Moisture diffusion and shrinkage characteristics of broad bean during low-temperature vacuum drying

Abstract

ABSTRACT Low-temperature vacuum dehydration could avoid heat injury because of high drying temperature, as well as prevent mechanical damage due to ice crystal during freezing. However, it is not clear for the effect of different temperatures and vacuum degrees on the dehydration process. The object of this work was to investigate the drying behavior of broad bean seed and to analyze the effect of moisture diffusion on the structure during low-temperature vacuum drying process. For this purpose, a test bench for low-temperature vacuum dehydration was designed and established. The nine dehydration operating conditions at temperatures of 0°C, 4°C, and 8°C and vacuum degrees of 95, 97, and 99 kPa were conducted. The key parameters including dehydration time, dehydration rate, shrinkage, effective moisture diffusivity were analyzed. Results showed that by increasing temperature and vacuum degree, the dehydration time obviously shortened, and the dehydration rate was significantly increased. The vacuum degree has an obvious influence on product shrinkage, especially in the thickness of broad bean seed. The average effective moisture diffusivity varied from 0.65 to 2.97 × 10−10 m2 s−1 under the tested drying conditions. And the high temperature and low vacuum degree were a better way for reducing the shrinkage of broad bean seed. To avoid structure collapse and cell rupture, the end moisture content of broad bean seed should be higher to 0.05 ~ 0.11.