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Effects of sub-optimal ripening temperatures on tomato fruit quality as determined by objective measurement Koskitalo, Leslie Norman

Abstract

Controlled environment experiments were conducted to study the influence of four night/day temperature regimes; 17.8/25.6, 7.2/18.3, 4.4/15.6 and 2.8/13.9°C on the quality of tomato fruits, Lycopersicon esculentum Mill. c.v. Early Red Chief, measured objectively at three harvest dates. Temperature effects on vegetative and reproductive growth and fruit cracking were also examined. In addition, the stability of carotenoid pigments of macerated and cubed fruit stored at -20° for 0, 10, 20 and 40 days was studied. Low air temperatures decreased plant growth, caused chlorosis of vegetative growth, and reduced the frequency of fruit cracking but had little effect on fruit weight. Flower formation continued at all temperatures with the exception of the 2.8/13.9 environment while fruit set occurred only at the two highest thermal regimes. Fruits harvested at 17.8/25.6 were considerably lower in total solids, reducing sugars and titratable acidities and had substantially higher pH values than fruit exposed to 7.2/18.3, 4.4/15.6 and 2.8/13.9. Temperature had little or no effect on fruit refractive indices and total pectic substances. The failure of total pectic substances to reflect the apparent firmness differences between treatments indicates that total pectic substances are not a satisfactory index of this quality parameter. Surface and internal lightness and yellowness declined with increasing temperatures and later harvests, while redness values increased. Fruit harvested at 17.8/25.6 attained a full red coloration in 7 days, while those exposed to 7.2/18.3 required about 14 days to reach a comparable level of colour development. Fruit exposed to 2.8/13.9 were of inferior colour as evidenced by high L and bL values and low aL values. The high degree of association between lightness and yellowness values under all treatment conditions suggests that surface colour and, to a lesser extent, internal colour can be adequately specified in terms of a constant and two, rather than three, variables. The high overall correlation coefficient obtained between surface and internal Lb/a ratios immediately indicated the possibility of utilizing surface Lb/a ratios to predict internal colour. Temperature and harvest dates influenced the relationship between internal and surface colour ratios as evidenced by the decrease in correlation coefficients with higher temperatures and later harvests. The effect of decreasing temperatures on tomato colour was found to be largely a function of temperature effects on lycopene synthesis. Colour values showed marked changes as total carotene concentrations increased up to about 55 μg/g fresh weight. Continued increases above this level were not accompanied by parallel changes in surface or internal colour. Temperatures and harvest dates affected all pigment concentrations with the exception of Ƴ-carotene and, for the most part, β-carotene. The temperature regimes ranked in order of decreasing fruit quality were as follows: 7.2/18.3; 17.8/25.6; 4.4/15.6; 2.8/13.9. Although of satisfactory coloration, fruits harvested at 17.8/25.6 were rated below the 7.2/18.3 fruit for reasons of lower dry matter, sugar and acid contents. Storage duration had little effect on carotenoid concentrations of cubed samples. In macerated samples, phytoene, phytofluene and ζ-carotene concentrations decreased with storage time. When fresh samples were analysed, all pigment concentrations with the exception of lycopene were found to be much lower in macerated than in cubed samples.

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