Slices of fresh apple, banana and strawberry were frozen at -20 oC and freeze-dried using a shelf temperature of 40 oC. Theoretical expressions were proposed to predict vapor transfer kinetics during the primary and secondary drying stages. In the former, a model that predicts the sublimation rate as a function of time, considering the increasing dried layer thickness, was used, which improves greatly the sublimation time equation offered in several textbooks without adding much complexity. In the latter, an analytical solution of the unsteady state diffusion equation was applied. Permeabilities were determined for the primary drying model at an absolute pressure of about 30 Pa, though the relevant kinetic coefficient combines permeability and the mass of ice to sublime relative to the dry matter (sublimation kinetic coefficient). In the secondary drying stage, diffusion coefficients of vapor in the dried layer were in the order of 10−09 m2s−1 for pressures of about 3-5 Pa. In both periods, agreement of predicted and experimental values was more than satisfactory. A minimum freeze-drying time of 12, 6.8 and 8.7 h, considering a final moisture content of 4% w/w, was calculated for apple, banana and strawberry, respectively. Normalized drying curves showed a faster sublimation rate for banana, intermediate for strawberry and slowest for apple. On the other hand, desorption curves showed a faster desorption rate for apple, intermediate for banana and slower for strawberry. In each period, the ordering of the relevant kinetic coefficients (sublimation and diffusion coefficients, respectively) represented the ordering of experimental curves.

Macro and micro-structural changes take place during food dehydration. Macro-structural changes encompass modifications in shape, area and volume. Studies of such changes are important because dehydration kinetics (essential for calculating industrial dryers) may be highly influenced by changes in food shape and dimensions. The overall changes in volume, surface area (“shrinkage”) and shape (Heywood factor, with provides a close description of food shape) were determined experimentally, and the results were correlated with simple expressions. Hence, although dehydration kinetics can be modeled with simplified overall shrinkage expressions, the possibility of selecting a suitable geometry and predicting the characteristics dimensions will provide higher accuracy. An additional unresolved problem is the lack of a general model that predicts macro-structural changes for various foods and diverse geometries. In this work, based on experimental data of sweet and sour cherries, and rose hip fruits, a simplified general model to predict changes in volume and surface area are proposed. To estimate how the changes in characteristic dimensions affect the kinetic studies, experimental drying curves for the three fruits by means of a diffusional model considered the following variants for the characteristic dimensions: (i) The radius of the fresh food, assumed constant; (ii) The radius of the partially dehydrated product; (iii) The radius predicted by the correlation for structural changes, especially volume, obtained in this work and generalized for the three fruits, and (iv) to demonstrate the need to study the macro-structural changes for all dehydrated foods, also be present the case of a restructured food.