Abstract

Drying temperature, air velocity, material thickness during hot air drying usually have a significant influence on drying kinetics and quality attributes of fruits and vegetables. However, under constant drying temperature, the influence of relative humidity (RH) of hot air on heat and mass transfer is often ignored in drying practice. On the other hand, the current literature about the influence of relative humidity of drying medium on drying kinetics and quality of fruits and vegetables are inconsistent or even contradictory. Usually, the relative humidity is roughly controlled based on experience. Therefore, it is necessary to summarize and reveal the influence mechanism of relative humidity of drying medium on drying kinetics and quality of fruits and vegetables. The RH effect mechanism on drying kinetics is analyzed in two aspects including heat transfer and mass transfer. Increasing RH intensifies heat transfer process while decreasing RH intensifies mass transfer process. Applied high RH in the initial drying stage and then decreasing RH can improve drying efficiency compared to the continuous dehumidification RH drying method. Additionally, step-down RH drying strategy method helps to mitigate the formation of casehardening on product surface and promote pore network, which is benefit for moisture transfer and diffusion. High RH during hot air has the potential to improve the pore network of materials, which is helpful for rehydration ratio. Additionally, low RH increases drying rate so that material surface generates a rigid crust or shell that fixes the volume. Low temperature but high RH drying medium can preheat the sample as soon as possible so as the energy which was used to heat the drying air can be saved. The applicable of step-down RH drying strategy is suitable for those food materials drying, whose β value is more than 1 in Weibull distribution model and Bi value in Bi-Di model is more than 0.1. In other circumstances, continuous dehumidification drying is suitable for food material drying. The auto RH control adjustment drying strategy has been investigated based on material temperature. Such RH control drying strategy provides theoretical foundation for enhancing drying efficiency and quality of fruits and vegetable materials.

摘要

热风干燥过程中的干燥温度、风速、物料厚度通常对果蔬的干燥动力学和品质属性有显着影响。然而，在恒定干燥温度下，干燥实践中往往忽略热空气相对湿度（RH）对传热传质的影响。另一方面，目前关于干燥介质相对湿度对果蔬干燥动力学和品质影响的文献并不一致甚至相互矛盾。通常，相对湿度是根据经验粗略控制的。因此，有必要总结和揭示干燥介质相对湿度对果蔬干燥动力学和品质的影响机理。从传热和传质两个方面分析了RH对干燥动力学的影响机理。增加 RH 会强化传热过程，而降低 RH 会强化传质过程。与连续除湿 RH 干燥方法相比，在干燥初期应用高 RH 然后降低 RH 可以提高干燥效率。此外，降压RH干燥策略方法有助于减轻产品表面表面硬化的形成，促进孔隙网络，有利于水分的传递和扩散。热空气中的高相对湿度有可能改善材料的孔隙网络，这有助于再水化率。此外，低 RH 会增加干燥速率，从而使材料表面产生固定体积的刚性外壳或外壳。低温高RH的干燥介质可以尽快预热样品，从而节省用于加热干燥空气的能量。降压RH干燥策略适用于食品物料干燥，Weibull分布模型中β值大于1，Bi-Di模型中Bi值大于0.1。在其他情况下，连续除湿干燥适用于食品材料的干燥。研究了基于物料温度的自动RH控制调节干燥策略。这种控制湿度的干燥策略为提高果蔬原料的干燥效率和质量提供了理论基础。