Evolution of milk composition, milk fat globule size, and free fatty acids during milking of dairy cows

The objective of this study was to measure milk composition (fat, protein, and calcium contents; fatty acid profile), milk fat globule size, and free fatty acid content throughout milking. Composition was measured from milk samples collected every 1 min during morning milking in 2 previously published experiments. Experiments 1 and 2 used 9 and 6 dairy cows, respectively. From the beginning to end of milking in experiments 1 and 2, lactose content decreased (-0.45 percentage units), as did protein content (-0.28 and -0.17 percentage units, respectively). In contrast, fat content increased (+5.66 and +5.57 percentage units, respectively). Milk fat globule size increased (+1.51 and +0.43 µm, respectively), whereas free fatty acid content (measured after 24 h of storage at 4°C) decreased quickly during the first minutes (-0.45 mEq/100 g of fat from time point 1 to time point 4 in experiment 1, and -0.85 mEq/100 g of fat from time point 1 to time point 5 in experiment 2), and then largely stabilized, with a slight tendency to increase toward the end of milking period in experiment 2 (+0.32 mEq/100 g of fat). The evolution of milk fatty acid composition depended on the experiment. From the beginning to the end of milking, the concentration of C16:0 consistently increased (+3.4 wt/wt % in experiment 1 and +3.3 wt/wt % from time point 2 to time point 7 in experiment 2), whereas the C18:1/C16:0 ratio increased during the first minutes of milking and then slightly decreased (-0.050 in experiment 1 and -0.031 from time point 2 to time point 7 in experiment 2). Calcium content decreased in experiment 2 (-58 mg/kg). In conclusion, milk composition changed greatly during milking, suggesting that different mechanisms are involved in synthesis and excretion, depending on the type of milk component.