Effects of adding polystyrene blocks in rectangular containers on radio frequency heating rate and uniformity of watermelon seeds

Abstract

Non-uniform heating is a main problem for applying radio frequency (RF) treatments to eliminate the seed-borne pathogens in seeds before commercial import/export. In this study, thermal and dielectric properties of watermelon seed coats and embryos with different moisture contents were measured for computer simulation. The simulation model of RF heated watermelon seeds with or without adding a polystyrene block was developed and validated based on experimental results using the commercial COMSOL software. The effect of adding different sizes of polystyrene block on RF heating rate and uniformity of watermelon seeds was also studied by the validated simulation model. The results showed that the difference in heating rate between first and second layers and average temperature of seeds increased from 0.63 to 18.78 °C/min and 40.74 to 58.60 °C with increasing thickness and height of polystyrene block increased from 5 to 30 mm and 4 to 28 mm. The volumetric RF heating uniformity index in seeds decreased with decreasing heating rate difference. Meanwhile, the height of 7.7 mm combined with thickness of 25 mm of the polystyrene block could provide the best RF heating uniformity for watermelon seeds. These findings from the established new methods to improve the heating uniformity could be used to develop an effective RF treatment protocol for controlling the seed-borne pathogens of watermelon seeds without reducing their quality.