The influence mechanism and control strategy of relative humidity on hot air drying of fruits and vegetables: a review

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.