Moisturizing strategy for enhanced convective drying of mushroom slices

Moisturizing strategy that controls the water evaporation rate of the wet material and thus the relative humidity in the drying chamber was proposed for hot air drying of mushroom slices for the first time. To assess the effectiveness of moisturizing strategy, drying kinetics, moisture diffusivities, mass transfer coefficients, surface color and rehydration capacity of dried products, energy and exergy efficiencies were evaluated. Mushroom slices were dried at constant drying temperature of 60 °C and air velocity of 3 m/s. The strategies of moisturizing for 0, 15, 30, and 45 min were applied in the initial drying stage. Compared to the continuous dehumidifying strategy (moisturizing time is 0 min), the strategies of moisturizing for 15 and 30 min reduced drying time by 5% and 15%, respectively. The proper moisturizing time (30 min) enhanced the heat transfer by increasing the enthalpy of the drying air, improved surface color, and increased rehydration capacity of dried mushroom slices by reducing cell shrinkage. The energy utilization and energy utilization ratio of drying process varied from 0.16 to 3.48 kJ/s and 0.065 to 0.572, respectively, while the exergy efficiency varied from 8.04% to 84.66%. The strategy of moisturizing for 30 min reduced 23.4% total energy utilization compared to continuous dehumidifying strategy. Both the highest average energy utilization ratio, i.e. 0.156 and average exergy efficiency, i.e. 70.19% were obtained for moisturizing time of 30 min. The increase of moisturizing time from 30 to 45 min showed an adverse impact on drying time, energy utilization ratio, exergy loss, and exergy efficiency due to the inhibition of water evaporation. This study shows that moisturizing strategy can be considered as a promising method to enhance energy and exergy efficiency of hot air dryer with the improvement of products quality and sustainability of drying systems.