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Size distribution of casein micelles in milk from dairy cows with different crossbreding levels of Holstein-Zebu cattle
Over the years, there has been an increasing interest in milk casein, because of their industrial importance. Casein micelle structure is crucial for gel formation in cheese and yogurt, and it is correlated with milk stability after of heating, freezing or drying. The present study evaluated the casein micelles mean size distribution in raw milk from crossbred Holstein-Zebu dairy cows, as follows: 1/2 Holstein-Zebu (n = 41); 9/16 Holstein-Zebu (n = 21); 5/8 Holstein-Zebu (n = 31); 3/4 Holstein-Zebu (n = 29); 7/8 Holstein-Zebu (n = 51); and 15/16 Holstein-Zebu (n = 27). The milk composition (protein, lactose, fat, nonfat and total solid contents), somatic cell count score, age, days in milk and milk production were also recorded. Size of casein micelles was determined by Photon Correlation Spectroscopy (PCS) after milk fat removal by centrifugation. The mean size of the casein micelles was 170.22 + 21.18 nm (121.8 – 235.6 nm). The logistic regression analysis was adjusted (P < 0.20) for days in milk, crossbreeding levels of Holstein-Zebu cattle, age, milk production, somatic cell count score, urea levels, fat, protein, lactose, and nonfat milk solids. The final model of the logistic regression analysis (P < 0.05) showed that the mean size of the casein micelles was associated with the crossbreeding levels of Holstein-Zebu cattle (odds ratio: 0.27; P = 0.001), somatic cell count score (odds ratio: -1.40; P = 0.001) and nonfat contents (odds ratio: -6.55;P = 0.033). To the best of our knowledge, this is the first study that access the effect of the crossbreeding levels of Holstein-Zebu on the mean diameter of casein micelles. This fact has important implications to production of milk-based products, and thus economic output of the dairy industry.
Keywords: casein, milk quality, raw milk