Effective hydroxylation of tangeretin from Citrus Peel (Chenpi) by edible acids and its improvement in antioxidant and anti-lipase activities

Tangeretin (TAN) is a biologically functional polymethoxylated flavonoid, extracted from the Citrus Peel (Chenpi). Although 5-demethyltangeretin (5-DT) generated through hydrolysis using a TAN precursor has superior biological function to TAN, its traditional preparation methods are difficult due to environmental hazard and low efficiency processes. In this study, the phosphoric acid (a kind of edible acids) was screened as catalysts of TAN hydrolysis conversion to 5-DT. The main conversion-influencing factors of reaction (material/liquid ratio, temperature and time) were investigated and the antioxidant activity of the modified products was determined by measuring DPPH (Diphenylpicrylbenzoquinone) free radical scavenging capacity and total reducing power. The phosphoric acid/ethanol (v/v) ratio of 7:3, material/liquid (g/L) ratio of 2:1, conversion temperature of 175 °C and conversion time of 2 h were attained as optimal conditions with a 93.71% conversion rate. Further, in vitro antioxidant and anti-lipase experiments confirmed that 5-DT was superior to TAN in total reducing power, scavenging capacity of DPPH free radicals (p < 0.01) and inhibition of lipase activities. Therefore, phosphoric acid is proposed as an alternative catalyst for the modification of TAN. Further, it provided technical support for the industrial production of 5DT and its application in functional foods.