恒温下相对湿度对果蔬热风干燥特性和品质的影响及调控

相对湿度作为干燥介质的重要参数，对干燥热质传质过程和干燥品质具有显著影响。但由于相对湿度对干燥过程的影响机理及优化调控机制尚不明确，导致相对湿度的调控方式多依靠经验，造成干燥效率低、品质差、能耗高等问题。对于传质过程，降低相对湿度能够增大对流传质系数，加快物料表面水分蒸发；而对于传热过程，升高相对湿度能够增大对流传热系数，加快物料升温速率。相对湿度较高时，物料升温速率快，内部水分迁移量增大，但表面水分蒸发量较小；而当相对湿度较低时，物料升温速率较慢，内部水分迁移量较小，但表面水分蒸发量较大。相对传热和传质过程的影响此消彼长，互相耦合。高相对湿度主要体现为对传热过程的影响，低相对湿度主要体现为对传质过程的影响。高相对湿度能够抑制物料表面的结壳，并能够提高复水性，降低收缩率。阶段降湿及多阶段降湿干燥方式下物料表面形成和保持了蜂窝状多孔结构，能够提高干燥效率和品质。基于监测物料温度的相对湿度调控方式被验证为较忧的相对湿度控制方式。阶段降湿干燥方式适用性的实质为：干燥过程中所体现出的对流传热热阻和内部导热热阻的相对大小，及对流传质阻力和内部传质阻力的相对大小，不同干燥条件和物料种类、厚度会影响以上传热传质阻力的大小，从而呈现出不同适应性的结果。当阶段降湿干燥过程中传热毕渥数>1且传热毕渥数>0.1时，说明阶段降湿干燥过程适用于此物料的干燥。该文综合论述了相对湿度对果蔬热风干燥过程中热质传递及干燥品质的影响，优化调控策略及适用性范围4个方面内容，明确了果蔬热风干燥过程中相对湿度的影响机理，为相对湿度的优化调控提供理论依据和技术支持。

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.