How steaming and drying processes affect the active compounds and antioxidant types of Gastrodia elata Bl. f. glauca S. chow

As both a traditional medicine and food material, fresh Gastrodia elata requires a curing process for quality improvement. The effects of steaming and various drying methods (sun-, hot-air-, microwave-vacuum-, freeze- and vacuum-drying) on the total phenolic, total flavonoid, ascorbic acid, adenosine, and phenolic compound contents, antioxidant activities (scavenging DPPH•, ABTS+•, OH• and reducing power) and microstructures were investigated in this study. The contents of adenosine and individual phenolic compounds were analyzed using high-performance liquid chromatography. The results showed that steaming had adverse effects on the total phenolic, total flavonoid, adenosine, parishin C, vanillyl alcohol, quercetin and cinnamic acid contents, while subsequent hot-air- and freeze-drying showed compensatory effects. Steaming significantly increased the levels of gastrodin, p-hydroxybenzylalcohol, p-hydroxybenzaldehyde, parishins (A, B and E) and catechin (by 3.4-, 1.1-, 1.1-, 3.8-, 6-, 1.4- and 1.5-fold, respectively, p < 0.05) compared to the fresh samples, which were further increased by hot-air- and freeze-drying. Hot-air- and freeze-drying significantly increased the levels of adenosine, gastrodin, p-hydroxybenzylalcohol, p-hydroxybenzaldehyde, parishins (A, B and C), vanillyl alcohol, catechin, caffeic acid, quercetin and cinnamic acid by 1.1-11.6-fold (p < 0.05) compared to steaming treatment. Steaming reduced all the antioxidant activities, which were restored partially by hot-air- and freeze-drying. Principal component and clustering analyses revealed the relationship among the samples, phenolics, and antioxidant activities, which suggested a steaming-then-drying action mechanism in which steaming changes enzymes and starch hydrolysis and drying promote condensation reactions. Collectively, steaming-then-hot-air- or freeze-drying is a promising method for enhancing the quality of Gastrodia elata for food applications.