Effect of dielectric properties on radio frequency heating uniformity of apple

As a new processing technology, RF heating has great potential in blanching of fruits and vegetables. Dielectric property is the main factor affecting the RF heating of fruits and vegetables. It is affected by frequency, temperature, water and other factors. According to the change of dielectric property, the blanching process of fruits and vegetables can be optimized and improved. In this paper, the dielectric properties of Fushi and Qinguan apple pulp at different temperatures (25, 40, 60, 80, 95 ℃)and different frequencies (13.56, 27.12, 40.68, 915, 2 450 MHz) are determined by using the open-ended coaxial probe technique. A RF heating system with the power of 6 kW and the frequency of 27.12 MHz is used to study the heating speed and uniformity of apple sample. Different electrode gaps (110, 120 and 130 mm) are set to heat the apple slice (20 mm × 20 mm × 50 mm). When the center temperature of the apple sample reaches 95 ℃, the heating is stopped. The temperature rise curve of apple sample from room temperature to 95 ℃ is recorded with fluorescent fiber temperature measuring system under different electrode gaps. The temperature distribution of the surface of the heated apple sample is measured by infrared thermal imager. The results show that the dielectric constant and loss factor of apple slice change significantly with temperature and frequency. At the same temperature, as the frequency (12-3 000 MHz) increases, the dielectric constant decreases first and then increases and then decreases. The loss factor reduces first increases then and decreases then; at the same frequency, there is no specific law for the dielectric constant and loss factor of apple. The temperature of the apple sample rises from 25 ℃ to 95 ℃, and the temperature rise curve approximates linear growth. When the electrode gap is 110mm, the temperature rise is the fastest. When the electrode gap is 130 mm, the temperature rise is the slowest, 120 and 150 s, respectively. The heating speed increases with the decrease electrode gap. During the process of RF heating, the temperature distribution of the apple sample is centered, that is, the center temperature is high and the edge temperature is low. The maximum temperature occurs at the center point, and the maximum temperature of the lower segment equidistant from the center is greater than that of the upper segment. The present study shows the great potential of RF heating for the blanching of fruit and vegetable. Future studies should aim to determine changes in the texture and nutrient contents of fruit and vegetable during RF heating.