A Biot number has been an excellent indicator for the heat or mass transfer performance during the agricultural production process. The difference between temperature and moisture can be determined between the inside and outside of the material, including heat transfer Bih and mass transfer Bim. Nowadays, a step-down relative humidity (RH) has been successfully applied to improve the drying efficiency and quality of some fruits and vegetables, such as carrots, American ginseng roots, and mushrooms. The high RH can be adopted in the step-down (RH) drying to pretreat the material, until the temperature increases to a high level. Afterward, the RH drying can be reduced to a low value for the surface moisture evaporation. The optimal angle of heat transfer can be selected for the high humidity in the early stage of step-down RH drying, leading to the rapid rise in the surface temperature. Once the surface temperature rises, the internal temperature increases in the way of heat conduction, and the migration and diffusion rate of water increases after the internal temperature rises. The mass transfer angle was also selected to inhibit the evaporation of water on the surface for the high humidity in the early stage, in order to prevent the surface from drying too fast and crust formation. Therefore, the difference between the surface and the internal temperature can be used to determine the dispersion and migration of the internal water into the surface in time, particularly for the effective dehumidification in step-down RH drying. The kind of fruit and vegetable materials can be the dominant factor in this case. It still remains unclear on step-down RH drying suitable for all fruit and vegetable materials. In this study, the applicability of step-down RH hot air drying was evaluated to clarify the drying characteristic, the effective moisture diffusion coefficient (Bih and Bim), color, rehydration ratio, and energy consumption of different thickness carrots (6, 12, and 18 mm) and logan under the step-down RH (first stage: 50% RH 30 min; second stage: 20% to end) and continuously dehumidification (RH<15%) with the drying temperature of 60 oC and the air velocity of 1.0 m/s. Results showed that the higher drying efficiency was achieved in the higher Deff value for the carrot and longan slices with the thickness of 6 mm in the continuous dehumidification, compared with stage one. By contrast, the stage dehumidification improved the drying efficiency with the higher Deff in the carrot slices of 12 mm or 18 mm. The Bih values of 6, 12, and 18 mm carrot slices were 0.582 7, 1.165 5, and 1.748 2, respectively. The maximum relative deviation was 2.7% between surface and internal temperature for the 6 mm carrot slices and Bim<1. The low RH greatly contributed to speeding up the drying rate, where the water inside the material was rapidly diffused to supply the surface. The maximum relative deviation between surface and internal temperature was 8.6% at the 12 mm carrot slices, and 13.8% at the 18 mm carrot slices, when Bim>1. Therefore, the step-down RH drying was necessary, due to the large water and temperature gradients on the surface and inside of the material. The Bim values of carrot slices with different thicknesses varied from 0.156 8 to 0.223 0. The Bim values of longan under the continuous dehumidification and step-down RH were 0.110 3 and 0.084 3, respectively. Furthermore, there was a small resistance of internal mass transfer in the longan (Bim<0.1), where the water quickly diffused to the flesh surface. Nevertheless, the flesh shrinkage, hard outer shell, and high RH outside caused great resistance to moisture migration during the drying process. The Bim values of carrot slices with different thicknesses were more than 0.1, indicating the large moisture gradient from the inside to the surface. Therefore, a high RH was adopted to reduce the evaporation rate of surface water for the high temperature. An excellent performance was achieved in the 6 mm carrot and longan under continuous dehumidification drying, indicating a better color, a higher rehydration ratio, and lower energy consumption. The carrot slices of 12 or 18 mm achieved better performance under step-down drying conditions. In conclusion, the applicability range of stage dehumidification drying was determined using heat and mass transfer Biot number, when Bih>1 and Bim>0.1 during step-down drying. The finding can provide theoretical and technical support for the different materials in the relative humidity control mode. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.
