Influence of dielectric and sorption properties on drying behaviour and energy efficiency during microwave convective drying of selected food and non-food inorganic materials

The average energy efficiency of microwave convective drying is strongly related to drying time and thus to drying kinetics. This study investigates the energy efficiency of materials with different material properties and drying rates. Four solid, porous materials (swede, potato, bread, and lightweight concrete) were dried under the same microwave convective drying conditions. Drying behaviour, temperature development, hygroscopicity, dielectric properties, and specific energy consumption were studied. Despite differences in kinetics and product properties, the specific energy consumption (MJ/kg evaporated water) was approximately the same for all materials at intermediate to high moisture contents. When drying was pursued into the hygroscopic region, however, the specific energy consumption increased. The results indicate that a relationship between microwave penetration depth and water activity of the materials may be found. Drying of the food materials was limited by burning either due to temperature runaway in the hygroscopic region or due to development of hot spots, possibly induced by material structure.