Modeling of Freeze-drying Kinetics for Apple, Banana, and Strawberry

Abstract

Fresh apple, banana, and strawberry slices were frozen at -20°C and freeze-dried using a shelf temperature of 40°C. Theoretical expressions were developed to predict vapor transfer kinetics during both sublimation and desorption periods. For sublimation, a model that accounts for the increasing dried layer thickness was employed to predict the sublimation rate as a function of time. This model significantly improves upon the time equation found in literature without adding substantial complexity. For desorption period, an analytical solution of the unsteady-state diffusion equation was applied. Permeabilities were determined for the sublimation drying model at an absolute pressure of approximately 30 Pa. However, the relevant kinetic coefficient combines permeability and the mass of ice to sublime relative to the dry matter (sublimation kineticcoefficient). In the desorption drying model, diffusion coefficients of vapor in the dried layer were on the order of 1×10-9 m²/s for pressures around 3-5 Pa. In both periods, the agreement between predicted and experimental values was highly satisfactory. A minimum freeze-drying time of 12, 6.8, and 8.7 hours, respectively, was calculated for apple, banana, and strawberry, considering a final moisture content of 4% w/w. Normalized drying curves revealed a faster sublimation rate forbanana, intermediate for strawberry, and slowest for apple. Conversely, desorption curves showed a faster desorption rate for apple, intermediate for banana, and slower for strawberry. In each period, the order of the relevant kinetic coefficients corresponded to the Arrangement of the experimental curves.