The self-association and thermal denaturation of caprine and bovine β-lactoglobulin

Milk components, such as proteins and lipids, have different physicochemical properties depending upon the mammalian species from which they come. Understanding the different responses of these milks to digestion, processing, and differences in their immunogenicity requires detailed knowledge of these physicochemical properties. Here we report on the oligomeric state of β-lactoglobulin from caprine milk, the most abundant protein present in the whey fraction. At pH 2.5 caprine β-lactoglobulin is predominantly monomeric, whereas bovine β-lactoglobulin exists in a monomer-dimer equilibrium at the same protein concentrations. This behaviour was also observed in molecular dynamics simulations and can be rationalised in terms of the amino acid substitutions present between caprine and bovine β-lactoglobulin that result in a greater positive charge on each subunit of caprine β-lactoglobulin at low pH. The denaturation of β-lactoglobulin when milk is heat-treated contributes to the fouling of heat-exchange surfaces, reducing yields and increasing cleaning costs. The bovine and caprine orthologues of β-lactoglobulin display different responses to thermal treatment, with caprine β-lactoglobulin precipitating at higher pH values than bovine β-lactoglobulin (pH 7.1 compared to pH 5.6) that are closer to the natural pH of these milks (pH 6.7). This property of caprine β-lactoglobulin likely contributes to the reduced heat stability of caprine milk compared to bovine milk at its natural pH.