Numerical and experimental study on the heat and mass transfer of kiwifruit during vacuum freeze-drying process

Abstract

In this work, the heat and mass transfer characteristics of kiwifruit samples during vacuum freeze-drying process have been investigated numerically and experimentally. The numerical simulation with moving grid method is adopted to simulate the radial and axial moving sublimation interfaces during vacuum freeze-drying. The simulated average moisture content and temperature of kiwifruit during drying are validated with the measured data, with the mean average error (MAE) of 14.7%. The findings demonstrate that the sublimation interface shifts toward the center during the drying process and the sublimation rate and pressure gradient are significant in the early stage of drying. After 10 h of drying process, the sublimation rate dropped by 80% due to the increase of the thickness of the dried region and the decrease of the shelf plate heat transfer. In addition, it is noted that the increasing shelf plate temperature and chamber pressure would cause an increase of sublimation rate, thereby reducing the drying time. Although increasing the sample thickness can improve the sublimation rate, the drying time becomes longer due to the presence of more ice crystals in thicker kiwifruit samples.