Evaluation of antioxidant capacity, structure, and surface morphology of ginger (Zingiber officinale) using different extraction methods

The antioxidant capacity of ginger depends on the type of variety, growing conditions, postharvest, drying method, extraction, and measurement, among others. The objective of the research was to compare the efficiency of ultrasound (US), magnetic agitation (AM), maceration (M), and reflux (R) extraction methods. In the GFD (fresh air-dried ginger) extract, the contents of total phenolic content (TFC), 6-gingerol (6-G), and 6-shogaol (6-S) were evaluated; as well as the antioxidant capacity by FRAP (ferric reducing antioxidant power) and IC(50) in DPPH (2,2-diphenyl-1-picrylhydrazyl radical). In addition, structural and morphological changes were evaluated with FTIR and SEM, respectively. The results for all extraction methods for TFC, 6-G, and 6-S were between 9.422 and 10.037 mg EAG/g dry matter (dm), 4.072–4.838, and 0.194–0.263 mg/g dm respectively, with the highest values for TFC and 6-G for M and 6-S for R. FRAP and IC(50) in DPPH values were between 0.172 and 0.192 mmol Fe(2+)/g dm and 0.531–0.722 mg dm/mL respectively, presenting higher antioxidant capacity in M and R1, R2, and R3 (1, 2, and 5 h, respectively). Extracts from M and R1 methods presented lower FTIR transmittance values and greater changes in their surface morphology, with folds and breaks in the starch granules by SEM images. It is concluded that extracts with higher antioxidant capacity are obtained using medium polarity solvents such as methanol and with M and R1 methods. Because of the longer extraction time and moderate thermal stress, the structure and surface morphology of the extracted GFD sample showed greater changes on the surface of the starch granules and, consequently, greater extraction of bioactive compounds.