Development of a Successive two-stage Radio Frequency Combined hot/cold air Drying of Squid Based on Drying Kinetics and Browning Index

In order to reduce the drying time and improve the quality of squid in traditional drying methods, a successive two-stage cold air (1 m/s, 22 °C)/hot air (1 m/s, 60 °C)-assisted radio frequency (CA-RF/HA-RF) drying method was developed with a 6 kW, 27.12 MHz RF heating system. Firstly, the drying kinetics and browning kinetics of squid subjected to hot air (HA), HA-RF and CA-RF drying were studied individually. HA-RF (0.86 g/(g·h)) drying resulted in a higher drying rate than solely HA (0.70 g/(g·h)) drying. However, due to the high drying temperature, the browning index of HA-RF drying was 28.23 higher than CA-RF drying. Meanwhile, CA-RF drying caused localized overheating of squid in the falling rate period, resulting in quality damage. Furthermore, a two-stage combined drying method (HA-RF)+(CA-RF) was brought out based on the drying and color changing kinetics. The two-stage combined drying method ensured a medium drying rate (0.77 g/(g·h)) and energy consumption (39.92 ± 2.67 kW·h/kg) while retained better color (Browning index = 35.13), hardness (318.08 ± 3.85 g) and springiness (0.56 ± 0.08). Moisture migration of squid in two-stage drying was investigated using low-field nuclear magnetic resonance method and superior microstructures were obtained by RF combined drying. In summary, the developed (HA-RF)+(CA-RF) successive two-stage drying process retains better color and lower energy consumption. The results provide potential methods to improve the efficiency and quality of aquatic products using combined RF heating technologies.