Development of protein–polyphenol conjugates via free radical grafting method: evaluation of the structural and functional properties

Abstract: Lipid oxidation is a common problem in emulsions. As beneficial emulsifiers but with low antioxidant ability, proteins are often used in combination with an antioxidant such as polyphenols. The formation of a protein-polyphenol conjugate makes this combination more effective. In this study, faba bean protein concentrate (FPC) was conjugated with the polyphenol tannic acid (TA) via the free-radical grafting method using a mixture of ascorbic acid and hydrogen peroxide. The structural and functional characteristics of the conjugates were determined in comparison with the protein alone, the protein mixed with only the radical initiator system, or with only TA without the radical initiator system. Surface plasmon resonance measurements showed a strong binding between the protein and TA, where the equilibrium binding constant was at least an order of magnitude better than the previous report of interactions between lentil proteins and polyphenols. Structural analysis of the conjugates was performed using intrinsic fluorescence, surface hydrophobicity, and free thiol group content. The fluorometry data demonstrated that TA interacted with both aromatic amino acids and the hydrophobic regions on the protein. The reduction of the free thiol group specified the interaction between TA and the sulfur-containing amino acids. The conjugate demonstrated a strong antioxidant capacity, which was measured by DPPH (72.4%), ABTS (89.2%), hydroxyl radical scavenging activity (81.6%), and Ferric reducing antioxidant power (4.18 µmol Trolox equivalent). Based on the presence of moles of TA, the antioxidant activity of the conjugate was found to be higher compared to pure TA. Such synergistic effect on antioxidant activity was also confirmed via electron spin resonance, which showed 98% hydroxyl radical scavenging activity for the conjugate (containing 0.14 mmol TA/g of protein) compared to 99% activity for pure TA. Therefore, the conjugate can be a potent antioxidant to delay lipid oxidation in protein-stabilized emulsions.