Analysis and modelling of temperature and moisture gradient for ginger slices in hot air drying

Abstract

To understand the temperature and moisture distribution of ginger slices during hot air drying, mathematical models were developed to simulate the heat and mass transfer in various drying modes, including constant hot-air drying modes (the drying temperatures were kept at 50, 60, 70, 80 °C, respectively) and multistage drying modes (the drying temperature was varied between 50 °C and 80 °C during a single drying process). The simulation results showed that the developed models fitted the experimental data well, which proved their effectiveness in various drying modes. The three-dimensional temperature and moisture distribution models also demonstrated that the low temperature gradient between the surface and the core had little effect on the drying results. In the case of a high drying temperature, the core part contained high moisture content even after drying. Among all drying modes, the L-H-L (the drying temperature was increased from 50 °C to 80 °C and then to 50 °C again) was the best if the maximum moisture difference during the drying process and the final moisture difference between the surface and the core were all considered. For the final product quality, this drying mode also performed well and resulted in a higher rehydration ratio, better microstructure, less color difference, higher total gingerol content, and good sensory evaluation. Therefore, it was necessary to keep the air temperature at a low level at the beginning and final stages for the quality and storage safety consideration. While in the middle stage, the drying temperature could be increased to a higher level to accelerate the drying speed.