In vitro potential of phenolic phytochemicals from black rice on starch digestibility and rheological behaviors

Various black rice materials (black rice flour, BF; dietary fiber-enriched extract, DE; phenolic-enriched extract, PE) were prepared from black rice, and their effects on in vitro starch digestibility were investigated in a wheat flour gel model. Specifically, the in vitro digestive behavior of the gel samples was continuously monitored from a rheological point of view. BF and DE did not inhibit digestive enzymes; however, PE exhibited IC₅₀ values of 24.12 mg/mL and 0.03 mg/mL against α-amylase and α-glucosidase, respectively. In addition, the predicted glycemic index (pGI) values of the gels with BF, DE, and PE at 20% replacement for wheat flour were as follows: control (wheat flour) ≈ BF > DE > PE. Moreover, a significant decrease in the in vitro viscosities of gels during intestinal digestion was observed in the order of BF, DE, and PE. As a result, PE showed the highest suppression effect on starch hydrolysis by inhibiting digestive enzyme. These results support phenolic compounds as more critical factors compared to dietary fiber for retarding in vitro starch digestibility of starch-based foods prepared with black rice.