Effect of viscoelastic relaxation on moisture transport in foods. Part II: sorption and drying of soybeans.

Abstract

The general fluid transport equation presented in Part-I of this paper is used for predicting moisture transport and viscoelastic stresses during sorption and drying of soybeans. Predicted drying curves were validated using experimental data obtained from literature (average absolute difference 6-13%). For drying temperatures used in the soybean processing industry (70-93 degrees C), smooth moisture profiles were obtained, which indicated Fickian (Darcian) transport. As the drying temperature approached the glass transition temperature (25 degrees C at 10% moisture content), the moisture profiles became sharper, which indicated non-Fickian (non-Darcian) transport. The viscoelastic stress profiles clearly exhibited the role of the force terms during imbibition and drying. Increase in drying temperature tends to decrease the stress relaxation function but reduction in moisture content during drying tends to increase it. The increase in stress due to the reduction in moisture content below 10% was not compensated by an increase in drying temperature. Drying of soybeans below 10% moisture content should be avoided in the industry because this will lead to thicker flakes that reduce the amount of oil recovery. During imbibition of soybeans, a high magnitude of stresses was obtained in the rubbery regions, which may cause critical regions prone to fissuring. The role of glass transition on stress development and critical region development was clearly observed during drying and imbibition of soybeans.