Milk Lipids: Lipolysis and Hydrolytic Rancidity

Abstract

Lipolysis, the hydrolysis of lipids, in milk produces free fatty acids (FFAs), which have both detrimental and desirable effects. The detrimental effects are due to the unpleasant flavors of short-chain fatty acids when present at high concentrations. However, under some circumstances and usually at lower concentrations, the short-chain FFAs impart desirable flavors to dairy products and other foods. For example, the characteristic flavor of some cheese varieties is due to their FFA content. Lipolysis is caused by lipases, which may be either the indigenous milk lipoprotein lipase or bacterial lipases, which are produced predominantly by psychrotrophic bacteria such as Pseudomonads growing in milk before heat treatment. Lipolysis by milk lipase can occur spontaneously in some milk when milk is cooled soon after harvest, but is more commonly induced by physical treatments that disrupt the milk fat globule membrane and allow the enzyme in the milk serum to access the milk fat inside the milk fat globule. Milk lipase is destroyed by pasteurization, but the bacterial lipases are heat-stable and hence can remain active in processed milk, even ultrahigh-temperature-treated milk, and dairy products and cause lipolysis during storage. The extent of lipolysis in milk and dairy products is usually measured by their FFA content.