Successive Two-Stage Hot Air-Drying with Humidity Control Combined Radio Frequency Drying Improving Drying Efficiency and Nutritional Quality of Amomi fructus

Amomi fructus (A. fructus) is a medicinal and edible herb due to its attractive aroma. In order to solve the challenges of severe cracking and aroma loss in hot air (HA) drying of Amomi fructus, a novel multi-stage successive combined drying technology, namely hot air drying with temperature and humidity process control (HADTHPC) followed by radio frequency (RF) drying, was developed in this study. The optimum moisture transition point, drying efficiency, and quality in terms of shell burst ratio (SBR), color, microstructure, volatile components, total flavonoid content (TFC), and antioxidant activity under three two-stage successive combined drying methods (HADTHPC + RF, HADTHPC + HA, HADTHPC + (RF-HA)) were investigated. Results indicated that the optimum moisture transition point of 40% was obtained considering RF heating rate and uniformity. HADTHPC + (RF-HA) reduced drying time by 55.5%, energy consumption by 47.80% compared with HADTHPC + HA, and improved drying uniformity by 5.16-32.00% compared to HADTHPC + RF, as well as retained the best color, microstructure, total flavonoid content, antioxidant activity, and lowest shell burst ratio. Flavor fingerprint established by headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) indicated that the relative contents of alpha-pinene, alpha-phellandrene, and myrcene in HADTHPC + (RF-HA) samples increased significantly compared with freeze-dried ones, which was associated with the intact preservation of glandular trichomes structure. Principal component analysis (PCA) indicated that the quality attributes and volatile components of samples with different drying methods could be well distinguished. Overall, HADTHPC + (RF-HA) is a promising drying approach achieving coupling of efficiency, energy consumption, and quality, providing a new strategy for accelerating the industrial processing of herbal medicines rich in volatile oils.