Studies On Radio Frequency Heating Uniformity In Low-moisture Agricultural Products

Radio frequency(RF) heating is considered as a potential postharvest technology for disinfesting agricultural products. Heating uniformity is the considerable problem in practical applications and production. In this study, three different comparison experiments were conducted to evaluate the heating uniformity in a 6 k W, 27.12 MHz parallel plate RF heating system with a free-running oscillator. The three sample conditions in RF heating experiments include four different thicknesses milled rice placed in the container, the milled rice of 9% and 11% moisture content with 4 cm thickness, and milled rice and soybean with 4 cm thickness and 9% moisture content in the half section of the container. Finally the milled rice sample with low moisture in the corner and high moisture in the center of the container was used to evaluate the heating uniformity improvement. The results showed that the milled rice temperature increased with increasing thickness. Temperatures in the center were lower than those in the corners and edges both in RF treated rice and soybeans. The samples of rice or with high moisture were heated more than those of soybean or with low moisture. The RF heating uniformity could be improved by placing the low moisture samples in the corner and high moisture ones in the center. In order to supply physical disinfesting methods, this study should provide useful information for designing practical RF treatment processes. Using the software COMSOL based on the finite element, the RF heating uniformity under the above experimental conditions was simulated. The experimental and simulation results are summarized below:(1) Studied the experimental temperature distribution of the same moisture content milled rice with different thickness. The four different thicknesses(2 cm, 4 cm, 6 cm, and 8 cm) of 9% w.b. moisture content agricultural products were heated under the electrode gap of 120 mm for 5 min. The results showed that the corner and edge heating was observed both in top and middle layers of samples with 4 thicknesses. The average temperatures both in top and middle layers increased with increasing thickness. Temperatures in the middle layer were higher than that in the top layer for each milled rice with 4 thicknesses. The heating uniformity was also gradually improved with increasing sample thickness.(2) Studied the experimental temperature distribution of the same thickness milled rice with different moisture content. The milled rice with 11% and 9% w.b. moisture content was conducted to evaluate the RF heating temperature distribution. The results showed that the similar heating pattern with edge and corner effects was obtained for two moisture samples separated in the container. The temperatures in higher moisture milled rice were higher than those in low moisture samples both in top and middle layers. The RF heating uniformity in the middle layer was better than that in the top layer both in two moisture samples.(3) Studied experimental temperature distribution of milled rice and soybean with the same moisture content and thickness. 9% moisture content milled rice and soybean with 4 cm thickness was used to compare the RF heating uniformity. The results showed that the similar temperature distributions with corner and edge heating were observed both for soybean and milled rice samples and in top and middle layers. The temperatures of milled rice were higher than those in soybeans.(4) Improvement of the RF heating uniformity with non-uniform moisture samples. Based on the pretreatment results and above observations, the RF overheating in corners was found in the rectangular container of dry samples. In order to determine and improve the RF heating uniformity, rice samples with 9% moisture content were placed in four corners of the container and the samples with 11% moisture content were filled in left space of the container. The results showed it was a good way to improve the RF heating uniformity.(5) Validated the model using the data of experiments, and obtained a reliable model. Using computer simulation to further improve the RF heating uniformity. Choosing the milled rice with 7%, 9%, and 11% moisture content placed at the four square corners of 6 cm×6 cm, respectively and changing the four square corners into 2 cm×2 cm;4 cm×4 cm;6 cm × 6 cm to evaluate the heating uniformity.