Fuzzy logic control of relative humidity in microwave drying of hawthorn

Abstract Previous studies showed that a saturated moisture could protect food materials from quality degradation during drying processes. To further interpret this phenomenon, a microwave drying system with online relative humidity measurement and intelligent control was built. With this system, the impact of relative humidity on the drying speed and product quality in microwave drying was investigated in this study. Constant temperature drying at 50 °C, 60 °C, 70 °C without relative humidity control were first tested and 60 °C was selected as the best drying temperature. Based on this temperature, four fixed relative humidity (5%, 30%, 50%, 70%) were attempted in four different drying processes, respectively. It was found that the middle relative humidity of 30% and 50% resulted in a better product quality. In contrast, a lower and a higher relative humidity degraded the product quality or slowed down the drying speed. A fuzzy logic controller was further designed to automatically adjust the relative humidity in microwave drying process in order to improve the drying effects. By carefully constructing the fuzzy rules and algorithms, an optimal product quality was achieved, while drying speed was also accelerated. Finally, a linear control method was designed to simulate the fuzzy logic control for convenient industrial application. The vitamin C content, hardness, water activity, rehydration rate, color difference, and sensory assessment were analyzed and compared for the final products. The developed methodology has the potential to be extended to other drying methods and various food materials, with readjustment of the fuzzy logic control parameters.