Structural changes and cholesterol-lowering in denatured whey protein isolate: Malic acid combined enzymolysis

Binding of bile salts (BS) and protein has been linked to cholesterol reduction. However, the interaction between the protein structural and binding of BS ability was unclear. The goals of this study were to investigate the relationship among the structure, morphology, and functional properties of hydrolytic peptides of whey protein isolate (WPI) for four groups: control (no treatment), malic acid acidolysis [WPI + MA(DL)], enzymolysis (WPI-E), and enzymolysis combined with MA acidolysis [WPI-E+MA(DL)]. We found that the combination of enzymolysis and acidolysis could significantly improve the bile salt-binding ability of WPI (p < 0.05) by changing the structure and morphology of the protein compared with that in the other groups. An increase in the degree of hydrolysis, reduction in sulfhydryl group content, and exposure of aromatic amino acids indicated that WPI bound to the bile salts through hydrogen bond interactions. Additionally, the increase in surface hydrophobicity and β-sheet, β-turn, and random structure contents according to Fourier transform infrared spectroscopy indicated an exposure of hydrophobic groups. This further confirmed that WPI exhibited higher bile salt-binding ability through hydrophobic interactions. Sodium dodecyl sulphate-polyacrylamide gel electrophoresi (SDS-PAGE), Tricine-SDS-PAGE, scanning electron microscopy, and particle size analysis revealed that the hydrophobic groups exposed to the protein surface were aggregated and formed fragmented polymers with a molecular weight of approximately 3.3 kDa. Moreover, differential scanning calorimetry and rheology revealed that these polymers exhibited higher thermal stability, apparent viscosity, and viscoelasticity, and the viscosity was positively correlated with binding ability. Thus, the relationship between the structure, morphology and functional characteristics of hydrolytic peptides in the study can provide good data for designing innovative new peptides that ameliorate cholesterol and lipid metabolism. And it has good application value for the development of healthy and functional food.