EFFECTS OF KAPPA-CASEIN GLYCOSYLATION ON HEAT-STABILITY OF MILK

A study was conducted to determine the relationship between the degree of glycosylation of kappa-casein (CN) and heat stability of milk at 1 40 degrees C between pH 6.2 and 6.9. Morning milk samples from individ ual Holstein cows (genotypes kappa-CN AA, beta-CN A(1)A(2), alpha(s1)- CN BB, beta-lactoglobulin (LG) AB) in mid-lactation were collected and analyzed for protein and somatic cell count. Samples of low somatic c ell count (<150 000 cells/ml) were skimmed and dialyzed overnight at 4 degrees C against bulk milk to equilibrate soluble components. In the first series of experiments, the heat clotting time vs pH (HCT-pH) cu rves were determined for 37 individual milk samples having various deg rees of glycosylation of kappa-CN, estimated through the N-acetylneura minic acid (NANA) content in mu g/mg of kappa-CN (NANA/kappa-CN). The mean NANA/kappa-CN, HCTmax, and HCTmin, were 50.3 +/- 22.3 mu g/mg, 17 .4 +/- 1.2 min and 3.3 +/- 0.7 min, respectively. The results of stati stical analysis showed that the variations in the degree of glycosylat ion of kappa-CN in normal milk did not significantly affect (P > 0.1) the heat stability parameters. The effect of NANA depletion on the HCT -pH profile was tested in the second series of experiments. HCT-pH pro files of 14 individual milk samples, untreated and neuraminidase-treat ed to extensively remove NANA associated with kappa-CN, were compared. The average NANA/kappa-CN before treatment, the HCTmax, and HCTmin, w ere 78.3 +/- 24.9 mu g/mg, 21.4 +/- 1.5 min and 3.6 +/- 0.8 min, respe ctively. As no effect of the desialylation was observed for HCTmin and pH(I) and as the effect on HCTmax, although significant (P less than or equal to 0.05), was very low (0.7 min on average), these results in dicate that the charges and the extent of hydrophilicity of the heat-i nduced kappa-CN/beta-LG complexes are not the crucial factors for the production of kappa-CN depleted micelles upon heating and that the gly cosylation of kappa-CN does not affect its heat-induced interaction wi th beta-LG.