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 alpha-lactalbumin at temperatures between 61 and 70 degrees C, whereas little denaturation of beta-lactoglobulin occurred at these temperatures. Above 70 degrees C, CYS-HCl promoted the irreversible denaturation of both alpha-lactalbumin and beta-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 similar to 61 and 70 degrees C, CYS-HCl initiates thiol-disulphide exchange reactions with the disulphide bonds on unfolded alpha-lactalbumin, but not with those on the native beta-lactoglobulin, thus alpha-lactalbumin irreversibly aggregates. At higher temperatures, CYS-HCl and the free thiol on unfolded beta-lactoglobulin initiates thiol-disulphide exchange reactions with the disulphide bonds on both a-lactalbumin and beta-lactoglobulin, rapidly aggregating both proteins via disulphide bonds and non-covalent interactions. (C) 2018 Elsevier Ltd. All rights reserved.