Study on Drying Control Strategy of White Radish Slice Based on Monitoring Medium Relative Humidity

Enhancing the drying rate and dried products quality, as well as energy efficiency, is very tempting for the drying industry. Recently, a lot of investigations have illustrated that the drying temperature, air velocity, and sample thickness have significant influences on the drying process. However, few investigations took into account the relative humidity (RH) as an important hot-air-drying parameter in the drying process. Therefore, in the current work, white radish slabs were used to explore the drying characteristics and quality under the drying condition of a constant RH, decreasing the RH step by step and decreasing RH automatically, together with a constant air velocity of 1 m·s^-1 and a drying temperature of 60 °C. Compared to continuous dehumidification, the step-down RH process was conducive to the material center temperature rise in the early stage of drying. When the material central temperature was increased and then the RH was reduced, the drying rate was increased and the overall drying time was shortened. The automatic-down RH control drying process includes three dehumidification processes. The respective RH control values were 40%, 30%, and 20% and the respective durations were 180 min, 90 min, and 60 min. The comprehensive quality evaluation showed that the comprehensive score of the automatic-down RH control process at 60 °C was the highest, which was 0.85. The L* and b* values of the automatic-down RH control were 26.0 and 1.67, respectively, which were better than those of the step-down RH, constant 20% RH, and constant 40% RH. The maximum rehydration ratio was 3.96 under the automatic-down RH control condition, and the quality was good. The lowest energy consumption under the condition of the automatic-down RH control was 0.90 kW·h·kg^-1. The present work contributes to a better understanding of the effect of the RH on the drying characteristics and quality of white radish slices, which is useful for enhancing the drying rate and dried products' quality as well as energy efficiency.