at various storage times at 1°C Packaging material` Storage ^ time (weeks) 0 HB  MHB  MLB  1 Mean 2 HB =  Time3 30  ^  60  ^  90  ^  120  54.43 + 1.57^41.67 + 0.55^41.70 + 1.74  38.56 + 1.70  38.46 + 2.18  1^50.15 + 4.12^31.87 + 2.82^33.12 + 1.28  32.25 + 1.19  31.56± 1.29  2^53.92 ±2.83^37.11 + 1.91^34.47 + 2.10  33.69 + 1.10  32.57 + 1.32  3^50.61 + 9.6/^35.37± 1.29^32.69 + 1.56  31.51 + 1.44  30.50 + 1.35  4^54.61 + 2.83^37.42 + 1.78^34.16 + 2.16  33.70 + 2.04  33.15 + 1.91  5^54.55 + 2.64^36.04 + 1.74^33.05 + 1.42  32.07± 1.29  32.36 + 1.13  6^50.82 + 1.93^33.99 + 1.08^30.94 + 0.73  30.04 + 0.95  29.10 + 0.85  54.43 + 1.57^41.67 + 0.55^41.70 + 1.74  38.56 + 1.70  38.46 + 2.18  1^54.72 + 2.72^38.01 + 1.15^32.02 + 0.13  32.83 + 0.74  35.05 + 1.22  2^40.08 + 1.95^33.22 + 2.84^30.81 + 2.48  29.86 + 2.34  30.13 + 1.24  3^57.07± 1.73^37.66 + 1.72^35.47 + 1.75  34.44 + 2.18  33.14± 1.70  4^53.41 + 3.48^33.65 + 1.81^32.23 + 1.37  31.74± 1.08  30.11 + 1.16  5^47.88 + 3.11^34.00 + 1.28^32.06 + 0.95  30.82 + 1.33  29.44± 1.22  6^51.89 + 3.62^35.37 + 1.77^33.43 + 2.00  32.40 + 2.03  31.24± 1.86  54.43 + 1.57^41.67 + 0.55^41.70 + 1.74  38.56± 1.70  38.46 + 2.18  1^54.63 + 1.20^39.23 + 1.09^35.99 + 0.92  33.06± 1.09  32.68 + 0.48  2^49.82 + 0.30^36.46 + 0.75^35.19± 1.23  34.23 + 0.54  32.83 + 0.75  3^51.12 + 0.33^33.78 + 0.37^31.30 + 0.93  30.25 + 0.20  29.36 + 0.13  4^47.47± 2.29^31.85 + 1.37^29.31 + 1.25  27.85 + 1.03  26.70 + 1.38  5^55.94 + 3.90^47.78 + 1.93^32.48 + 2.16  34.58 + -  31.40± 1.65  6^54.48 + 1.48^37.05 + 0.76^34.74 + 0.57  32.13 + 0.68  32.12 + 0.64  O  O  O  (n = 60) + standard error of the mean. High barrier; MHB = Medium high barrier: MLB = Medium low barrier . 3Time exposed to air.  Table 4.16^Hunter a valuesl for puree grom MAP Newtown apple slices at various storage times at luC Time3  Packaging^Storage time materials^(weeks) HB  MHB  NB  90  60  30  120  0  -2.27  t  0.143  3.23  !  0.81  2.98 1 0.51  1.3?  1  0.20  5.35^I 0.10  1  3.30  t  0.86  7.65  1  0.82  8.58  I 0.63  8.67  1  0.28  8.50  1 0.30  1 0.31  8.05  1  0.31  8.22  1 0.30  2  2.93  1  1.15  8.63  1  0.11  8.73  3  3.89  I  0.20  8.18  1  0.31  8.86 4 0.11  8.14  t  0.11  8.11  f 0.36  4  3.36  1  0.38  8.89  •  0.30  8.78  t 0.30  8.73  t  0.30  8.59  t 0.36  5  3.32  1  0.60  8.97  1  0.66  8.91  1 0.73  8.61  1  0.74  8.72  t 0.73  6  2.97  1  0.31  6.81  1  1.75  8.19  t^0.19  8.57  !  0.25  8.32  t 0.22  0  -2.2?  t  0.11  3.23  1  0.84  2.98  t 0.51  1.37  1  0.20  5.35  1 0.10  1  3.55  1  0.0,1  10.31  t  0.32  8.?? 4 0.78  9.61  1  0.11  10.66  1 0.37  2  1.58  t  0.48  7.86  1  0.59  7.70  1 0.58  7.72  1  0.85  7.55  i 0.58  3^,  3.16  t  0.11  10.04  1  0.61  9.50  1 0.79  9.60  1  0.60  7.26  t 1.92  1  2.28  t  0.38  8.71  1  0.52  8.98  t 0.50  8.69 1  0.31  8.18  j0.31  5  2.68  t  0.13  9.64  t  0.88  9.73  1 0.93  9.72  1  0.92  9.62  t^1.02  6  3.61  t  0.57  9.21  t  0.11  7.30  1^1.86  7.80  1  1.32  7.78  1^1.19  0  -2.2?  1  0.14  3.23  t  0.81  2.98  t 0.51  1.37  t  0.20  5.35  t 0.10  1  -0.76  t  0.27  9.37  t  0.12  10.01  t 0.35  9.53  1  0.63  9.92  f 0.57  2  1.59  t  0.61  8.91  +^0.51  8.66  / 0.18  8.75  t  0.26  8.10  + 0.23  3  2.52  t  0.16  9.10  1  0.20  9.16  t 0.31  8.90  4  0.31  8.68  ! 0.31  1  3.75  t  2.07  7.61  t  0.39  7.88  t 0.30  7.69  1  0.45  7.15  ! 0.37  5 6  2.21  t  0.68  10.28  t  0.53  10.65  1^0.51  11.61  1  -  10.22  i 0.60  3.12  +^0.35 .^.  9.55  .+  0.11  7.89  t 1.62  8.36  t  1.36  8.32  t 1.01  1 2 Mean (n = 60) + standard error of the mean.  HB + High barrier; MHB = Medium high barrier; MLB = Medium low barrier. JTime exposed to air.  Table 4.17^Hunter b^valuesl for puree grom MAP Newtown apple slices at various storage times at 1vC Packaging..^ mates-Lel&^Storage^ ^ time (weeks) H8  MHB  MB  .^3^. Time (mxn) 30  60  90  120  0  16.68 + 0.59  16.56 t 0.23  16.60 1 0.63  15.96 + 0.80  16.34 t 1.10  1  14.10 t 0.94  11.42^+^1.15  12.18 t 0.34  11.91^+ 0.39  11.59 + 0.42  2  16.85 + 0.67  13.71^+ 0.41  13.07 t 0.66  12.34 + 0.31  12.52 + 0.56  3  14.57 + 0.70  12.43 + 0.42  11.84 t 0.58  11.32 1 0.61  11.04^0.53  4  15.65 + 0.65  13.35 t 0.49  12.24 t 0.61  12.05 t 0.62  11.9? + 0.75  5  16.27 t 0.81  13.15 t 0.86  12.02 t 0.78  11.73 1 0.81  8.701^2.48  6  15.19 + 0.52  12.79 t 0.36  11.49 t^0.10  11.38 + 0.27  10.97^0.16  0  16.68 + 0.59  16.56 t 0.23  16.60 + 0.63  15.96 1 0.80  16.34 t^1.10  1  16.C6 t 0.13  14.64 + 0.19  12.05 ± 0.44  12.75 + 0.42  13.47 g 0.22  2  13.92 t 1.06  11.90 t 0.87  11.22 t 0.85  11.47^+^1.11  11.50 + 0.34  3  17.21^t 0.80  14.12 + 0.79  12.95 t 0.79  12.71^+ 0.68  12.34 1 0.93  4  16.13 + 0.80  12.89 t 0.83  11.87 t 0.44  1l.29. ^0.45  5  15.49 t^1.15  13.37 ± 0.93  12.22 + - 0.69 12.43 + 0.75  11.98 + 0.87  11.39 1 0.83  6  16.20 t 0.36  13.29 + 0.38  12.76 + 0.59  13.18^t^0.^24  11.94^+^0.61  0  16.68 t 0.59  16.56 t 0.23  16.60 t 0.63  15.96 +  0.e0  16.34 +^1.10  1  14.42 t 0.31  15.46 + 0.67  14.33 + 0.43  12.73 + 0.57  12.60 + 0.22  2  16.04 + 0.31  13.31^t 0.42  13.11^t 0.46  13.03 ± 0.23  12.56 1 0.36  a  15.92 t 0.40  12.95 t 0.33  11.87 t 0.43  11.35 + 0.37  10.95 1 0.31  4  14.54 t 0.36  11.92 t 0.63  11.06 + 0.55  10.39 .+0.58  9.82 + 0.62  5  18.14 t 0.96  14.46 I 0.86  13.22 t 1.10  14.35 t -  12.42 * 0.85  6  16.84 t 0.68  14.23 t 0.48  13.47 t 0.44  12.04 + 0.18  12.27 t 0.45  1 Mean (n = 60) + standard error of the mean. 2  HB + High barrier; MHB = Medium high barrier; MLB = Medium low farrier. Time exposed to air.  ,--.. o 1-,  102  Table 4.18 Statistical significance probability values for Hunter L, a and b values of puree from MAP Newtown apple slices stored for periods up to 6 weeks at 1°C Hunter Source oficolour variation value L  a  b  Holding time2(min) 0  30  60  90  120  Block  ns3  ns  0.027  ns  ns  P. film  ns  ns  ns  ns  ns  S. time  ns  ns  0.001  0.001  0.001  Film x s. time interaction  0.006  ns  ns  0.041  0.030  Block  ns  ns  ns  ns  ns  P. film  ns  0.031  ns  ns  us  S. time  0.001  0.001  0.001  0.001  0.001  Film x s. time interaction  0.006  ns  ns  ns  ns  Block  ns  ns  0.020  ns  0.025  P. film  ns  ns  ns  ns  ns  S. time  ns  0.001  0.001  ns  0.001  Film x s. time interaction  0.002  0.003  0.029  0.029  ns  113. film = packaging film; S. time = storage time (weeks) 2Time exposed to air. 3ns = not significant at 5%.  103  Apple pies, prepared from ascorbic acid (0.5 to 1.5%) - treated apple slices which were stored in 100% N2 or 50% CO2 (balance N2) at 5°C for 3 weeks, had sensory quality attributes comparable to those prepared from fresh apple slices (O'Beirne, 1988). 4.2.5  TEXTURE MEASUREMENT  4.2.5.1 INSTRON PLUNGER TEST Instron plunger test was used to measure the bioyield point force (Newtons), deformation to the bioyield point (mm), firmness (ratio of bioyield point to deformation; N/mm), rupture point force (Newtons), deformation to rupture point (mm) and the slope (ratio of rupture point force to deformation; N/mm). 4.2.5.1.1 BIOYIELD POINT FORCE As shown in Table 4.19, the bioyield point for apple slices packaged in HB, MHB and MLB film - type pouches increased during the first two weeks of storage to levels as high as 76 N and and then dropped to about 42 to 48 N in the third week of storage. Such changes in bioyield point force with storage time (p < 0.001) were statistically significant (Table 4.20). The bioyield point force values for apple slices from all film types of pouches stored for 6 - weeks were slightly lower than the values for unstored apple slices. The increase of bioyield point force of slices stored for a few weeks, can be attributed to increases in cell turgidity, since high cell turgidity has been related to high bioyield point force (Mohsenin, 1986). Cell turgidity would increase because of the reduction of cell water loss inside the pouch. The reduction of bioyield point of apple slices stored for 3 weeks or more may be caused by increased cell wall failure (Mohsenin, 1986). 4.2.5.1.2 RUPTURE POINT FORCE Table 4.20 shows that the rupture point force for apple slices were significantly different for storage time (p < 0.001), packaging films (p = 0.010) and for  104  Table 4.19 Changes of Instron plunger test - bioyield point force (Newtons) of MAP Newtown apple slices during storage at 1°C Storage time (weeks) 0^53.80  Packaging material' HB  + 1.49a2  MHB  MLB  53.80 + 1.49a  53.80 + 1.49a  1^72.10 + 1.18b  67.16 + 2.58b  69.58 + 0.75b  2^75.12 + 2.21b  76.15 + 1.41c  75.43 + 1.66b  3^42.41 + 1.97c  45.95 + 1.59a  47.64 + 3.21a  4^45.23 + 0.97c  46.16 + 0.79ad  44.81 + 2.15a  5^42.48 + 0.72c  41.97 + 1.10d  46.02 + 0.72a  6^45.54 + 2.11c  45.68 + 2.67d  47.39 + 2.41a  P  3  <^0.014  1HB = High barrier; MHB  0.050  0.009  Medium high barrier; MLB = Medium low barrier.  2Mean (n = 64) ± standard error of the mean. The means within the same column followed by different letters are significantly different by Tukey's test at or below the corresponding probability given beneath. 3The maximum probability level among significant p - values for pairs of means calculated by Tukey's test.  105  Table 4.20 Analyses of variance of Instron - plunger test texture measurements of MAP Newtown apple slices during storage at 1°C  Texture^Sources of parameter^variation Bioyield point force  SS  DF  MS  F-ratio  Block Packaging film Storage time P. film x storage time Error  38.27 47.50 12257.18  3 2 6  12.76 23.75 2042.86  0.74 1.38 118.91  0.531 0.259 0.001  196.18 1030.96  12 60  16.34 17.18  0.95  0.504  Block Packaging film Storage time P. film x storage time Error  0.10 1.16 21.13  3 2 6  0.03 0.58 3.52  0.33 5.87 35.74  0.802 0.005 0.001  2.46 5.92  12 60  0.21 0.10  2.08  0.032  20.57 29.77 3166.36  3 2 6  6.86 14.89 527.73  1.33 2.90 102.62  0.272 0.063 0.001  186.21 308.55  12 60  15.52 5.14  3.02  0.062  Deformation to bioyield point  Ratio of bioyield point force to deformation (firmness)  Block Packaging film Storage time P. film x storage time Error  106  Table 4.20 continued  Texture^Source of parameter^variation Rupture point force  SS  DF^MS  F-ratio  Block Packaging film Storagetime P. film x storage time Error  19.15 95.05 14528.98  3 2 6  6.39 47.52 2421.50  0.67 5.01 255.22  0.572 0.010 0.001  393.48 569.27  12 60  32.79 9.49  3.46  0.001  Block Packaging film Storage time P. film x storage time Error  61.02 32.96 232.06  3 2 6  20.34 16.48 38.67  1.06 0.86 2.02  0.372 0.428 0.077  236.53 1147.85  12 60  19.71 19.13  1.03  0.434  117.64 343.09 6778.53  3 2 12  39.21 171.55 1129.76  1.25 5.45 35.88  0.301 0.007 0.001  3701.53 1889.01  6 60  308.46 31.48  9.80  0.001  Deformation to rupture point  Ratio of rupture point force to deformation Block Packaging film Storage time P. filrn x storage time Error  107  the packaging film and storage time interaction (p < 0.001). As shown in Table 4.21, rupture point force of slices increased during the first 2 weeks of storage, regardless of the packaging film, and then dropped to values close to those for unstored slices with further storage to 6 weeks. The reduction in the rupture point force of apple slices stored for 3 weeks or more may be caused by the breakdown of the pectic substances in the middle lamellae (Bourne, 1982b). 4.2.5.1.3  DEFORMATION TO BIOYIELD POINT AND DEFORMATION TO RUPTURE POINT  Differences in deformation to the bioyield point were statistically significant (Table 4.20) for different packaging films (p = 0.005), for storage time (p < 0.001) and for the packaging film and the storage time interaction (p = 0.032). Deformation to bioyield point decreased in storage (Table 4.22). Unstored apples had higher deformation values while deformation to bioyield point values of MAP apple slices were lower. An increase in cell turgidity of MAP apple slices may reduce cell deformation. Deformation (mm) to rupture point (Table 4.23) did not change significantly in storage for any of the experimental factors considered (Table 4.20). But similar to the deformation to bioyield point, deformation to rupture point also was the highest for unstored apples and low for MAP apple slices (Fig. 4.16). Deformation to rupture point increased during the six week storage period. The increase of deformation is considered as an indication of increasing fruit softness (Mohsenin, 1986; Hamann, 1983). Therefore, softness of MAP apple slices increased with storage time. Acidity of ascorbic acid (Baurnfiend Pinkert, 1970) and acidification by CO 2 (Weichmann, 1986; Kader et al., 1989) can cause hydrolysis of middle lamellae and a loss of firmness.  108  Table 4.21^Changes of Instron plunger test - rupture point force (Newtons) of MAP Newtown apple slices during storage at 1°C Packaging materiall  Storage +-ima weeks)  3  MHB  MLB  54.82 + 1.23a 0. 68.07 + 1.86b il 81.96 + 0.72b  54.82 + 1.23a  ns  70.60 + 2.01b b 76.19 + 2.70b  0.03  2  54.82 + 1.23a 2 T 78.04 + 1.00b a + 1.57b 82.49  3  45.14 + 1.71c  47.78 + 1.09de  4  48.63 + 0.88ac  49.13 + 0.32ad  46.11 + 1.95ac a 45.69 + 2.76c  42.70 + 0.74c  42.53 4^1.03e  46.89 + 0.68ac 0.007  51.59 + 2.25a  49.09 + 2.08cd a  0 1  HB  a  a  .  a  a  6  P  3  -;^0.002  a  a  a  a  a  0.017  P  a  a  a  iLlw 46.28 + 1.55ac  a  ns ns ns  ns  0.013  1HB = High barrier; MHB = Medium high barrier;^MLB = Medium low barrier. 2Mean (n = 64) + standard error of the mean. The different upper letters indicate that the means within the same column are signigicantly different by Tukey's test at or below the corresponding probability given beneath while the different lower letters indicate that the means within the same row are significantly different by Tukey's test at or below the corresponding probability given across. 3The maximum probability level among significant p - values for pairs of means calculated by Tukey's test. ns = not significant.  109  Table 4.22 Changes of Instron plunger test - deformation (mm) to bioyield Roint of MAP Newtown apple slices during storage at luC Storage time (weeks)  0  Packaging Materiall HB  MHB  MLB  P  4.05 + 0.11d  4.05 + 0.11a  1  4.05 + 0.112a (A. 2.34 + 0.16b  2.88 + 0.09b  2.80 + 0.104c  2  2.45 + 0.06b  2.60 + 0.11c  3  2.57 + 0.14bc  2.56^+^0.11g,  2.46 + 0.13b 0. 2.89 + 0.24bd  4  2.58 + 0.06bc a2.49^+ 0.08b  2.35 + 0.09c 2.87 + 0.28bd ns 75 2.78 + 0.12cd 3.33 + 0.17acd 0.002 b -Z.3.66 + 0.35adb 2.98 + 0.22sid ns  LA  0.  a  5 6 P  3  3.12^+ 0.17c (.1 0.039  a b  1r  a b  a  ns 0.024 ns ns  --a---  0.032  0.021  1 HB  = High barrier; MHB = Medium high barrier; MLB = Medium low barrier.  2 Mean  (n = 64) ± standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability shown beneath while the different lower letters indicate that the means within the same row are significantly different by Tukey's test at or below the corresponding probability given across.  3 The maximum probability level among significant p - values  for pairs of means calculated by the Tukey's test. ns = not significant.  110  Table 4.23 Instron plunger test - deformation (mm) to rupture point of MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging material' HB  MHB  MLB  0  5.00 + 0.062  5.00 ± 0.06  5.00 + 0.06  1  0.12 3.45±0.12  3.78 + 0.11  3.60 + 0.12  2  0.11 3.46±0.11  3.52 + 0.14  3.52 + 0.20  3  3.34 ± 0.17  3.36 ± 0.06  4.03 + 0.23  4  3.57 + 0.18  3.03 + 0.06  3.44 -I- 0.19  5  0.29 4.48±0.29  3.65 + 0.21  4.00 + 0.44  6  3.95 + 0.25  3.80 + 0.17  4.11 + 0.22  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier 2Mean (n = 64) + standard error of the mean.  100 ^ 90 80 70 60 --50 40 30 20 10 0 0  ^ ^ ^ 7 4^5^6 2 1 DEFORMATION (nun) HB - film pouch, 1 week (I - IV, slices from one pouch) MLB - film pouch, 5 weeks(V - VIII, slices from one pouch)  Pig. 4.16 A profile of force-deformation curves of MAP Newtown apple slices in NB (week 1) and MLB (week 5) pouches  112  4.2.5.1.4 FIRMNESS (RATIO OF BIOYIELD POINT FORCE TO DEFORMATION) Firmness (ratio of bioyield point force to deformation) of MAP apple slices changed significantly during the 6 - week storage period (p < 0.001). The firmness values for unstored and stored apple slices under MAP are presented in Table 4.24. When the slices were stored for 1 to 2 weeks, the firmness values rose from about 31 N/mm regardless of the film type for the pouches. At the third week, the firmness values of slices in all of the pouch types dropped to 19 to 20 N/mm and remained approximately in this range for the remaining storage time. 4.2.5.1.5 RUPTURE POINT FORCE TO DEFORMATION RATIO As shown in Table 4.25, rupture force to deformation ratio values of slices in all of the film pouches increased in weeks 1 and 2 of storage and then decreased. Differences in rupture force to deformation ratio (N/mm) were statistically significant for storage time (p < 0.001), packaging films (p = 0.007) and for the packaging film and storage time interaction (p < 0.001). At the end of the 6 - week storage period apple slices in high barrier film pouches (HB and MHB) had higher ratio values than the apple slices in MLB film pouches. 4.2.5.2 INSTRON TEXTURE PROFILE ANALYSIS (TPA) As shown in Table 4.26, Instron measured TPA - fracturability of apple cylinders  was statistically different for package film types (p = 0.010), storage time (p < 0.001) and the packaging film and storage time interaction (p < 0.001). Fracturability of slices in all pouches with different films decreased with storage (Table 4.27). Fracturability of unstored apple slices was about 174 N. After the second week, apple slice fracturability decreased dramatically and, at the sixth week, it ranged from 97 to 104 N.  113  Table 4.24 Changes of Instron plunger test - firmness (ratio of bioyield point force to deformation, N/mm) of MAP Newtown apple slices during storage at 1°C Storage^ Packaging material' time (weeks)^HB^ MHB^MLB 0  13.28 + 0.27a2  13_28 + 0.27a  13.28 + 0.27a  1  33.66 + 2.81c  25.30 + 1.36b  25.80 + 0.96b  2  31.83 + 0.54c  30.99 + 1.03c  31.19 + 2.43b  3  17.24 + 0.34b  19.58 + 0.38de  17.50 + 1.27ac  4  18.19 -I- 0.66b  20.80 + 0.66bd  15.94 + 1.27c  5  17.63 + 0.53b  15.97 + 0.33ae  14.84 + 0.68c  6  15.23 ± 1.00b  14.11 + 1.42ae  16.12 + 1.15c  P3 .:  0.001  0.021  0.008  "HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 64) + standard error of the mean. Means within the same column followed by different letters are significantly different by Tukey's test at or below the corresponding probability given beneath. 3The maximum probability level among significant p - values for pairs of means calculated by Tukey's test. ns = not significant.  ^ ^ ^  ^+  114  Table 4.25 Changes of Instron plunger test - ratio of rupture point force to deformation (N/mm), of MAP Newtown apple slices during storag,_1 at luC Storage time (weeks)  HB  MHB^MLB^P 3  11.88 + 1.27 2 a^11.88 + 1.27ae 11.88 + 1.27a^ns -6^7N--^ + 0.68b^21.92 + 1.11a 0.038 1.49b^20.38 1 25.28 0  a  ^a  ^+  Packaging materiall  ^  b  + 1.56c^23.00 + 1.37b^ns 0.91b^24.10 25.19 2 CA^ u^u 3 13.70 + 0.14a^15.06 + 0.37dfg 12.04 + 0.22  ^ca^  ^+  b^  b^  c  .  + 0.31bd 13.05 + 0.60a 0.005 0.73a^16.69 4 14.26 a + 0.71aef 13.02 + 1.74a^ns 0.37a^12.22 12.92 5 u^a U 6 13.78 + 0.72a^14.69 + 1.15ed 11.37 + 0.64a ^ns u^a u 3 0.001^0.031^0.001  . ^a^ b^ ^+  P  :^  1 HB = High barrier; MHB = Medium high barrier; MLB = Medium  low barrier.  2 Mean (n = 64) + standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability given beneath while the different lower letters indicate that the means within the same row are corresponding probability given across. 3 The maximum probability level among significant p - values  for pairs of means calculated by Tukey's test. ns = not significant.  115  Table 4.26 Analyses of variance of Instron texture profile analysis of MAP Newtown apple slices during storage at 1°C Texture  Sources of variation  SS DF  MS  802.12 3 1960.91 2 75912.78 . 6  267.37 980.46 12652.13  1.36 4.98 64.30  0.264 0.010 0.001  10497.31 12 11806.63 60  874.78 196.78  4.45  0.001  3 2 6  135.42 513.22 7562.26  0.61 2.30 33.91  0.613 0.109 0.001  3560.25 12 13381.93 60  296.69 223.03  1.33  0.226  3 2 6  59.17 312.79 10805.70  0.42 2.20 75.93  0.742 0.120 0.001  1804.55 12 8538.61 60  150.38 142.31  1.06  0.412  F-ratio  Fracturability Block Packaging film Storage time Packaging film x storage time Error Hardness-1 Block Packaging film Storage time Packaging film x+ storage time Error  406.26 1026.43 45373.56  Block Packaging film Storage time Packaging film x storage time Error  177.52 625.58 64834.21  Hardness-2  ^  116  Table 4.27 Changes of TPA - fracturability (Newtons) of MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging materiall HB  ^  MHB^MB^P 3  0^173.46 + 1.882a 173.46 + 1.88a 173.46 + 1.88a a^a^a 1^166.87 + 10.71a 185.72 + 9.78a 171.09 + 5.28a^ns a^a^a 2^167.73 + 17.78a 110.71 + 1.85b 104.00 + 4.39b 0.011 a^ b^  In  3^106.97 + 1.53b^101.30 + 1.12b 104.16 + 5.90b^ns  ^a^-6^0.  4^107.50 + 2.00b^127.43 4 15.81b 99.26 + 5.79b^ns a^ a^-55^106.86 + 3.11b^105.70 + 2.05b 101.71 + 6.43b^ns  ^a  a^a  6^104.02 + 3.43b^101.17 + 4.75b^96.72 + 6.82b^ns  ^5,^ a^ a  3 P^0.001^0.001^0.001  1 HB = High barrier; MHB = Medium high barrier; MLB = Medium  low barrier.  2 Mean (n =64) + standard error of the mean. The different upper letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability given beneath while the different lower letters indicate that the means within the same row are significantly different by Tukey's test at or below the corresponding probability given across. 3 The maximum probability level among significant p - values  for pairs of means calculated by the Tukey's test. ns = not significant.  117  Fracturability, a product of high hardness and low cohesiveness, is a secondary textural parameter and represents the force with which the material fractures (Larmond, 1975). The continuous decrease of fracturability of apple slices during the 6 - week storage period may be due to increased cell turgidity as a result of moisture retention in the sealed MA pouches. Tijskens (1979) reported that high cell turgidity decreased axial compression strength. Results of this study show even though fracturability decreased it remained high (> 97 N) during storage for 6 weeks, regardless of the film type. N.Y. 653 breeding line of freshly harvested apples had fracturability of only 41 N (Bourne, 1986). TPA hardness-1 (Table 4.28) and hardness-2 values (Table 4.29) for apple slices decreased significantly with storage (p < 0.001). Hardness-1 and hardness-2 values for unstored apples were 100 N and 127 N, respectively. By the second week of storage, hardness-1 values rose to 145 N for apple slices in MLB film pouches to 160 N for apple slices in MHB film pouches. Similarly, hardness-2 values rose to 188 N for apple slices in  MHB film pouches to 172 N for apple slices in HB and MLB film pouches. During the second week, both the hardness-1 and hardness-2 values dropped dramatically and this drop continued until the end of storage. Hardness-1 and hardness-2 values for apple slices at the sixth week ranged from 100 to 110 N and 61 to 65 N, respectively. Changes in hardness-1 and hardness-2 values during the storage period did not change appreciably with respect to different film type pouches. Hardness is the maximum force applied to attain a given deformation during a given compression cycle. Hardness of a solid food is defined as the force required to compress a food between the molar teeth (Larmond, 1975). Bourne (1986) found that apple tissue with very high water activity (aw = 0.99) produced typical TPA force - time curves and always had higher hardness than fracturability values. Apple slices used in this study had typical - TPA curves for apple tissue with high tissue moisture.  118  Table 4.28 Changes of TPA - hardness-1 (Newtons) of MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging materiall HB  MHB  MLB  0  126.72 + 0.532a  126.72 +^0.53a  126.72 + 0.53a  1  172.22 + 10.26b  188.35 + 15.54b  172.39 -I- 8.95b  2  132.31 + 14.15a  95.79 + 4.65a  99.37 + 2.83a  3  129.00 + 1.75a  129.32 + 13.58a  117.02 + 7.95a  4  121.82 ±2.52a  122.40 + 2.09a  113.00 ± 6.66a  5  112.42 +4.84a  112.28 --1-^3.12a  110.24 +6.82a  6  102.80 + 1.50a  110.24 + 6.78a  99.48 + 7.59a  P3 (  0.009  o.00t  (Loot  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 64) + standard error of the mean. 3The maximum probability level among siginificant p - values for pairs of means calculated by the Tukey's test. ns = non significant. a-bMeans within the same column followed by different letters are significantly different by Tukey' test at or below the corresponding probability shown beneath.  119  Table 4.29 Changes of TPA - analysis hardness-2 (Newtons) MAP Newtown apple slices during storage at 1°C  Storage time (weeks)  Packaging material'  HB  MHB  MLB  0  100.25 + 0.522a  100.25 + 0.52a  100.25 + 0.52a  1  147.14 + 7.66b  159.73 ± 11.56b  145.37 + 12.45b  2  96.84 + 9.73ac  71.01 + 4.89c  73.51 + 2.05ac  3  80.22 + 1.89ad  78.26 + 9.17ac  72.31 -1- 6.10c  4  76.23 + 2.15c  77.45 + 1.91ac  70.10 + 4.97c  5  71.37 + 3.24d  70.83 + 2.20c  68.99 + 4.73c  6  65.27 ± 1.48d  63.89 + 4.79c  60.73 + 5.90c  p3 '  0.024  0.032  0.046  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 64) ± standard error of the mean. The different letters indicate that the means within the same column are significantly different by Tukey's test at or below the corresponding probability given beneath. 3The maximum probability level among significant p - values for pairs of means calculated by Tukey's test.  120  The peak heights of two compression cycles of TPA curves for individual slices from the same apple were equal while peak heights were different for different slices. Slices from relatively small apples consistently had taller curves. In other words, smaller slices required higher forces during the compression cycle while slices from larger apples required lower forces to attain the same compression (Fig. 4.17). Although the apples were sorted mechanically by the packer according to the size some size variation was evident. The unsymmetrical shape of the second compression cycle shows the presence of a plastic deformation in the apple (Mohsenin, 1986). Low 0 2 atmospheres (Kader, 1986; Kader et al., 1989; Knee, 1980; Weichmann, 1986) retard softening. Bolin and Huxsoll (1989) reported that peach slices stored in anaerobic atmospheres had a firmer texture than those stored in aerobic modified atmospheres. In this study, the relatively high firmness of apple slices packaged in the two low 0 2 permeable film type (HB MHB) pouches may be attributed partly to the low 0 2 partial pressures in the microatmosphere. High water activity of the apple tissue may have been the major factor which determined textural changes of apple slices during the relatively short storage period. Apple tissue loses its textural quality before the limit of its' sensory acceptability (Tijskens, 1979). Overripe and ripe apples did not have any differences in puncture test measured hardness (Blanpied Blak, 1977). Therefore, data on textural quality of apple slices in modified atmospheres should be used to conclude appropriateness of particular storage conditions for apple slices but not to determine acceptability of apples slices. 4.2.6  SENSORY EVALUATION  During the training sessions, the sensory evaluation of MAP apple slices by panelists included apple odour in mouth, sweetness, sourness, sweet sour balance, firmness at finger tips, crispness (sound created in mouth), force needed to shear by teeth in the mouth, surface whiteness, off-flavour and overall acceptability (see Appendix).  175:. 150: 125: 100  DEFORMATION (mm) Pig. 4.17 TPA - force-deformation curves of MAP Newtown apple slices ( ical TPA curves for apple slices, regardless of storage)  122  After the chewing of apple slices for a few times, apple odour in the mouth was determined by exhaling through nostrils and the mouth closed. Unstored apple slices had distinct pear-like odour. This odour was less intense in apple slices after the second week of storage. Panelists rated apple odour to be good to fair (Table 4.30). During the CA and MA storage of apples, flavour volatile production is reduced and flavour intensity drops dramatically (Patterson et al., 1974; Lidster et al., 1983; Brackmann, 1989; Yahia et al., 1990). As shown in Tables 4.31 and 4.32, sweetness and sourness of apple slices did not change appreciably during storage. Ratings for sweetness averaged slightly sweet while that for sourness averaged moderately sour during the storage of apple slices. Changes in the soluble solids and titratable acidity were not appreciable (Table 4.09) and thus only small differences in sweetness and sourness in stored slices would be expected. On the other hand, these two attributes affect the perceived strength of each other. Therefore, a greater change in one attribute may be balanced by the other. Sensory sweet sour/balance was rated also to be good to fair during storage and decreased slightly with the storage time (Table 4.33). As shown in Table 4.34, apple slice firmness determined by the finger pressure  method did not change appreciably during the storage. Panelists rated stored slices from pouches of all film types to be firm to very firm. Instron analysis data in the present study (Table 4.24) showed that apple slices had high firmness values throughout the storage period. The force required to shear apple tissue by teeth in the mouth has been considered as a component of apple crispness (Watada 8z Abbott, 1982). In this study, panelists rated the force required to tear apple tissue in the mouth as moderate to high throughout the storage period (Table 4.35). Sensory crispness is recognized a combination of mechanical properties and acoustic properties which occurs during the chewing action (Vickers, 1988). In this study, panelists were asked to rate the strength of sound created during initial chewing.  123  Table 4.30 Sensory panel ratings for apple odour (in mouth) of MAP Newtown apple slices during storage at 1°C Storage time (weeks) ^  Packaging material' HB  MHB  0  3.5 + 0.352  3.5 + 0.35  3.5 + 0.35  1  3.2 + 0.36  3.1 + 0.33  3.0 + 0.39  2  2.5 + 0.14  3.1 + 0.21  2.5 + 0.30  3  2.9 + 0.25  2.8 + 0.48  2.9 + 0.32  4  2.5 + 0.30  2.9 + 0.33  2.9 + 0.16  5  3.1 + 0.39  3.2 + 0.40  3.1 ± 0.38  6  2.6 -I- 0.20  2.7 + 0.31  2.6 + 0.27  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) + standard error of the mean.  MLB  124  Table 4.31 Sensory panel ratings for sweetness of MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging mat eriall HB  MHB  MLB  0  2.8 + 0.352  2.8 + 0.35  2.8 + 0.35  1  2.6 + 0.16  2.9 + 0.38  2.6 + 0.27  2  2.9 + 0.26  2.8 + 0.28  2.4 + 0.23  3  2.2 + 0.27  2.1 + 0.28  2.4 + 0.30  4  2.6 + 0.27  2.9 + 0.39  2.9 + 0.33  5  2.7 + 0.40  2.4 + 0.36  2.4 + 0.35  6  2.3 ± 0.31  2.4 + 0.37  2.5 + 0.31  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) ± standard error of the mean.  125  Table 4.32 Sensory panel ratings for sourness of MAP Newtown apple slices during storage at 1°C Storage time (weeks) ^  Packaging material' HB  MHB  MLB  0  3.0 + 0.242  3.0 + 0.24  3.0 + 0.24  1  3.4 + 0.27  3.2 + 0.21  3.3 + 0.20  2  3.2 -I- 0.32  3.0 + 0.21  3.3 + 0.31  3  4.3 + 0.44  3.2 + 0.18  3.9 + 0.33  4  3.4 + 0.34  3.8 + 0.25  3.5 + 0.29  5  3.4 + 0.40  3.7 + 0.25  3.8 + 0.30  6  3.5 + 0.28  3.5 + 0.42  3.7 + 0.37  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean = 6) + standard error of the mean.  126  Table 4.33 Sensory panel ratings for sweet/sour balance of MAP Newtown apple slices during storage at 1°C Storage time (weeks) ^  Packaging material'  HB  MHB  MLB  0  3.3 + 0.262  3.3 -k 0.26  3.2 + 0.26  1  2.6 + 0.20  2.5 + 0.28  2.6 + 0.35  2  2.4 ± 0.19  2.4 -I- 0.23  2.0 + 0.34  3  2.4 + 0.28  2.6 + 0.30  2.4 + 0.35  4  2.6 + 0.27  2.4 + 0.30  2.5 + 0.30  5  2.8 + 0.35  2.7 + 0.49  2.5 + 0.44  6  2.4 + 0.23  2.5 + 0.36  2.5 + 0.38  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) ± standard error of the mean.  127  Table 4.34 Sensory panel ratings for firmness (at finger tips) of MAP Newtown apple slices during storage at 1°C Storage time (weeks) ^  Packaging material' HB  MHB  MLB  0  4.8 + 0.112  4.8 + 0.11  4.8 + 0.11  1  4.3 + 0.19  4.5 + 0.22  4.5 + 0.20  2  4.5 + 0.20  4.7 + 0.17  4.2 + 0.31  3  4.5 + 0.16  4.6 + 0.17  4.3 + 0.28  4  4.2 + 0.21  4.3 + 0.16  4.2 + 0.18  5  4.2 + 0.15  4.2 + 0.18  4.1 + 0.20  6  4.2 + 0.16  4.3 + 0.16  4.1 + 0.22  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) + standard error of the mean.  128  Table 4.35 Sensory panel ratings for shear force required between teeth in mouth for MAP Newtown apple slices during storage at 1°C  Storage time (weeks) ^  Packaging material' HB  MHB  MLB  0  3.7 + 0.322  3.7 + 0.32  3.7 + 0.32  1  3.6 + 0.20  3.5 + 0.37  3.5 + 0.23  2  3.7 + 0.18  3.4 + 0.30  3.4 + 0.34  3  3.6 + 0.31  3.6 ± 0.29  3.1 + 0.28  4  3.5 + 0.38  3.5 + 0.37  3.5 + 0.31  5  3.6 + 0.38  3.4 + 0.38  3.3 + 0.26  6  3.4 + 0.35  3.5 + 0.36  3.5 ± 0.38  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier.  2Mean (n _, 6) + standard error of the mean.  129  Apple slice crispness decreased significantly (Table 4.36) during the 6 - week storage time (p = 0.004). However, panelists consistently rated apple slices held for periods up to 6 weeks to be crisp. Crispness evaluation of apple slices held in different film types showed that the crispness of apple slices stored in HB and MHB film - pouches did not change with time but it changed significantly (p = 0.028) for apple slices in MLB film pouches during storage (Table 4.37). The relatively high headspace 02 levels in the MLB pouches must have been responsible for the loss of textural firmness. Surfaces of apple slices remained white to slightly browned (Table 4.38) and apple surface whiteness changed very slightly during the storage period of 6 weeks. Off-flavour of apple slices increased during storage (Table 4.39). At the 5th week of storage, some panelists perceived an fermented or alcohol-like off-flavour. Anaerobic respiration in apple slices packaged in the two low 02 permeability film pouches (HB MHB) may be responsible for the alcoholic or fermented off-flavour. Ethanol and acetaldehyde had been reported to be the cause off-flavour of apples in storage (La Belle, 1981). Yeasts are naturally occurring on fruits and in fruit juices and carry out alcohol fermentation (Deak, 1991). Overall sensory acceptability of apple slices decreased as storage time progressed (Table 4.40). Apple texture remained relatively unchanged. Apple surface colour and offflavour were the major attributes that determined sensory unacceptability of apples. After 6 weeks in storage, apple slices of some replicates of MHB film pouches were slimy while some replicates of MLB film pouches had a moldy smell. Therefore, high initial contamination  may lead to mold growth in early storage period at low 02 partial pressures. This same factor contributes to the disadvantage of using 02 permeable films for packaging. Sensory quality of apples is directly related to the soluble solids content (Watada & Abbott, 1982). The relatively high soluble solids content maintained by apple slices during storage may have contributed to the lack of statistically significant sensory ratings.  130  Table 4.36 Analysis of variance of sensory panel ratings for crispness of MAP Newtown apple slices during storage at 1°C Source of variation  SS  Panelists Packaging film Storage time Packaging film x storage time Error  DF  MS  F -ratio  1)  2.80 0.81 5.40  3 2 6  0.933 0.405 0.900  3.72 1.61 3.58  0.773 0.209 0.004  1.85  12  0.154  0.61  0.822  15.08  60  0.251  131  Table 4.37 Sensory panel ratings for crispness (sound created in mouth) of MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging material'  HB  MLB  MHB  0  4.3 + 0.24a2  4.3 -I- 0.24a  4.3 ± 0.24a  1  3.3 + 0.42a  3.8 + 0.25a  3.4 + 0.21a  2  3.5 + 0.35a  4.0 + 0.18a  3.3 ± 0.30b  3  3.4 + 0.23a  4.0 + 0.09a  3.4 ± 0.15a  4  3.6 + 0.22a  3.8 + 0.25a  3.6 + 0.13a  5  3.7 ± 0.24a  3.7 -I- 0.19a  3.8 ± 0.12a  6  3.6 + 0.24a  3.4 + 0.41a  3.8 ± 0.19a  3 P ‘  ns  ns  0.028  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 4) + standard error of the mean. 3The different letters within the same column indicate that the means are significantly different by Tukey's test at or below the probability given beneath. ns = not significant.  132  Table 4.38 Sensory panel ratings for appearance (whiteness) of MAP Newtown apple slices during storage at 1°C  Storage time (weeks)  ^  Packaging materiall HB  MHB  MLB  0  4.6 + 0.152  4.6 + 0.15  4.6 + 0.15  1  4.1 + 0.28  4.7 + 0.14  4.5 + 0.26  2  4.2 + 0.24  4.5 + 0.20  4.4 + 0.26  3  4.1 + 0.19  4.6 + 0.13  4.4 + 0.14  4  4.3 + 0.14  4.4 + 0.17  4.4 + 0.17  5  4.2 ± 0.23  4.3 + 0.15  4.5 + 0.18  6  4.4 + 0.24  4.7 + 0.12  4.6 + 0.15  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) + standard error of the mean.  133  Table 4.39 Sensory panel ratings for off-flavour of MAP Newtown apple slices during storage at 1°C Storage time (weeks)  Packaging material' HB  MHB  MLB  0  4.8 + 0.252  4.8 + 0.25  4.8 + 0.25  1  4.6 + 0.41  4.5 + 0.41  4.4 + 0.41  2  4.2 + 0.46  4.2 + 0.41  4.2 + 0.46  3  4.2 + 0.52  4.5 + 0.31  4.5 + 0.32  4  4.2 + 0.52  4.2 + 0.34  4.5 + 0.41  5  4.0 + 0.15  4.3 + 0.41  4.5 + 0.41  6  3.5 ± 0.50  4.2 + 0.31  4.2 + 0.49  "HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) + standard error of the mean.  134  Table 4.40 Sensory panel ratings for overall sensory acceptability of MAP Newtown apple slices during storage at 1°C Storage time (week)  Packaging material'  HB  MHB  MLB  0  4.3 + 0.262  4.3 + 0.26  4.3 + 0.26  1  3.3 + 0.26  3.5 + 0.33  3.4 + 0.33  2  2.3 + 0.34  3.0 + 0.43  2.9 -I- 0.55  3  2.7 + 0.59  2.9 + 0.55  2.8 + 0.59  4  3.3 + 0.23  3.1 + 0.38  3.2 + 0.36  5  3.1 + 0.38  3.1 + 0.40  3.2 + 0.35  6  2.7 + 0.38  2.9 + 0.26  2.8 + 0.31  1HB = High barrier; MHB = Medium high barrier; MLB = Medium low barrier. 2Mean (n = 6) + standard error of the mean.  135  5. CONCLUSION The sealed gas - impermeable respiratory chambers used in this study provided approximate conditions employed in the pouches of MAP apple slices. The measurement of the respiration rate of apple slices inside the chambers was convenient. However, the method did not consider the effect of build up of excess CO 2 levels on respiration rates. The coelution of argon with 0 2 during gas chromatographic separation may have affected the determination of respiration rates. Thus, a gas analysis technique which can distinguish each gas separately is required. This study also confirmed the fact that the selection of the packaging films based on the respration-permeation equilibrium can be used only when the headspace 0 2 is not high enough to cause quality deterioration such as aerobic microbial spoilage and surface discolouration by browning. Further, this study proved the finding by Powrie et al. (1990b) that high barrier packaging films must be used in modified atmosphere packaging of 0 2 sensitive commodities in order to preserve quality. The Arrhenius relationship for apples showed that both varieties of apples could be stored at 1 to 15 0 C without any chilling injury. However, the influence of temperature on the storage of Newtown apples merits further investigation. The importance of the Arrhenius plot with respect to short - term storage of apple slices is questionable. Even the most chilling sensitive apple cultivars take about three months to show the effects of chilling injury (Wang, 1990).  The determination of transition 0 2 levels for commodities showing biphasic respiratory curves was important. However, it's significance in the evaluation of commodity respiration such as anaerobiosis needs to be evaluated. Oxygen levels around the transition 0 2 level, depending on the specific commodity requirement and the length of storage, may provide the optimum microatmosphere 0 2 concentration to be maintained in the package. For example, with respect to commodity, MAP - apple slices may be successfully stored with the headspace 0 2 levels below the transtion 0 2 level while commodities like broccoli which  136  produce off-flavour in suboptimum 02 rnicroatmospheres, may need an 02 level just above the transition 02 level. Results of the packaging study showed that HB film - type pouches resulted in unacceptable respiratory gas compositions within the pouches with very high (about 35%) CO2 levels and possibly zero 02 levels (about 0.30% 02 measured could be argon). MHB film pouches had excess CO2 levels and probably just sufficient 02 levels in the headspace. On the other hand, the MLB film allowed an acceptable CO2 level and a 02 level sufficient to carry out aerobic respiration without anarobiosis. But in storage at the sixth week, apple slices in some MLB pouches had a moldy smell. Both low permeability film pouches (HB and MHB) pouches had slimy apple slices. Apple slices from MHB film pouches maintained good consumer quality for the first four weeks. Surface colour preservation and crisp texture in the mouth of apple slices from low permeability film pouches seemed to be better during the first 3 Weeks. Therefore, although all three packging films resulted in good package performance for the most of the storage time, under the conditions employed, MHB - 6 mil PE would be the choice for packaging. Unavailability of packaging films with exact permeability requirements hindered achieving of the best quality preservation with MAP apple slices. The most appropriate film for packaging should have been a film with 02 and CO2 permeability properties in between those of 6 and 4 mil polyethylene. A suitable film for packaging of apple slices should be more moisture permeable than any of the films used. Similarly, in comparison to 02 more CO2 should permeate through the film (the ratio of 02 to CO2 transmission through the film is 1: 4 or 5 for presently available films). Even Canada Fancy apples were not of good quality for packaging studies since browning due to bruising could be seen underneath the skin of some apples. These apple defects would not have been detected without peeling . Apple slices from all the packages, after the 5th week of storage, had excessive moisture. Therefore, apple slices seem to be water clogged. These apple slices produced  137  excess juice after maceration for chemical analysis and this was also felt by sensory panelists during maceration in the mouth. Although chemical and textural properties changed for apple slices stored in different packaging films and with increasing storage time, apple slices maintained these properties within the well acceptable ranges reported in the literature for different apple varieties. Flavour may be the major attribute that affected the acceptability of MAP apple slices. Ascorbic acid sucessfully inhibited browning of Granny Smith apples. However, the same concentration (5%) and the same dipping period (3 min) was not sufficient for Newtown apples. Therfore, it is important that low browning cultivars be used in MAP. Under this particular circumstance, the selection of the variety made the major difference in storage quality of MAP apple slices. This study also showed that the maintenance of partial anaerobiosis in storage was important in retaining microbial quality and colour preservation of the MAP apple slices. 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Trans ASAE. 31: 920-925. Yang, S.F. and Hoffman, N.E. 1984. Ethylene biosynthesis and its regulation in higher plants. Annu. Rev. Plant Physiol. 35:155-159.  149  Zagory, D. and Kader, A.A. 1988. Modified atmosphere packaging of fresh produce. Food Technol. 42(9): 70-77. Zemlianukhin, A.A. and Ivanov, B.F. 1978. Metabolism of organic acids of plants in the conditions of hypoxia. In "Plant Life in Anaerobic Environments," (Ed.) D.D. Hook and R.M.M. Crawford. Ch. 9, pp. 203-219. Ann Arbor Science Publishers, Inc., Ann Arbor, MI.  150  APPENDIX  151  SENSORY EVALUATION SCORE SHEET - APPLE SLICES NAME  DATE ^ SAMPLE NO.  Please Make a vertical line on each horizontal line to indicate the intensity of each attribute. Cross or name off - flavor/5 and/or off odor/5 you noticed. Please rinse between samples and feel free to make any comments.  Apple odor^5^4^3^2^1 ^4^ in mouth^4 ^ 4 •Excellent Very^Good^Fair^poor good 5^4^3^2^1 Sweetness^+ ^ 4 ^ 4- ^ 4 ^ + Very^Sweet^Moderate Slight^Not sweet^sweet^sweet^sweet Sourness^5^4^3^2^1 4- ^ 4 ^ 4 ^ 4 ^ + NOt sour slight^Moderate Sour^Very sour^sour  sweet/sour^5^4^3^2^1  balance^+ ^ 4 ^ 4 ^ 4 ^ 4 Excellent Very^Good^Fair^Poor good  Firmness^5^4^3^2^1 (at finger^f ^ + ^ + ^ 4 ^ 4 tip)^Very^Firm^Slight^Soft^Very firm^soft^soft Crispness^5^4^3^2^1 (sound in^+- ^ f ^ f ^ + ^ -4 mouth)^Very^Crisp^Moderate Slight^Not crisp^crisp^crisp^crisp Force needed^5^4^3^2^1 to tear^+ ^ 4 ^ 4 ^ 4 ^ 3(in mouth)^Very^High^moderate Slight^Low high Whiteness^5^4^3^2^1 (Surface)^tt ^ 4 ^ 1- ^ 4 ^ + NO^very^slight^Brown^Very 0010r^511111ht tan^tan^brown Off-flavor^5^4^3 ^2^1 1- ^ 4 ^ 4^4 ^ + Absolute- very^slight^moderate Strong ly none^slight^storng  overall^5^4^3^2^1  acceptability I- ^ + ^ + ^ + ^ + very^Acceptab-^Moderate^Slight/ Unaccepaccepta-^le^accepta-^accept- table ble^ able^ble Bland (lack of flavor)  OFF-FlAVORS Alcoholic/fermented Musty(old/stale) ^ COMMENTS:  ^Bitter ^  

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