Interactions between different genetic variants of beta-lactoglobulin and kappa-casein A during heating of skim milk

Milk samples (5 mL) from cows, homozygous for kappa-casein A and either homozygous A (n = 14) or B (n = 12) or heterozygous AB (n = 14) for beta-lactoglobulin (beta-lg), were heated at 90 degrees C for 0-10 min, followed by ultracentrifugation at 100000g. The native whey proteins in the supernatant were thereafter analyzed by fast performance liquid chromatography (FPLC), and the decrease of whey proteins was recorded. The reaction orders for the heat-induced loss of beta-lg were found to be 0.53, 0.65, and 0.91 for beta-lg A, beta-lg A+B, and beta-lg B, respectively, in milk containing the A variant of kappa-casein. The rate of heat-induced loss, expressed as the half-life (t(1/2)) of the reaction, was found to be 340 and 270 s for beta-lg A and B, respectively, in milk from homozygous cows. In milli from heterozygous cows (beta-lg A+B), this value was 330 s. After correction for the casein number, the t(1/2) value for beta-lg B was still notably lower than for beta-lg A. The results, therefore, showed that the concentration of beta-lg B decreased more rapidly than beta-lg A in skim milli. containing kappa-casein A, probably due to differences in hydrophobicity between the genetic variants of beta-lg.