Spontaneous lipolysis

Lipolysis has been classified as spontaneous or induced. This distinction is made because different measures have to be taken to correct the problem. Induced lipolysis is most frequently defined as lipolysis initiated in raw milk by some form of mechanical agitation. Traditionally, spontaneous lipolysis has been defined as lipolysis caused by the cooling of raw milk. The cooling requirement is no longer strictly adhered to, and lipolysis in raw milk is said to be spontaneous if rancidity develops without apparent mechanical agitation (Downey 1980A,B). The distinction between spontaneous and induced lipolysis is not always clear, and both may occur at the same time. 

Downey (1980) reasoned that although milk lipoprotein lipase is present in sufficient amounts to cause extensive hydrolysis and potential marked flavor impairment, this does not happen in practice for the following reasons (1) the fat globule membrane separates the milk fat from the enzyme, whose activity is further diminished by (2) its occlusion by casein micelles (Downey and Murphy 1975) and by (3) the possible presence in milk of inhibitors of lipolysis (Deeth and Fitz-Gerald 1975). The presence in milk of activators and their relative concentration may also determine whether milk will be spontaneously rancid or not (Jellema 1975 Driessen and Stadhouders 1974A Murphy et al. 1979 Anderson 1979). 

Deeth, H. C. and Fitz-Gerald, C. H. 1975. Factors governing the susceptibility of milk to spontaneous lipolysis. Int. Dairy Fed. Doc. 86, 24-34. 

Milk when freshly secreted from a healthy udder has <0.5 pmol FFA/ml (Connolly et al., 1979 Brathen, 1980). These adds result from incomplete synthesis rather than lipolysis. Under proper handling and storage conditions, only small increases in the FFA level should occur. In some cases, however, substantial increases are observed, which result from either induced or spontaneous lipolysis. Induced lipolysis results when the milk lipase system is activated by physical or chemical means. Spontaneous lipolysis is defined as that which occurs in milk which has had no treatment other than cooling soon after milking (Tarassuk and Frankel, 1957). 

Lipolysis in milk is affected by inhibiting and activating factors. As discussed above, proteose peptone fraction of milk can inhibit milk LPL while apolipoproteins stimulate the enzyme. This is particularly important in spontaneous lipolysis however, proteose peptone 3 has been shown to inhibit lipolysis induced by homogenization, sonication, and temperature activation (Arora and Joshi, 1994), while protein components of the milk fat globule membrane inhibit lipolysis caused by bacterial lipase (Danthine et al., 2000). Several exogenous chemical agents can also inhibit lipolysis (Collomb and Spahni, 1995). For example, polysaccharides such as X-carrageenan at 0.3 g/1 effectively inhibits lipolysis in milk activated by mechanical means or temperature manipulation (Shipe et al., 1982) and lipolysis caused by the lipase from P. fluorescens (Stern et al., 1988). 

Milk that undergoes lipolysis without being subjected to any of the treatments described above has been referred to as naturally active, susceptible, spontaneously lipolytic or spontaneous in contrast to normal milk in which no lipolysis occurs. 

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). 

In feeding trials with lipid supplements, Astrup et al. (1980) observed that palmitic or myristic acid significantly enhanced spontaneous lipolysis but stearic acid and fatty acids with a chain length shorter than myristic acid had no effect. These workers found that feeding rapeseed oil to underfed cows reduced the susceptibility of their milk to lipolysis, while Chazal and Chilliard (1985) reported that supplementation with non-protected lipids, particularly highly unsaturated oils such as rapeseed, increased the level of FFAs in milk. Protected oil supplements have been found to lead to reduced lipolysis in milk (Astrup et al., 1979) or to have little effect on FFA level (Urquhart et al., 1984). 

An in-depth review of the effects of feed and nutrition on spontaneous lipolysis was published by Jellema (1980). 

Other factors. A cow s hormonal balance can affect the susceptibility of her milk to spontaneous lipolysis (Fredeen et al., 1951 Kastli et al., 1967 Bachman et al., 1988). The oestrus cycle appears to have little effect on spontaneous lipolysis (Fredeen et al, 1951) but may affect lipase activity in the milk (Kelly, 1945). In contrast, treatment of cows with oestradiol and progesterone has been shown to lead to increased lipolysis in the milk (Bachman, 1982 Heo, 1983 Bachmann eta/., 1985) but no change (Bachman, 1982) or a transient increase (Bachmann et al., 1985) in total lipase activity. It appears that the increased lipolysis in milk following hormonal treatment, or in milk from cows with ovarian cysts, may not be typical spontaneous lipolysis as cooling is not needed to initiate it (Bachman, 1982) a lipase other than lipoprotein lipase, possibly a bile salt-stimulated lipase, may be responsible for such lipolysis (Heo, 1983 Bachmann et al., 1985). Treatment of cows with bovine somatotropin has been reported to have no significant effect on milk lipoprotein lipase activity (Azzara et al., 1987). 

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). 

Bachman et al. (1988) reported a low repeatability (0.22) of spontaneous lipolysis in the milk of cows from lactation to lactation. However, according to Chazal and Chilliard (1987c), the repeatability between two successive lactations explained 30-40% of the variation in FFA data. These authors concluded that spontaneous lipolysis evident in late pregnancy is dependent on an intrinsic factor repeatable from lactation to lactation as well as an extrinsic factor, probably linked to diet. 

Supplementation of cows diets with zinc has been found to reduce spontaneous lipolysis significantly (Hermansen et al., 1995). The authors suggested that zinc deficiency may be a potential risk factor for spontaneous lipolysis. However, corroborative evidence is required before zinc supplementation could be recommended for reducing spontaneous lipolysis. 

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). 

Thus, four factors have been shown to contribute to the susceptibility of a milk to spontaneous lipolysis lipase activity, milk fat globule vulnerability, activating factors and inhibiting factors, with the balance of the last two being most important (Deeth and Fitz-Gerald, 1975a Sundheim, 1988 Cartier and Chilliard 1990). Sundheim (1988) concluded that these factors could explain 80 87% of lipolysis induced by cold storage. 

Bachman, K.C. 1982. Effect of exogenous estradiol and progesterone upon lipase activity and spontaneous lipolysis in bovine milk. J. Dairy Sci. 65, 907-914. 

Cartier, P., Chilliard, Y. 1990. Spontaneous lipolysis in bovine milk combined effects of nine characteristics in native milk. J. Dairy Sci. 73, 1178-1186. 

Chazal, M.P., Chilliard, Y. 1987a. Effect of breed of cow Friesian and Montbeliarde on spontaneous and induced lipolysis in milk. J. Dairy Res. 54, 7-11. 

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