Abstract: Drying uniformity and microwave energy utilization rate (MEUR) are important indicators for evaluating microwave energy absorption and utilization of materials. Drying uniformity and MEUR during drying are the main factors affecting product quality and processing cost, respectively. During the microwave foam drying of raspberry berries the electric field intensity, temperature and microwave energy absorption distribution pattern of the material layer was analyzed by numerical simulation to improve the drying uniformity and MEUR. The influence of microwave intensity, layer thickness and circulating drying time (CDT) on the drying characteristics, drying uniformity (temperature and water content uniformity) and MEUR of foamed pulp were investigated by theoretical analysis and single-factor experiments. The results indicated that the electric field distribution uniformity (EFDU) of the material layer was significantly improved with the increase of microwave intensity (P < 0.05). The EFDU and absorption characteristics of the material layer had a significant influence on the uniformity of temperature and water content distribution (P < 0.05). Drying uniformity was positively correlated with microwave intensity (2-6 W/g) but negatively with layer thickness (3-7 mm) and CDT (2-6 min). With the increase of microwave intensity, the drying uniformity gradually increased, and with the increase of layer thickness and CDT, the drying uniformity gradually decreased. MEUR increased initially and then decreased with the increase of microwave intensity, layer thickness and CDT. These results provide a theoretical basis for studies on the energy-efficient production of microwave dried berry fruit powder with high quality.

Key words: raspberry berry, microwave, foam drying, energy utilization, uniformity