Effect of citrus peel on phenolic compounds, organic acids and antioxidant activity of soy sauce

Soy sauce contains a number of bioactive components, which have been shown to possess strong antioxidative properties. Citrus peel (CP) possesses a complex mixture of flavonoids and has also show good antioxidant activity. In this study, we investigated the effect of CP on the dynamic changes of enzymatic activities in the koji and the changes in the phenolic compounds, organic acids, and antioxidant activity of soy sauce during moromi fermentation. The results show that adding CP in koji-making is helpful to increase the beta-glycosidase activity and not effect on the activities of other enzymes in the koji. Moreover, the total flavonoid content (TFC), total phenolic content (TPC), contents of aglycone-form isoflavones, polymethoxylated flavones and organic acid, and the antioxidant activity of soy sauce brewed with CP were found to be higher than those of soy sauce brewed without CP during moromi fermentation, which showed strong positive correlation with antioxidant activities. This study suggests that adding CP in koji-making is a simple and effective method to improve functional and bioactive properties of soybean products, which can also avoid the waste of resources.