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

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 degrees 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. B-Lactoglobulin dimers and its complex with alpha-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. (C) 2012 Elsevier Ltd. All rights reserved.