Lipolysis induced

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). 

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. 

H. Okuda, C. Morimoto, and T. Tsuji-ta. Relationship between cyclic AMP production and lipolysis induced by forsko-lin in rat fat cells, J. Lipid Res., 1992, 33, 225-231. 

The lipolysis induced by catecholamines increases free fatty acid (FFA) levels, which, when entering the myocardium during an infarct, increases the heart s oxygen needs to metabolize these acids when the myocardium can least afford it. P-Blockers reduce this 02 consumption and workload, apparently by shifting metabolism toward carbohydrates. 

Moreover DBcAMP did not inhibit the hydrolysis of cAMP by heart phosphodiesterase, indicating a failure to bind to the enzyme. Since DBcAMP is not degraded by the diesterase its biological activity should not be influenced by those materials which either inhibit (theophylline) or stimulate (insulin, nicotinic acid, imidazole) the enzyme. The effect of these compounds on llpolysls produced by DBcAMP has been studied with somewhat conflicting results. Theophylline was found to potentiate DBcAMP-Induced lipolysis24,79 and nicotinic acid inhibited DBcAMP-induced lipo-lysls27,79. Insulin and imidazole have been reported to have either no effect or to inhibit lipolysis induced by DBcAMP. These results are difficult to interpret at this time. 

In the Netherlands, milk from every farmer is tested twice a year for the extent of lipolysis, using the BDI method. However, the BDI method only detects long chain FFA, which does not induce off-flavor. On the other hand, headspace sampling does detect the short chain FFA. The aim of this study is to compare the BDI method to headspace sampling. 

Raw milk was heated at 40°C and mixed in a blender for 1 min. This milk was added in different quantities (0-5 ml) to fresh raw milk to induce lipolysis. After 3 days, the milk was analyzed using the BDI method and headspace sampling. 

Increased lipid synthesis/inhibi-tion of lipolysis Activation of lipoprotein lipase (LPL)/induc-tion of fatty acid synthase (FAS)/inactivation of hormone sensitive lipase (HSL) Facilitated uptake of fatty acids by LPL-dependent hydrolysis of triacylglycerol from circulating lipoproteins. Increased lipid synthesis through Akt-mediated FAS-expression. Inhibition of lipolysis by preventing cAMP-dependent activation of HSL (insulin-dependent activation of phosphodiesterases )... 

Ketteihut, I.C. Goldberg, A.L (1988). Tumor necrosis factor can induce fever in rats without activating protein breakdown in muscle or lipolysis in adipose tissue. J. Clin. Invest. 81, 1384-1389. 

Cortisol-induced lipolysis not only provides substrates for gluconeogenesis (formation of glucose from noncarbohydrate sources) but it also increases the amount of free fatty acids in the blood. As a result, the fatty acids are used by muscle as a source of energy and glucose is spared for the brain to use to form energy. 

The regulation of fat metabolism is relatively simple. During fasting, the rising glucagon levels inactivate fatty acid synthesis at the level of acetyl-CoA carboxylase and induce the lipolysis of triglycerides in the adipose tissue by stimulation of a hormone-sensitive lipase. This hormone-sensitive lipase is activated by glucagon and epinephrine (via a cAMP mechanism). This releases fatty acids into the blood. These are transported to the various tissues, where they are used. 

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, W. K. 1975. Identity of the major lipolytic enzyme activity of bovine milk in relation to spontaneous and induced lipolysis. Int. Dairy Fed. Doc. 86, 80-89. 

Deeth, H.C., Fitz-Gerald, C.H. 1978. Effect of mechanical agitation of raw milk on the milk-fat globule in relation to the level of induced lipolysis. J. Dairy Res. 45, 373-380. 

Buffaloes milk contains an LPL similar to cows milk LPL and in comparable quantities. A higher proportion is located in the cream (e.g., 23% compared to 12% for cows milk Balasubramanya et al., 1988). Bha-vadasan et al. (1988) found no relationship between the extent of lipolysis and LPL activity in either species. Lipolysis by LPL is inhibited by proteose peptone fractions 3, 5 and 8 from buffaloes milk, with the PP3 fraction being the most inhibitory (Ram and Joshi, 1989). As in cows milk, lipolysis can be induced by shaking or homogenization (Sammanwar and Ganguli, 1974). 

Continuous pumping, particularly with aeration, causes damage to the milk fat globule membrane and subsequent lipolysis to an extent dependent on the type of pump (Downes et al., 1974 Kirst, 1980). Pumping of raw milk through ultrafiltration membranes (Hicks et al., 1990), and sudden release of pressure and the use of a milk exit temperature of >7°C during concentration of raw milk by reverse osmosis can also induce lipolysis (de Boer and Nooy, 1980 Barbano et al., 1983). Factory separation of cream, where the cream is partially homogenized as it leaves the separator bowl at relatively... 

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. 

The data of Gudding (1982) suggest that the elevation of FFAs may depend on the cause of mastitis, as relatively higher levels of FFAs were observed in milk from quarters infected with Staphylococcus aureus. When mastitis is induced experimentally by intramammary infusion of endotoxins or bacteria, the increases in FFAs correspond closely with the increases in SCC and other indices of mastitis (Salih and Anderson, 1979 Fitz-Gerald et al., 1981 Ma et al., 2000). Murphy et al. (1989) concluded that the increased lipolysis in mastitic milk is due to increased susceptibility of the milk fat. 

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