Lotus seed drying by intermittent phased varying power microwave under vacuum

Abstract: Lotus seeds are widely serving as functional food and Chinese herbal medicine, due mainly to their high nutritional value rich in protein, starch, and dietary fiber. Nevertheless, drying has been an important but weak link in the processing of lotus seeds. Microwave drying has generally been used in the drying of fruits and vegetables, showing high efficiency, energy saving, high product quality, and automatic control. Currently, the constant power microwave easy to operate and control is therefore commonly used to dry most of food, but the treated food is easy to be over-dried or burned. Correspondingly, the original nutrients can be reduced, due to the high speed of heat and mass transfer generated in the late drying stage of constant power microwave. Great efforts have also been made on microwave vacuum microwave drying to greatly maintain quality appearance of food. However, the commonly-used hot air drying consumes high energy to dry fresh lotus seeds with low efficiency, often failing to gain high quality. In this study, two intermittent microwave vacuum dryings were designed to improve the quality of dried lotus seeds, including constant and stage-varying power microwave drying. A systematic investigation was implemented to optimize the drying parameters of vacuum degree and microwave intermission mode, as well as the impact on drying characteristics. Additionally, traditional hot air drying was also introduced as a reference. The results demonstrated that the stage-varying power microwave drying performed much better than the traditional hot air drying and constant power microwave drying. The optimal drying parameters were achieved for the stage-varying power microwave drying with a vacuum degree of 0.1 MPa, and a microwave intermission mode of 1.5 min-on /1.5 min-off. Two stages were contained to obtain a safe water content of lotus seeds, where the first stage worked for 3 min under the microwave power of 1 600 W, and the second stage worked for 3 min under the microwave power of 1 200 W. Specifically, the concentrations of starch were 43.43 g/100g and 42.73 g/100g within lotus seeds for the hot air drying and stages-varying power microwave drying, respectively, indicating an insignificant difference between them. But there was a significant difference in the concentration of protein and crude fibers within lotus seeds under two drying. The concentration of protein in lotus seeds using stage-varying power microwave (19.37 g/100g) was higher than that of hot air drying (18.57 g/100g). More importantly, the crude fibers within lotus seeds using stage-varying power microwave drying (14.60 g/100g) remained much higher than those using hot air drying (6.94 g/100g), increasing as high as 2.1 times. The external quality of lotus seeds was also better under the stage-varying power microwave drying. Additionally, the drying time was greatly shortened with an energy consumption of 7 848 kJ/kg, compared with hot air drying (465 012 kJ/kg). It inferred that the stage-varying power microwave drying gained a better quality of dried lotus seeds with less energy consumption. The presented findings can provide a potential guideline for drying and processing of lotus seeds.