Effective Antioxidant Activities of Anthocyanins as Affected by pH of Antioxidant Assays

Abstract

This study analyzed the effect of pH on antioxidant activities of anthocyanins. Five black glutinous rice bran crude extracts (BBCs) with different anthocyanin content were compared. They were referred to as Moo Ser (MS), Saked (SK), Phayao (PY), Phichit (PC) and Kao Kor (KK). The antioxidant activities were evaluated using 2 in vitro antioxidant assays conducted at different pH, DPPH (pH 5.5) and FRAP (pH 3.6). Total anthocyanin content (TAC) of KK (23 g/kg) was the highest, followed by PC (13 g/kg), PY and SK (both 11 g/kg), and MS (10 g/kg). The pH of the assays affected the activity of the anthocyanins as the KK showed DPPH scavenging activity comparable to other BBCs but exhibited significantly greater FRAP antioxidant activity. To eliminate possible interference by other active components in the BBC, anthocyanin-rich isolate (ARI) was then isolated from KK. The ARI contained 90 % cyanidin-3-glucoside as analyzed by HPLC. The ARI was dissolved in 5 different buffer solutions (pH 1, 3, 5, 7, and 9) and incubated for 1 h, 3 days, and 1 week before the evaluation of antioxidant activities. All ARI-containing buffer solutions exhibited comparable DPPH scavenging activity. However, the ones at pH 5 and 7 solutions were less reactive than others when tested with the FRAP assay. These results indicated that anthocyanin is a good antioxidant not only in an acidic environment but also in higher pH ranges. This suggests that the utilization of anthocyanins as natural antioxidants in products such as food and cosmetics is not limited by the pH of the products.