An Investigation of the Mechanism of Malondialdehyde-Induced Oxidative Modification of Yak Milk Proteins

Changes in yak milk protein structure and nutritional functions are investigated to reduce nutritional quality degradation during yak milk processing. Yak milk proteins are oxidized using different concentrations (0, 0.5, 1, 3, 5, 10 mmol L −1 ) of malondialdehyde (MDA). Indexes of oxidation (e.g., carbonyl and sulfhydryl group contents) are reported, and changes in yak milk proteins and their functions caused by oxidation are investigated. With increased MDA concentration, carbonyl contents increase, total sulfhydryl and free sulfhydryl contents tend to first decrease and then increase, and disulfide bond contents decrease; solubility decreases, surface hydrophobicity tends to enhance and then weaken, particle sizes first increase and then decrease, the zeta potential first trends down and then up, and dimerized tyrosine contents trend down. Proteomics identified 1456 proteins; protein variability was greatest at 1 mmol L −1 MDA, with 173 proteins with reduced and 275 proteins with increased function; the 10 mmol L −1 MDA group showed the least protein variability, with 190 proteins with reduced and 156 proteins with increased function; all protein functions were concentrated in biochemical processes, cellular components, and molecular functions. Compared with 0 mmol L −1 , 11, 11, 12, 12, and 7 pathways were up-regulated in the 0.5, 1, 3, 5, and 10 mmol L −1 MDA groups; the number of down-regulated pathways was 9, 5, 7, 17, and 10, respectively.