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Thermo-physical properties of apples and prediction of freezing times Ramaswamy, Hosahalli Subrayasastri


In two varieties of apples, Golden Delicious and Granny Smith, the temperature dependence of different thermo-physical properties has been investigated. Detailed regression equations are given to cover the variations of thermal conductivity, apparent specific heat and thermal diffusivity of apples with temperature both above and below the freezing point. Tissue density has been studied at four different temperatures. The thermo-physical properties determined in this study have been employed to predict the freezing times of apples under various conditions of freezing using different models reported in the literature. The freezing conditions included for both variety of apples were: five freezing systems viz., freezing in air at -21 to -25°C and at -28 to -30°C, freezing by immersion in ethylene glycol at -18 to -20°C and -20 to -24°C and by immersion in liquid nitrogen at -197°C; three container sizes, viz., cans of size 300x407, 307x409 and 401x411; two initial product temperatures, 16-25°C and 1-7°C; and two target temperatures, -10° and -18°C. Two types of prediction methods were used, the analytical methods of : Plank (194]) , Nagaoka et al. (1955), I. I. R. (1972), Mellor (1976), Cleland and Earle (1979b), and the numerical methods with constant as well as varying thermal properties. The predicted values of freezing times by the different models were compared with experimental values and the relative merits of each model discussed. Based on an analysis of the prediction errors, a modification of Plank's equation to give the least error was suggested as follows [equation omitted]. The mean overall prediction error of the suggested model was 6.64% which was less than 5% beyond the experimental error of 2.38%.

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