Experimental Study of Combined Fluidized Bed and Microwave Drying Applied to Soybeans and Pumpkin Seeds

Introduction: Microwave technology has emerged as a partial alternative to fossil fuels for processing particulate materials. When combined with fluidized bed systems, it offers opportunities to optimize processes in the food industry, contributing to sustainability and improved product quality.
Methods: A pilot-scale fluidized bed dryer equipped with two magnetrons was used to study drying behavior in soybeans and pumpkin seeds. Samples were soaked in tap water (1:8 w/w) for 9 hours at room temperature. Initial moisture content was 62.4% for soybeans and 68.7% (w.b.) for pumpkin seeds. Air convection was generated using a 4 HP blower, with heating via four electric resistors set at 50 °C. Air velocities were 4.0 and 3.4 m/s, equal to twice the minimum fluidization velocity for each product. Thermographic images (Testo® 875i, Germany) were taken every 5 minutes. Drying rate and moisture content curves were constructed over time.
Results: For soybeans, weight loss after 30 minutes was 32.2% without microwaves, 42.5% at 350 W, and 34.5% at 750 W. Microwave application at low power enhanced weight loss, though no further improvement was seen at higher power.
For pumpkin seeds, weight loss reached 42.0% without microwaves, 45.8% at 350 W, and 48.2% at 750 W. Here, microwave power positively influenced drying efficiency, suggesting better energy coupling and surface moisture removal with increasing power.
Conclusions: Weight loss behavior varied depending on the product matrix. Overall, microwave-assisted fluidized bed drying improved moisture removal compared to conventional drying, particularly at moderate power levels.