Oxidation promotes cross-linking but impairs film-forming properties of whey proteins Original Research Article

The objective of the study was to investigate the impact of oxidation on the film-forming properties of whey protein isolate (WPI). Sequential heating (70–90 °C) then oxidation (0.1 mM FeCl3/1 mM ascorbate/0–20 mM H2O2) (H → O) or vice versa (O → H) were conducted to oxidize/unfold WPI at pH 6.8 and 8.0 before casting. The resulting films were characterized through mechanical, microstructural, and protein electrophoretic analyses. Oxidation promoted protein cross-linking mainly through disulfide bonds. Tensile strength (TS) and elongation at break (EAB) of films decreased for WPI oxidized by higher concentrations of H2O2. Film solubility (protein leachability) at pH 3–7, ranging from 20 to 40%, was unaffected by H2O2 up to 5 mM but reached almost 100% at above 5 mM H2O2 except at pH 4–5. β-Lactoglobulin dimers and its complex with α-lactalbumin were abundant in O → H WPI films and polymers of WPI dominated in H → O films. Microstructural images confirmed that oxidation promoted crumbly structures thereby explaining the reduced film-forming capability.