The impact of an in vitro procedure that mimics the physiochemical changes occurring in gastric and small intestinal digestion on the antioxidant capacity and bioaccessibility of phenolic compounds from 16 types of buckwheat-enhanced wheat breads was assessed. The methodology was based on the Global Antioxidant Response (GAR) which combined bioaccessible antioxidant capacity of the soluble fraction from digestible portion measured by the standard Trolox Equivalent Antioxidant Capacity (TEAC) assay and antioxidant capacity of the insoluble fraction from the undigested portion by the QUENCHER method. The bioaccessibility of the phenolics was measured in the soluble fraction with Folin-Cicalteu reagent and in the insoluble fraction by modified QUENCHER method. The studies showed almost 20-fold higher GAR values as compared to the antioxidant capacity of the respective undigested reference breads. The bioaccessible antioxidant capacity of soluble fraction from digestible portion increased significantly whereas the undigested residue displayed antioxidant capacity that accounted for up to 15 % of the GAR. The bioaccessible phenolics accounted for up to 90 % of the total phenolics after digestion and were highly correlated with GAR results of buckwheat-enriched wheat breads. Our results indicate that in vitro digestion is the crucial step that releases of high amount of phenolic antioxidants. The combination of QUENCHER assay with Total Antioxidant Capacity (TAC) and Total Phenolic Content (TPC) assay estimated on Folin-Ciocalteu reagent has been useful for the determination of the bioaccessible antioxidant activity and phenolics of the soluble and insoluble fraction of buckwheat-enhanced wheat breads.
Keywords: Bioaccessibility model; Bioaccessible phenolic compounds; Buckwheat-enhanced wheat bread; Gastrointestinal digestion; Global Antioxidant Response (GAR).