Effects of κ-casein glycosylation on heat stability of milk

Abstract

A study was conducted to determine the relationship between the degree of glycosylation of κ-casein (CN) and heat stability of milk at 140 °C between pH 6.2 and 6.9. Morning milk samples from individual Holstein cows (genotypes κ-CN AA, β-CN A 1A 2, α s1 - CN BB, β-lactoglobulin (LG) AB) in mid-lactation were collected and analyzed for protein and somatic cell count. Samples of low somatic cell count (< 150 000 cells/ml) were skimmed and dialyzed overnight at 4 °C against bulk milk to equilibrate soluble components. In the first series of experiments, the heat clotting time vs pH (HCT-pH) curves were determined for 37 individual milk samples having various degrees of glycosylation of κ-CN, estimated through the N-acetylneuraminic acid (NANA) content in μg/mg of κ-CN (NANA/κ-CN). The mean NANA/κ-CN, HCT max, and HCT min were 50.3 ± 22.3 μg/mg, 17.4 ± 1.2 min and 3.3 ± 0.7 min, respectively. The results of statistical analysis showed that the variations in the degree of glycosylation of κ-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 profiles of 14 individual milk samples, untreated and neuraminidase-treated to extensively remove NANA associated with κ-CN, were compared. The average NANA/κ-CN before treatment, the HCT max, and HCT min were 78.3 ± 24.9 μg/mg, 21.4 ± 1.5 min and 3.6 ± 0.8 min, respectively. As no effect of the desialylation was observed for HCT min and pH I and as the effect on HCT max, although significant ( P ≤ 0.05), was very low (0.7 min on average), these results indicate that the charges and the extent of hydrophilicity of the heat-induced κ-CN/β-LG complexes are not the crucial factors for the production of κ-CN depleted micelles upon heating and that the glycosylation of κ-CN does not affect its heat-induced interaction with β-LG.