This might also explain the significantly higher extent of κ-CN dissociation in UHT milk ( Figure 1). The UHT treatment seemed to trigger significant dissociation of G-κ-CN, which could not be attained under heating at 90☌ for 6 min. The mean serum:milk κ-CN GD ratio of UHT milk was >1.0, indicating that the dissociation of G-κ-CN was favored, in contrast to that of raw milk and heated milk (90☌ for 6 min). However, in UHT milk, the extent of dissociation of G-κ-CN increased greatly. With heating at 90☌ for 6 min, dissociation of NG-κ-CN was still favored, as in raw milk, but proportionally more G-κ-CN was present in the serum. The mean serum:milk GD ratios were 0.83 ± 0.02 in heated milk (90☌ for 6 min) and 1.06 ± 0.01 in UHT milk. Heat treatments, particularly UHT treatment, resulted in a greater proportion of G-κ-CN in the serum phase. Seasonality had significant effects on the partitioning of G-κ-CN between the micelle and the serum phase, the extent of whey protein–casein micelle association under both heating conditions, and the casein micelle size of the UHT milk. Indeed, UHT treatment might have triggered significant dissociation of G-κ-CN, resulting in aggregation among the casein micelles and increased apparent mean casein micelle diameter. Compared with heating at 90☌ for 6 min, UHT treatment (140☌ for 5 s) induced greater dissociation of κ-casein, a similar extent of whey protein denaturation, a lower extent of whey protein–casein micelle association, and a larger increase in casein micelle size. We observed significant between-year effects on protein, total calcium, ionic calcium, pH, and casein:total protein ratio, which might have resulted from different somatic cell counts in the 2 years. ![]() ![]() The ratio of casein to total protein did not vary significantly over the season, but late-season milk had the highest content of glycosylated κ-casein (G-κ-CN) and the lowest content of α-lactalbumin in both years. Seasonality also had significant effects on milk calcium, ionic calcium, fat globule size, buffering capacity, and ethanol stability, but not on casein micelle size. ![]() Fat, protein, and lactose contents varied consistently during the year in patterns similar to those of the lactation cycle. We determined seasonal variations in the composition and characteristics of bovine milk, as well as heat-induced changes in the physicochemical properties of the milk, in a typical seasonal-calving New Zealand herd over 2 full milking seasons.
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