Developing a flipping device for combined radio frequency-hot air treatment to improve the heating uniformity of pear slices

The non-uniformity of radio frequency (RF) heating is a major technical bottleneck restricting the development of large-scale drying processes for high-moisture agricultural products. To effectively improve the RF heating uniformity of pear slices, this study designed and developed a flipping device suitable for use in an RF heating system. Effects of three key operating parameters, namely electrode gap (100, 110, and 120 mm), hot air temperature (50, 60, and 70 °C), and flipping time interval (15, 30, and 60 min), on drying rate, heating rate, heating uniformity index ( λ ), and sample colour were systematically investigated. Based on a comprehensive evaluation of drying performance and heating uniformity, the best-performing operating condition within the experimental range was identified as an electrode gap of 110 mm, a hot air temperature of 60 °C, and a flipping time interval of 30 min. Subsequently, comparative tests were conducted among flipping assisted RF drying (HAFRFD), hot air-assisted RF drying (HARFD), hot air and flipping drying (HAFD) and hot air drying (HAD). The results showed that HAFRFD achieved a clearly higher drying rate than the other three drying methods, and its heating uniformity index λ was also significantly lower than that of HARFD, indicating that the introduction of the flipping device effectively improved RF heating uniformity. In addition, pear slices subjected to HAFRFD exhibited the best colour quality. This combined drying method may provide an effective technical solution to address the issues of uneven heating and quality deterioration during RF drying of high-moisture agricultural products.