Changes in milk fat globule physical properties and milk fatty acid composition throughout the lactation cycle of Laoshan goat

The objective of this study was to investigate the effect of lactation stage on the physical properties of milk fat globules (MFGs) and profiles of milk fatty acids (FAs) of milk from Laoshan dairy goats. Goat colostrum and milk samples were collected on days 1, 3, 30, 90, 150, and 240 postpartum. Milk fat content was found to significantly decrease during the first 90 d of lactation and increase thereafter as lactation progressed (P < 0.01). Average area particle size and volume particle size of MFGs showed trends similar to those of the milk fat content of goat milk with the extension of lactation, and the lowest levels were 2.39 mu m and 3.97 mu m on day 90, respectively (P < 0.01). The zeta-potential of MFGs significantly increased before 30 d of lactation, with a slight drop on day 90. It increased thereafter and tended to stabilize in the late stages of lactation (P < 0.01). The content of several saturated fatty acids (SFAs) (C10:0-C16:0) in early lactation, especially in colostrum, was lower than that in other stages of lactation (P < 0.05). Branched-chain fatty acids of the iso and anteiso series, including C13:0 anteiso, C15:0 anteiso, and C15:0 iso, were observed at high contents in mid-lactation goat milk (P < 0.01). As lactation progressed (3 d to 240 d), the content of SFAs significantly increased (P < 0.01), while the content of monounsaturated fatty acids and polyunsaturated fatty acids significantly decreased (P < 0.05). Furthermore, the content of C18:0 significantly increased during the first 30 d of lactation and decreased thereafter as lactation progressed (P < 0.01). Our results indicated that lactation stage has a pronounced effect on the physical properties of MFGs and the FA profiles of goat milk. Lay Summary The composition and physical properties of milk fat affect the quality and sensory properties of dairy products. However, changes in the physical properties of milk fat globules (MFGs) and profiles of milk fatty acids (FAs) of goat milk throughout the lactation period remain unclear. In the current study, we found that milk fat content and particle size of goat MFG decreased significantly during 90 d of lactation and increased thereafter as lactation progressed. The content of saturated FAs significantly increased, while that of monounsaturated FAs and polyunsaturated FAs significantly decreased from colostrum to late-lactation milk. The characteristics of goat MFGs and milk FAs were affected by the lactation period, which may provide valuable insights into the utilization of MFGs from dairy goats.