Modeling atmospheric freeze-drying of food products operated above glass transition temperature

Abstract

A mathematical model based on mass and heat balance equations was developed for simulating atmospheric freeze-drying of food products. When fresh fruit slices are freeze-dried at temperatures far above the glass transition temperature, structural deformation and melt-back would occur due to the glass-rubber-liquid transition. Based on this observation, an atmospheric freeze-drying system was modeled by considering the in-drying product as a uniform mixture of the rubber/liquid and ice phases that could exhibit a specific vapor pressure. Temperature- and moisture content-dependent apparent vapor pressures of the apple slices were obtained experimentally and plotted in a diagram. Notably, the apparent vapor pressure was significantly different from the vapor pressures of pure water/ice at the corresponding temperatures, particularly at around the glass transition curve. The simulation was implemented with the developed single zone model by applying these obtained apparent vapor pressures. This approach could predict drying kinetics with reasonable accuracy and simplified equations. The parameters applied in this simulation were further confirmed to be applicable to various operating conditions (air temperature from –10 to 10 ºC; air flow rate from 0.1 to 0.5 m/s).