Effects of condensed vs hydrolysable tannins on gluten film strength and stability

Abstract

Depending on their structure, tannins are known to strongly complex proteins and alter their functionality. This study aimed to determine effect of polymeric condensed tannins from sorghum (proanthocyanidins, PA), versus tannic acid (hydrolysable tannins) on rheological properties and stability of wheat gluten films and foams; monomeric catechin was used for comparison. Gluten films were solution-cast and assessed for tensile strength, aqueous solubility, and stability to protease hydrolysis. Wheat flour batter and isolated gliadins, glutenins, and starch viscosity and stability at room temp and under cook-cool cycles were assessed. PA-treated film, but not tannic acid or catechin films, showed increased (p < 0.05) tensile strength (2.2X vs control), and decreased aqueous solubility and protease degradation at pH 2.0, suggesting PA extensively cross-linked gluten. Both PA and tannic acid dramatically improved batter stability vs control and catechin (8% vs 38% separation at 60 min), although PA was more effective over 4 h. Surprisingly, in a cook-cool cycle, PA, but not tannic acid, dramatically increased peak and final paste viscosity (53% and 35%, respectively, vs control). Using isolated flour components, this was found to be largely due to PA uniquely cross-linking denatured gliadin fraction during the heating cycle, likely through hydrophobic interactions with exposed hydrophobic amino acid residues. Our findings indicate that differences in tannin conformation lead to different gluten protein binding mechanisms, which can be uniquely used to expand gluten functionality. The sorghum derived polymeric PA interact more strongly with gluten proteins than tannic acid.