Effect of freeze-drying conditions on shrinkage and porosity of dehydrated agricultural products

Structural proper ties such as particle density bulk (or apparent ent) density and porosity of apple, banana, carrot and potato were investigated after freeze-drying under various drying conditions. The materials were freeze-dried at various pressures, to achieve sample temperature ranging between -50 and -5 degrees C. Fbr temperatures above the glass transition temperature of the concentrated amorphous solution (T'(g)= -45 degrees C), the freeze-dried materials decreased in size. The bulk density of the freeze-dried materials was increased with temperature and an exponential equation was used to express this influence. The final porosity decreased as the sample temperature increased for all the materials examined, ranging between 70 and 95%. The final porosity was related to the complex viscosity of frozen apple and potato which decreased linearly as the temperature increased in the temperature range examined. (C) 1998 Elsevier Science Limited. All rights reserved.