Ventilation improving drying uniformity of germinated brown rice under continuous microwave conditions

Abstract: Germinated brown rice (GBR) is a type of brown rice with sprout length of 1-2 mm, which is germinated in the appropriate temperature and humidity. GBR contains a large number of bioactive compounds, such as Gamma-aminobutyric acid, inositol hexaphosphate and glutathione, and has high physiological activity and highly nutritious, sanitarian and pharmacological effects. As a result, GBR is known as active rice for commodity. Fresh GBR is produced by soaking the brown rice in a mild water with germination temperature of 35-37 oC and final moisture content of about 30.0% (wet basis). It is a necessary process to dehydrate the fresh GBR to safe moisture content of 14% (wet basis) to prevent the quality deterioration, such as the degradation of nutritious compounds, spoiling and sour smell. Dried GBR should have enough bioactive compounds, complete shape and normal color (favored golden yellow) to meet the requirement of consumers. Hot air drying is the conventional method to dry GBR, which is the most common drying method with long drying time, high energy consumption and poor quality. Microwave drying technology is a feasible technology for the drying of GBR with high efficiency, easy control and wide feasibility. However, non-uniformity of drying seriously influences the drying quality of dehydration material and the utilization of microwave energy. In order to improve the uniformity of GBR in a continuous microwave dryer, the combination of the platform test, computer simulation and theoretical analysis was employed to analyze the microwave energy distribution in the material layer of the microwave dryer. The effects of ventilation velocity on the uniformity of GBR during microwave drying were investigated. The distribution and utilization of microwave energy in material layer of microwave dryer depend on the strength of electrical field and changes of dielectric properties of material that are the function of temperature and moisture, which play the key role in influencing the distribution and changes of temperature and moisture content of GBR. The results showed that the electric field intensity in the material layer in an already established microwave dryer did not proportionally increase with the microwave input power due to low transmission efficiency and high reflection loss of electromagnetic wave. In the microwave drying process of GBR, the ventilation airflow may blow away the vapor from GBR. The excessively low ventilation velocity had weak capacity of carrying vapor in a microwave drying cavity. In the case of excessively high ventilation velocity, the coupling of the dryer structure and the airflow movement may cause the uneven flow distribution in material layer. The appropriate ventilation velocity was in range of 0.5-1.0 m/s, which could promote the uniformity of the electromagnetic wave distribution. The feedback loss and transmission efficiency of microwave between feed ports of waveguide resulted in a serious decline of the utilization of microwave energy in material layer. There was obvious distribution difference of moisture and temperature in GBR layers in microwave drying. The microwave drying technology with ventilation may weaken the non-uniform consequence of electric distribution to improve the drying uniformity of GBR and assure its drying quality.