Lipase spontaneous lipolysis

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). 

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). 

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., Paquet, D. 1990. Inhibiting and activating effects of skim milks and proteose-peptone fractions on spontaneous lipolysis and purified lipoprotein lipase activity in bovine milk. J. Dairy Sci. 73, 1173-1177. 

Sundheim, G. 1988. Spontaneous lipolysis in bovine milk combined effects of cream, skim milk, and lipoprotein lipase activity. J. Dairy Sci. 71, 620-626. 

Sundheim, G., Bengtsson-Olivecrona, G. 1987a. Isolated milk fat globules as substrate for lipoprotein lipase study of factors relevant to spontaneous lipolysis in milk. J. Dairy Sci. 70, 499-505. 

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). 

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). 

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