HPLC-DAD characterisation of phenolic compounds from Andean oca (Oxalis tuberosa Mol.) tubers and their contribution to the antioxidant capacity

A qualitative and quantitative characterisation of the main non-anthocyanin phenolic compounds from two different colored oca (Oxalis tuberosa Mol.) genotypes with potential antioxidant capacity was carried out by high performance liquid chromatography with photodiode array detection (HPLC-DAD). Phenolic compounds were fractionated in two main fractions: an aqueous (Faq) and a ethyl acetate fraction (Fea). In addition, the contribution of these phenolic fractions to the antioxidant capacity was evaluated. The Faq revealed the presence of caffeic, vanillic and cinnamic acid derivatives, flavan-3-ols and flavones derivatives, as the main non-anthocyanin phenolic compounds for both genotypes. Anthocyanins for the purple genotype were significantly present in this fraction. Acid hydrolysis revealed the presence of vanillic, caffeic and cinnamic acids and malvidin in Faq. The Fea was composed mainly of caffeic and cinnamic acid derivatives as well as flavan-3-ols, flavones and flavanone derivatives. Based on their UV-Vis spectral data the flavan-3-ols, flavones and flavanones detected in both fractions seem to correspond to bound forms of catechin, luteolin and apigenin and naringenin, respectively. The Faq fractions were the major contributors to the ABTS antioxidant capacity (77-82%). The results obtained in the present study suggest that oca tubers could potentially be considered beneficial for human health and for potential industrial applications.