Lipoprotein lipase

Milk contains quite high levels of an indigenous lipase, lipoprotein lipase (LPL). Like many enzymes in milk, LPL enters milk from the cow's blood. The physiological role of LPL is in the metabolism of plasma triglycerides. Milk contains sufficient LPL activity to cause perceptible rancidity very quickly under optimal conditions. The reason milk does not become rancid is ude to compartmentalisation of enzyme and substrate. Most (>80%) LPL is associated with the casein micelles while its substrate, triacylglycerols, is protected by the milk fat globule membrane (MFGM). If this compartmentalisation is compromised (as, for example, would occur if the MFGM of raw milk is damaged by excessive pumping or homogenisation), then enzyme and substrate come together and high levels of free fatty acids are liberated.

LPL is relatively non-specific for fatty acid type but specific for fatty acids esterified at the sn-1 and sn-3 positions of triacylglycerols. Since, in bovine milkfat, short chain fatty acids are preferentially esterified at the sn-3 position, LPL appears to release them preferentially.

Much LPL activity is lost on pasteurization of milk but 78C x 15 s is required for complete inactivation. However, residual LPL activity in pasteurized milk is usually not of significance. LPL is of most importance during the ripening of cheeses made from raw milk. Levels of lipolysis in cheeses made from raw milk are usually higher than cheeses of the same variety made from pasteurized milk.

Further reading:

Olivecrona, T., Vilaro, S. and Olivecrona, G. (2003). Lipases in milk. In Advanced Dairy Chemistry-1 Proteins, Part A, 3rd edition, P.F. Fox and P.L.H. McSweeney (eds), Kluwer Academic Publ., New York, pp. 473-494.