Improvement of the Structure and Antioxidant Activity of Protein–Polyphenol Complexes in Barley Malts Using Roasting Methods

Proteins and polyphenols are important components in barley malt. During the roasting process of barley malt, proteins and polyphenols interact and influence each other, ultimately altering the nutritional profile and functional properties of barley malt. In this research, polyphenol-free proteins and protein-polyphenol complexes were extracted from barley malt subjected to varying degrees of roasting. The antioxidant activity of protein-polyphenol complexes was assessed by ABTS, FRAP, and ORAC assays. The structural characteristics of the proteins were examined through UV, FL, CD, FTIR, and SEM. We found that roasting enhances the solubility of globulin, prolamin, and glutenin and facilitates the binding of these proteins with polyphenols. Conversely, the impact of roasting on albumin exhibits a trend opposite to that observed in the other three proteins. The antioxidant activity of protein-polyphenol complexes was significantly higher than that of polyphenol-free proteins. Additionally, the microenvironment of the amino acid residues of the four proteins exhibited increased polarity following the roasting process, and the structural conformation of albumin, globulin, and glutelin transitioned from an ordered to a disordered state. Our results indicate that roasting enhances the antioxidant activity of protein-polyphenol complexes by altering the secondary and tertiary structures of these proteins, thereby exposing more hydrophobic side-chain groups inside the proteins and offering more binding sites for polyphenols.