Cooling causes changes in the distribution of lipoprotein lipase and milk fat globule membrane proteins between the skim milk and cream phase

Lipoprotein lipase (LPL) activity and free fatty acid levels were studied in freshly milked, uncooled milk from individual Danish Holstein or Jersey cows, or after storage for up to 24 h at either a cooling temperature (4°C) or at the milking temperature (31°C). Upon cooling for up to 24 h, LPL activity increased in the cream phase, whereas the activity in the skim milk was steady, as observed for Jersey cows, or increased, as seen for the Holsteins. Storage at 31°C decreased the LPL activity in both the cream phase and the skim milk phase. The increase in free fatty acid levels was found to depend on LPL 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 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 globules were isolated from all milk fractions and analyzed. Major changes in the distributions of proteins between the skim milk phase and the cream phase were observed after cooling at 4°C for 4 h, where a total of 29 proteins between the 2 breeds was found to change their association with the milk fat globule membrane (MFGM) significantly. Among these, the MFGM proteins adipophilin, fatty acid-binding protein, and lactadherin, as well as the non-MFGM proteins β-casein, lactoferrin, and heat shock protein-71, were identified. Adipophilin, lactadherin, and lactoferrin were quantitatively more associated with the MFGM upon cold storage at 4°C, whereas β-casein, fatty acid-binding protein, and heat shock protein-71 were found to be less associated with the MFGM upon cold storage.