Aqueous Extract of Gembili (Dioscorea esculenta) Tubers Exceeds Its Ethanol Fractions in Phenolic Content and Antioxidant Capacity

Gembili (Dioscorea esculenta) tubers are used in Indonesia mainly as a source of inulin, recovered from hot-water extracts by ethanol precipitation, but how antioxidants partition during this step is unknown. A hot-water extract (AE) was compared with its ethanol-insoluble (EIF) and ethanol-soluble (ESF) fractions for total phenolic (TPC) and flavonoid (TFC) contents and for 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and ferric reducing antioxidant power (FRAP) activities. AE had the highest TPC (214.9 ± 6.1 mg gallic acid equivalents/g) and TFC (159.7 ± 6.5 mg quercetin equivalents/g), the lowest IC50 values (DPPH 30.0 ± 0.8; ABTS 28.5 ± 1.9 µg/mL), and the highest FRAP (269 ± 42 mg ascorbic acid equivalents/g). EIF exceeded ESF in all assays except FRAP (p < 0.001), and antioxidant capacity followed TPC across nine determinations (DPPH IC50, r = −0.94). The inulin-rich precipitate therefore retains part of the antioxidant capacity of the tuber.