Spontaneous milk

The reason that rancidity is not more prevalent in market milk is due to the fortuitous fact that spontaneous rancidity can be prevented or reduced by mixing such milk within 1 hr after milking with four to five times its volume of normal milk (Tarassuk and Henderson 1942). Since usually only about one out of five cows in a herd produces spontaneous milk, this defect is almost automatically eliminated or reduced. It is clear, however, that farmers with only a few cows are likely to encounter spontaneously rancid milk during the lactation period. 

Spontaneous milk can be produced by most, if not all, cows but, because of the individuality of cows and their response to various factors,... 

The sooner spontaneous milk is cooled and the lower the temperature to which it is cooled, the more lipolysis that occurs (Tarassuk and Richardson, 1941 Bachman and Wilcox, 1990a) if cooling is delayed, the extent of lipolysis is reduced (Dunkley, 1946 Kitchen and Cranston, 1969). Once the milk is cooled, spontaneous lipolysis proceeds during cold storage and the rate of lipolysis increases if the temperature is raised (Tarassuk and Richardson, 1941). As with induced lipolysis, the rate of spontaneous lipolysis is high initially but levels olf later. An FFA level of up to 10 meq/1 can be obtained (in extreme cases) after 24 hours storage at 5°C. 

Feed and nutrition. Both the quality and quantity of feed influence the tendency of a cow to produce spontaneous milk (Fredeen et al., 1951 Jellema, 1980). The milk of most cows on a low plane of nutrition has an enhanced susceptibility (Gholson et al., 1966 Astrup et al., 1980). The effect of low energy intake is particularly marked when cows are in late lactation (Stobbs et al., 1973 O Brien et al., 1996) but can also be considerable in early lactation (Dillon et al., 1997). The cow s body condition has not been found to be a reliable indicator of the susceptibility of her milk to spontaneous lipolysis (Ortiz et al., 1970). 

Milk production. In general, low-yielding cows are more likely to produce spontaneous milk than are high-yielding animals (Hunter et al., 1968 Ortiz et al., 1970 Jellema and Schipper, 1975 Chazal and Chilliard, 1986 Saito, 1992). The milk yield, like season, cannot be considered an independent variable as it is dependent on other factors such as stage of lactation and quality and quantity of feed (Ahrne and Bjorck, 1985). 

The breed of the cow, generally, does not appear to affect its propensity to produce spontaneous milk (Chilliard, 1982). For example, Chazal and Chilliard (1987a) found no difference between Friesian and Montbeliarde cows in relation to spontaneous lipolysis. Bachman et al. (1988), however, found that the milk of Jerseys is more susceptible than that of Holsteins. There also appears to be some within-breed heritability of spontaneous milk production (Deeth and Fitz-Gerald, 1976 Jurczak, 1996). 

The presence of an inhibitory factor (or factors) in milk has been suggested to explain the lack of lipolysis in normal milk and the inhibition of lipolysis when normal milk is mixed with spontaneous milk (Dunkley and Smith, 1951). It has been demonstrated that normal skim milk contains a heat-stable, dialysable inhibitor (Deeth and Fitz-Gerald, 1975a), and that proteose-peptone 3 is an effective non-competitive inhibitor (Anderson, 1981 Cartier et al., 1990). The inhibitors prevent lipolysis by blocking the lipase-milk fat globule membrane interaction (Deeth and Fitz-Gerald, 1975a). 

Spontaneous milk is capable of developing oxidized flavor within 48 h of milking without the presence of contaminating iron or copper. Bruhn et al, (1976) reported that 12-20% of raw milk samples are in this category. 

Spontaneous oxidation of milk fat, which has been known for over 60 years (Corbett and Tracy, 1943), is influenced by heredity, stage of lactation and feeding practices (Shipe, 1964). Some cows consistently produce spontaneous milk, others occasionally, and others not at all (Parks et al, 1963). Differences between milk from the different quarters of the same cow may occur. 

Spontaneous milks which are labile to oxidation without added Cu or Fe. 

It has been proposed that spontaneous milks have a high content (10 times normal) of xanthine oxidase (XO). Although addition of exogenous XO to non-susceptible milk induces oxidative rancidity, no correlation has been found between the level of indigenous XO and susceptibility to oxidative rancidity. The Cu-ascorbate system appears to be the principal pro-oxidant in susceptible milk. A balance between the principal antioxidant in milk, a-tocopherol (Chapter 6), and XO may determine the oxidative stability of milk. The level of superoxide dismutase (SOD) in milk might also be a factor but there is no correlation between the level of SOD and the propensity to oxidative rancidity. 

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