Polyphenols in amaranth (A. manteggazianus) flour and protein isolate: Interaction with other components and effect of the gastrointestinal digestion

Abstract

In this work, there were analysed the interaction between phenolics present in amaranth flour (F) and amaranth protein isolate (I) with other components, as well as the effect of the gastrointestinal digestion on them (Fd and Id). Extractions were performed under different conditions (temperature, acid, organic solvent, alkali). Methanol/water extracts (25 °C and 80 °C) from F showed the presence of isoquercetin/rutin, quercetin, kaempferol and two unidentified peaks (II and III). In the presence of acid (much more evident at 80 °C), the extraction of some components increased: catechin, 4-hydroxibenzoic acid, isoquercetin/rutin, II, III. When methanol/acetone/water extraction was performed, p-coumaric acid and a new unidentified peak (IV) were observed. About 15% of the total phenol –namely; p-coumaric, rutin/isoquercetin, and kaempferol– were linked to the protein fraction. After the proteins were isolated (I), the amount of some of the compounds which were originally present in a soluble form (e. g. catechin) and in the protein-bound fraction were decreased. Simulated gastrointestinal digestion of flour released some phenolics (catechin, phenolic acids) that were ligated to proteins, and they significantly incremented the ORAC and ABTS activity of most of the extracts. Isoquercetin/rutin, quercetin and kaempferol remained after digestion. Extracts from the digested protein isolate presented differences in the composition and lower ORAC and/or ABTS activities for some of them. The study of the effect of the simulated gastrointestinal digestion process on bioaccessibility and on antioxidant activity (an aspect that, to our knowledge, has not been previously studied on amaranth polyphenols) yielded promising results, which suggest that amaranth flour is a potential antioxidant functional ingredient.