Modeling dehydration and rehydration of cooked soybeans subjected to combined microwave-hot-air drying

Pre-cooked soybeans were subjected to convective hot-air, microwave and combined microwave-hot-air dehydration. Three microwave levels (210, 300, 560 W) and three air temperatures (160, 180, 200 °C) were examined. Drying kinetics, rehydration kinetics and colour change were investigated relative to microwave level and air temperature. Combined microwave-hot-air drying decreased the drying time required when compared to drying with either hot-air or microwave energy alone. Predictive models were developed to describe dehydration and rehydration kinetics. Dehydration rate, rehydration rate and total colour change of rehydrated product generally increased with microwave level and air temperature. Within the studied range, optimal drying occurred for the lowest levels of both microwave and air temperature studied, i.e. microwave power = 210 W, air temperature = 160 °C. Industrial relevance: Application of microwave energy simultaneously with convective hot-air drying to pre-cooked soybeans was up to four times faster when compared to convective drying alone, and up to twice as fast when compared to microwave drying alone. Rehydration time for soybeans subjected to combined microwave-hot-air drying was 50-60% less than for legumes subjected to either convective or microwave drying alone. Therefore, application of combined microwave-hot-air drying to pre-cooked soybeans resulted in significant saving in process time, while also producing a dehydrated product with fast rehydration properties.