Hybrid microwave-hot air tunnel drying of onion slices: Drying kinetics, energy efficiency, product rehydration, color, and flavor characteristics

Hybrid microwave-assisted hot air (MW-HA) tunnel drying at 100, 150 and 200 W at 50 °C, 60 °C or 70 °C was evaluated with gathered data on the associated drying kinetics, energy consumption/efficiency and product quality as compared with conventional hot air drying (HA) in the same tunnel. A pilot-scale 1.5 x 4 kW magnetron microwave tunnel supplemented with a four-duct convective hot air dryer was used to dry onion slices. In the hybrid microwave (MW-HA) mode, the multistage microwave and hot air drying were used simultaneously during the drying process, while only hot air was used in HA mode, and the two were compared for drying performance and quality. Hybrid MW-HA reduced the drying time by over 90%, and increased the energy efficiency by 3.0-16.0 times as compared to HA. The rehydration capacity of all samples were well described by a first order kinetic model and MW-HA dried product had better rehydration characteristics. Verma model and Midilli & Küçük model were the best ones for describing both MW-HA and HA drying kinetics of onion slices, and the effective moisture diffusivity of MW-HA was two order of magnitudes higher than HA drying. MW-HA also improved color, flavor, antioxidant and total phenolic compounds (TPC) retention in the dehydrated product.