Rate Analysis of the Freeze Drying of Liquid Foods by a Modified Uniformly Retreating Ice Front Model

Abstract

The freeze drying of liquid foods was analyzed by a modified uniformly retreating ice front (URIF) model, where the unfrozen water and the surface resistance of the material were considered. The apparent thermal conductivity estimated by the data for drying was approximately twice as large as the thermal conductivity measured by the steady state method. The vapor permeability and surface resistance was separately estimated from the apparent thermal conductivity, the volume fraction of ice crystals in the sample, and from the drying data. The vapor permeability was higher as the initial water content was higher and the freezing temperature was higher. The surface resistance was higher as the initial water content was lower and the freezing temperature was lower. The drying rate and the temperature of the frozen layer were well described by the modified URIF model with the apparent thermal conductivity, the vapor permeability and the surface resistance.