Microwave-Assisted Atmospheric Freeze Drying of Green Peas: A Case Study

Atmospheric freeze drying (AFD) is based on the sublimation of ice due to a pressure gradient (convective drying), and is a dehydration process for temperature-sensitive products. Since the process is slow in general, microwave radiation (MW) was applied in order to increase the sublimation in fluid and fixed bed conditions at drying temperatures of −6°C, −3°C, and 0°C. The modified Weibull model was used to describe the drying behavior for all investigations. With 280 Watt power supplied to the magnetrons, it was possible to reduce drying time by approximately 50%. The drying efficiency was approximately 30%, while the SMER was increased by 0.1 to 0.3 kgwaterkWh−1, which gives better energy efficiency for the microwave drying system used in this investigation. The product quality (color reduction and particle size/porosity) was well preserved in fixed bed drying at −6°C and −3°C, while the product quality was reduced significantly in microwave AFD experiments at 0°C and in a fluid bed. The drying rates of AFD in a fluid bed condition were not as high as those in a fixed bed. MW-AFD in a fixed bed condition at temperatures of −6°C and −3°C performed best regarding product quality, drying time, and process control.

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