Optimizing a Successive Two-Stage Drying Process for Exocarpium citri grandis: Hot Air Drying with Integrated Temperature-Humidity Control Followed By Radio Frequency Drying

A novel successive two-stage drying strategy, namely hot air drying with integrated temperature and humidity control (HAD-ITHC) combined with radio frequency (RF) drying, was proposed on large-size Exocarpium citri grandis (ECG) to overcome time-consuming and heat-sensitive component degradation. After optimizing the moisture transition point, two different second-stage drying conditions (hot air drying-assisted RF (HAD-ITHC + RF-HA) and vacuum drying-assisted RF (HAD-ITHC + RF-VD)) were conducted while comparing to HAD-ITHC based on drying efficiency and product quality. The RF heating characteristics during the second stage were improved by decreasing the moisture content during the HAD-ITHC first stage. However, after the moisture content of 40% continuing HAD-ITHC increased the crusting, shrinkage, structural damage, and reduced the RF drying efficiency. As a result, the 40% point was selected as the optimized moisture transition point. The optimal drying conditions for the second stage were electrode gap of 150 mm, hot air temperature of 60 ℃ under RF-HA, and electrode gap of 100 mm, atmospheric-vacuum pressure duration of 4 min:5 min under RF-VD. Compared to HAD-ITHC, introducing RF shortened the drying time (30.01–37.15%) and improved flavonoids content (5.92–9.41%), especially HAD-ITHC + RF-VD. However, the sequential vacuum-atmospheric pressure cycle of RF-VD caused severe crumpling and volatile components (VOCs) losses. The better preservation of flavonoids and VOCs, and uniformity of heating and final moisture distribution were achieved under HAD-ITHC + RF-HA. Overall, HAD-ITHC + RF-HA was a promising successive two-stage drying approach to ECG with higher drying efficiency and better quality.