Lipid oxidation-mediated non-enzymatic glycation and its impact on the digestibility of milk proteins.

Background: This study explored the influence of milk fat on non-enzymatic glycation of proteins and protein digestibility by analyzing the accumulation of glycation products and changes in protein structure. Results: The contents of POV and MDA increased significantly on day 7 of storage in milk model groups with different milk fat concentrations (0, 2, 4, 6%) and the increase was more obvious in the high-fat (6%) group. In the high-fat group, the contents of glyoxal and methylglyoxal, the intermediate products of non-enzymatic glycation increased at a high level. The peak shift and the appearance of new peaks in the infrared spectrum further indicated enhanced glycation coupled with lipid oxidation. In addition, the decrease in fluorescence intensity, the increase in ultraviolet absorption, the reduction in surface hydrophobicity, and the change of protein secondary structure in the high fat group indicated obvious changes in the microstructure of proteins. The cleavage site of protease was reduced because of the alterations in protein advanced structure and the occupation of amino acids digestion sites, which subsequently reduced the digestibility of proteins. This phenomenon was more obvious in the early stage of gastrointestinal digestion. Conclusion: High levels of milk fat promoted non-enzymatic glycation by accelerating lipid oxidation, which in turn induced glycation, cross-linking, and aggregation of milk proteins. These structural changes further hindered the action of digestive enzymes, ultimately reducing the digestibility of milk proteins.