Fermentation enhances the in vitro antioxidative effect of onion (Allium cepa) via an increase in quercetin content

► Antioxidative effects of onion extract were enhanced after fermentation. ► We used the crude enzyme extract from Aspergillus kawachii for fermentation. ► By fermentation, quercetin-3,4′-diglucoside and -4′-glucoside were declined. ► Increased compounds were quercetin-3-glucoside and quercetin, respectively. ► Quercetin showed antioxidative activity and neuronal cell protective effects.