Moisture equilibrium data for desorption of water from apples was determined at 20°, 30", 40°, SO" and 60°C. The rehydration 40°C isotherm was also found. The experimental procedure was a gravimetric dynamic method. Sorptive capacity decreases as temperature increases. Rehydration of the dried material results in hysteresis.The data are interpreted in thermodynamic terms. Specific surface and monolayer concentrations are also found. Pore size distribution studies show that the most frequent pore radius at 20°C and 40°C are, respectively, 26 A and 22 A. Hysteresis data are compared with what would be expected from the cellulose wall, indicating that solubility and other effects increase sharply the sorptive capacity of the fruit. A comparison of desorption and adsorption characteristics shows that dehydration results in serious damage of the fruit structure.INTRODUCTION ACCORDING to a recent review (Bruin and Luyben, 1980) data on the influence of temperature on sorption isotherms are relatively scarce. However, the importance of these data in considering drying problems is. self-evident. Drying is a coupled heat and mass transfer process, in which product temperature rises from room temperature and approaches the drying air temperature. In drying, packaging or storing modelling, the coupling of heat and mass transfer is done mathematically via the equilibrium relationship (Rotstein and Comish, 1978a, b). Even for a qualitative discussion of these processes there is a need to know how sensitive the equilibrium moist air-foodstuff is to temperature change.
Water is desorbed from apples in two stages. The initial stage is characterized by a very fast release of water. The second stage corresponds to low moisture contents and a low rate of desorption. The process can be mathematically modeled in different ways depending on whether or not the true tissue structure is taken into account. These alternative ways are reviewed, compared and used to analyze the mechanisms involved.where i stands for the components water vapor in gas phase, air in gas phase, liquid water and solid. The total P is considered constant and 1 : i "v is considered negligible, Thus, Eq (1) can be written, for an uni-dimensional process :
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