Effect of adding low levels of β-mercaptoethanol on the disulphide bonds of κ-casein and β-lactoglobulin solutions

Low levels of β-mercaptoethanol (up to 0.5%, v/v) were added to κ-casein and β-lactoglobulin solutions (5 mg mL−1), mixtures thereof, and milk. The reduction of disulphide bonds was monitored over time for up to 24 h, using micro-fluidic electrophoresis, at temperatures ranging from 20 to 50 °C. Lower concentrations of β-mercaptoethanol and a shorter reaction time were required to reduce κ-casein to the same extent as β-lactoglobulin. This suggests that the intermolecular disulphide bonds of κ-casein were more accessible than the intra-molecular disulphide bonds of β-lactoglobulin. The reduction of β-lactoglobulin at any β-mercaptoethanol concentration and reaction time was enhanced as the temperature increased up to 50 °C. The level of reduced κ-casein increased with increasing temperature during the first 2 h; however, as the reaction time was prolonged, the level of monomeric κ-casein decreased. This suggests that monomeric κ-casein can re-form disulphide bonds via either re-oxidation or thiol-disulphide exchange reactions.