Seasonal variations in composition, properties, and heat-induced changes in bovine milk in a seasonal calving system

摘要

We determined seasonal variations in the compositionand characteristics of bovine milk, as well as heat-inducedchanges in the physicochemical properties of themilk, in a typical seasonal-calving New Zealand herdover 2 full milking seasons. Fat, protein, and lactosecontents varied consistently during the year in patternssimilar to those of the lactation cycle. Seasonality alsohad significant effects on milk calcium, ionic calcium,fat globule size, buffering capacity, and ethanol stability,but not on casein micelle size. The ratio of casein tototal protein did not vary significantly over the season,but late-season milk had the highest content of glycosylatedκ-casein (G-κ-CN) and the lowest content ofα-lactalbumin in both years. We observed significantbetween-year effects on protein, total calcium, ionic calcium,pH, and casein: total protein ratio, which mighthave resulted from different somatic cell counts in the 2years. Compared with heating at 90°C for 6 min, UHTtreatment (140°C for 5 s) induced greater dissociationof κ-casein, a similar extent of whey protein denaturation,a lower extent of whey protein–casein micelleassociation, and a larger increase in casein micelle size.Indeed, UHT treatment might have triggered significantdissociation of G-κ-CN, resulting in aggregationamong the casein micelles and increased apparent meancasein micelle diameter. Seasonality had significant effectson the partitioning of G-κ-CN between the micelleand the serum phase, the extent of whey protein–caseinmicelle association under both heating conditions, andthe casein micelle size of the UHT milk.