Oxidation of Whey Proteins during Thermal Treatment Characterized by a Site-Specific LC-MS/MS-Based Proteomic Approach

Thermal treatment is often employed in food processing to tailor product properties by manipulating the ingredientfunctionality, but these elevated temperatures may accelerate oxidation and nutrient loss. Here, oxidation of different whey proteinsystems [alpha-lactalbumin (alpha-LA),beta-lactoglobulin (beta-LG), a mix of alpha-LA and beta-LG (whey model), and a commercial whey proteinisolate (WPI)] was investigated during heat treatment at 60-90 degrees C and a UHT-like treatment by LC-MS-based proteomic analysis.The relative modification levels of each oxidation site were calculated and compared among different heat treatments and samplesystems. Oxidation increased significantly in protein systems after heating at >= 90 degrees C but decreased in systems with highercomplexity [pure protein (alpha-LA >beta-LG) > whey model > WPI]. In alpha-LA, Cys, Met, and Trp residues were found to be most proneto oxidation. In beta-LG-containing protein systems, Cys residues were suggested to scavenge most of the reactive oxidants andundergo an oxidation-mediated disulfide rearrangement. The rearranged disulfide bonds contributed to protein aggregation, whichwas suggested to provide physical protection against oxidation. Overall, limited loss of amino acid residues was detected after acidichydrolysis followed by UHPLC analysis, which showed only a minor effect of heat treatment on protein oxidation in these proteinsystems