Hydroxyl Radical-Stressed Whey Protein Isolate: Chemical and Structural Properties

Many whey protein-containing foods are prepared in the presence of reactive oxygen species generated during processing. The objective of the present study was to determine chemical and structural changes, including carbonyls, sulfhydryls (SHs), dityrosine, surface hydrophobicity, turbidity, and cross-linking, in whey protein isolate (WPI) exposed to FeCl3/H2O2 hydroxyl radical-generating systems (HRGS) at room temperature (20 °C). Protein carbonyl content in WPI increased (P < 0.05) with increasing concentrations of H2O2 when incubated for up to 10 h; total SH groups decreased (P < 0.05) in a similar fashion. The HRGS-oxidized WPI also showed a higher dityrosine content, surface hydrophobicity, and turbidity (P < 0.05) than nonoxidized WPI. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed substantial losses of β-lactoglobulin, α-lactalbumin, and bovine serum albumin, and a concomitant formation of protein polymers in oxidized WPI. The protein-oxidation indexes were all significantly correlated (P < 0.01). These oxidation-induced changes demonstrate high susceptibility of WPI to oxidative stress at room temperature and may explain variations in functionality of whey proteins often observed in formulated foods.