Simulation of heat and mass transfer shrinkage and quality of yam slices dried using infrared combined hot air

This study aims to accurately reveal the heat and mass transfer of yam slices during infrared and hot air drying (IR-HAD). A multi-field IR-HAD model was established to couple the temperature and humidity fields in the yam slices using finite element (FE) software COMSOL 6.1, considering the shrinkage and deformation. The heat and mass transfer was simulated at different temperatures (50℃, 60℃ and 70℃) and then verified by a series of experiments. A systematic investigation was implemented to clarify the effects of different temperatures on the quality (color difference, rehydration ratio, polysaccharide content and allantoin content) of yam slices. The results showed that (1) the volume ratios of yam slices increased with the increase in drying temperature, which were 34.55%, 37.23%, and 39.04% at the drying temperatures of 50°C, 60°C, and 70°C, respectively. (2) The R² values of the simulated temperature and moisture content considering shrinkage and deformation were 0.973, 0.976, 0.981, and 0.983, 0.976, and 0.974, respectively, indicating better agreement with the test ones. (3) The temperature field of yam slices presented consistent simulation and experimental drying. The surface temperature of the yam slice was higher than the internal temperature at the early stage of drying (0-15 min). The peripheral temperature was slightly higher than the center, leading to the forced hot-air heating. The infrared radiation further heated the inside of the yam slices, as the drying proceeded. As such, the internal and center temperatures were gradually higher than the external and peripheral ones. The moisture vapor diffusion of yam slices also showed a tendency to increase and then decrease during drying. There was an increase in the temperature of the yam slice and the rate of water evaporation in the pre-drying period (0-60 min); The temperature of the yam slice was stable in the mid-drying period (60-120 min). The infrared hot air was mainly used to evaporate the latent heat, and the amount of water evaporation and diffusion increased gradually to the maximum; The surface of the yam slice formed a hard layer in the post-drying period (120-195 min), leading to the less water evaporation and diffusion. At the same time, there was an uneven moisture concentration of yam slices during the whole drying. The internal moisture concentration was higher than the surface ones, where the center concentration spread to the outside. The reason was that the heating occurred mainly on the surface and periphery of the yam slices, leading to the vaporization of moisture in the formation of an internal humidity gradient. (4) The quality of dried yam slices was evaluated by the coefficient of variation. The highest performance was achieved at 60 ℃, with a color difference of 7.49, a rehydration ratio of 2.65 kg/kg, a polysaccharide content of 24.17 mg/g, and an allantoin content of 2.66 μg/g. This finding can provide a strong reference for the simulation of various materials in infrared and hot-air drying.