Milk supply hydrolytic rancidity

The principal lipase in bovine milk is a lipoprotein lipase (LPL Chapter 8) which is associated predominantly with the casein micelles and is isolated from its substrate, milk fat, by the MFGM, i.e. the enzyme and its substrate are compartmentalized. However, even slight damage to the membrane permits contact between enzyme and substrate, resulting in hydrolytic rancidity. The enzyme is optimally active at around 37°C and pH 8.5 and is stimulated by divalent cations, e.g. Ca (Ca complex free fatty acids, which are strongly inhibitory). The initial turnover of milk LPL is c. 3000 s i.e. 3000 fatty acid molecules are liberated per second per mole of enzyme (milk usually contains 1-2 mg lipase 1 , i.e. 10-20 nM) which, if fully active, is sufficient to induce rancidity in about 10 s. This never happens in milk due to a variety of factors, e.g. the pH, ionic strength and, usually, the temperature are not optimal the lipase is bound to the casein micelles the substrate is not readily available milk probably contains lipase inhibitors, including caseins. The activity of lipase in milk is not correlated with its concentration due to the various inhibitory and adverse factors. 

The frequency and severity of lipolysis increases in late lactation, possibly owing to a weak MFGM and the low level of milk produced (which may aggravate agitation) this problem is particularly acute when milk production is seasonal, e.g. as in Ireland or New Zealand. 

Some cows produce milk which is susceptible to a defect known as spontaneous rancidity - no activation treatment, other than cooling of the milk, is required the frequency of such milks may be as high as 30% of the 

Mixing of normal milk with susceptible milk in a ratio of 4 1 prevents spontaneous rancidity and therefore the problem is not serious except in small or abnormal herds. The incidence of spontaneous rancidity increases with advancing lactation and with dry feeding. 

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