Influence of relative humidity on the drying characteristics and quality of fruits and vegetables during constant temperature hot air drying as well as controlling strategy

Drying temperature, air velocity and humidity of drying medium are the three important parameters during hot air drying of fruits and vegetables. Among them, drying temperature and air velocity are positively related to drying efficiency. Relative humidity (RH) is usually used to reflect the size of the humidity or humidity content of the medium at the constant drying temperature and total pressure. However, it is still unclear on the influence mechanism of RH on drying. The manual regulation of relative humidity cannot fully meet the large-scale production, due to the low quality, drying efficiency, and high energy consumption. Therefore, this study aims to clarify the influence and control mechanism of RH on the heat and mass transfer during hot air drying of fruits and vegetables, in order to reduce the emission and loss for energy saving with high efficiency. The drying quality was optimized in the control of RH in four aspects. The low RH was used to increase the mass transfer coefficient and the evaporation of water on the surface of the material. While the high RH was to increase the convective heat transfer coefficient and the heating rate of the material. In the high RH, there was the increase in the temperature rise rate of the material, and the internal water migration, but there was the low evaporation of surface water. In the low RH, there was a low heating rate of the material and the internal water migration, but there was a high evaporation of surface water. As such, there was the coupled influence of RH on the heat and mass transfer. High RH was mainly reflected in the heat transfer, whereas, low RH was in the mass transfer. High RH was used to avoid crust formation on the surface of the material, in order to improve the rehydration for the less shrinkage rate. The honeycomb porous structure was formed and maintained on the surface of the material under the step-down dehumidification and multi-stage dehumidification drying, leading to high drying efficiency and quality. An optimal RH control was achieved using material temperature. The stage dehumidification and drying were attributed to: the relative magnitude of thermal resistance to heat transfer and internal heat conduction, and the relative magnitude of mass transfer resistance and internal mass transfer resistance during drying. The heat and mass transfer resistance depended mainly on the drying conditions, types and thicknesses of materials. The stage dehumidification and drying were suitable for the drying of this material at Bi_h>1 and Bi_m>0.1. The influence mechanism was given on the relative humidity during hot air drying of fruits and vegetables. The finding can also provide the theoretical basis and technical support to the influence and control mode of RH in the hot air drying of fruits and vegetables.