Cooling Causes Changes in the Distribution of Lipoprotein Lipase and Milk Fat Globule Membrane Proteins between the Skim Milk and Cream Phase

Summary

The development in lipoprotein lipase (LPL) activity and free fatty acids were studied in freshly milked non-cooled milk from individual Danish Holstein or Jersey cows, or after storage for up to 24 h at either cooling temperature at 4ºC or at the milking temperature at 31ºC. LPL activity increased in the cream phase upon cooling, while the activity in the skimmed milk was either steady, as observed for Jersey cows, or increased, as seen for the Holsteins, in the skim milk upon cooling for up to 24 h. Storage at 31ºC decreased the LPL activity in both the cream phase and the skim milk phase. The increase in free fatty acids was found to depend on potential lipoprotein lipase activity, incubation temperature, substrate availability, and incubation time. Furthermore, the migration of milk proteins between the skim milk phase and the cream phase upon cooling of bovine milk from Jersey cows or from Danish Holstein cows was studied using proteomic methods involving 2-dimensional gel electrophoresis and mass spectrometry. Proteins associated with the milk fat globule membranes were isolated from all milk fractions and analyzed. Major changes in the distributions of proteins between skimmed milk and the cream phase were observed after cooling at 4ºC for 4 h, where a total of 29 proteins were found to change their association with the milk fat globule membrane significantly in the two breeds. Among these, the milk fat globule membrane-associated proteins adipophilin, fatty acid binding protein, and lactadherin as well as β-casein, lactoferrin, and HSP-71 were identified. Adipophilin, lactadherin, and lactoferrin were quantitatively more associated with the milk fat globule membrane upon cooling to 4°C, whereas β-casein, fatty acid binding protein, and HSP-71 were found to be less associated with the milk fat globule membrane when cooled.