The effect of thiol reagents on the denaturation of the whey protein in milk and whey protein concentrate solutions

Adding cysteine (CYS-HCl) to skim milk, whey protein isolate (WPI), or milk/WPI solutions promoted the irreversible denaturation of α-lactalbumin at temperatures between 61 and 70 °C, whereas little denaturation of β-lactoglobulin occurred at these temperatures. Above 70 °C, CYS-HCl promoted the irreversible denaturation of both α-lactalbumin and β-lactoglobulin. A greater effect was observed at higher CYS-HCl concentrations at any given pH, or at higher pH at any given CYS-HCl concentration. Similar effects were observed with glutathione and 2-mercaptoethanol. It is proposed that at temperatures between ∼61 and 70 °C, CYS-HCl initiates thiol-disulphide exchange reactions with the disulphide bonds on unfolded α-lactalbumin, but not with those on the native β-lactoglobulin, thus α-lactalbumin irreversibly aggregates. At higher temperatures, CYS-HCl and the free thiol on unfolded β-lactoglobulin initiates thiol-disulphide exchange reactions with the disulphide bonds on both α-lactalbumin and β-lactoglobulin, rapidly aggregating both proteins via disulphide bonds and non-covalent interactions.