Effects of non-covalent interactions between the milk proteins on the rheological properties of acid gels

When skim milk (SM) and whey-protein-enriched skim milk (WPE-SM) were heated (80 °C, 30 min) with N-ethylmaleimide (NEM, 0–0.8 mm), the levels of residual native whey proteins increased to ∼70%, whereas the levels of disulphide-bonded whey proteins decreased to <10%. Acid gels prepared from heated SM with added NEM had slightly lower firmness than those made from control heated SM because the former gels lacked intermolecular disulphide bonds. In contrast, acid gels made from heated WPE-SM with added NEM had higher firmness than those prepared from control heated WPE-SM. This implies that, even without intermolecular disulphide bonds, non-covalent interactions can be sufficient to produce acid gels with firmness higher than gels with disulphide bonds. Nevertheless, disulphide interactions can be more important than non-covalent interactions in influencing the yield properties of the gels because acid gels without disulphide bonds can be fractured more easily than those with disulphide bonds.