Functional Breads Enriched With Amaranth Flour and Grape Pomace Peels: Protein Fraction Bioaccessibility and Antioxidant Properties

The overall objective of this work was to evaluate the effect of the addition of amaranth flour (AF) and/or red grape pomace peels (GP) on the protein fraction bioaccessibility and antioxidant activity of phosphate buffer soluble fractions. Breads containing 20% w/w AF, 5% w/w GP, or a mixture of both, which had shown acceptable sensory characteristics in previous studies, were subjected to simulated gastrointestinal digestion (SGID) using the INFOGEST consensus protocol. The original and digested products were characterized, focusing on the peptide/polypeptide fractions and their in vitro antioxidant properties. AF-containing bread showed the presence of high molecular weight compounds in the SDS-PAGE profiles, probably due to albumin/gluten protein interaction, and small increases in phosphate buffer (35 mmol. L−1, pH 7.8) solubility with a decrease in molecules > 10 kDa. While the presence of GP also modified the electrophoretic profile, appearing some new bands (< 45 and < 20 kDa) and also caused a diminution of < 10 kDa PBS-soluble molecules. After SGID, the proteolysis degree (TNBS method) was lower for GP-containing breads (17%–18% vs. 21%–25%). Digests from breads added with AF and/or GP also presented high molecular compounds. The presence of AF and/or GP improved the antioxidant potency (IC50 values for ORAC and ABTS scavenging decreased by half or even more) for breads before SGID. After SGID, the antioxidant potencies of the buffer-soluble fraction increased much more without effect due to the addition of AF or GP. Amaranth protein appears to play a protective role over polyphenols during gastrointestinal digestion.