Milk lipolysis

Besides changing the natural flavor of milk, lipolysis may produce a variety of other effects. One of the most noticeable of these is the lowering of surface tension as lipolysis proceeds (Schwartz 1974). Fatty acids, especially their salts, and mono- and diglycerides, being good surface-active agents, depress the surface tension of milk (see the discussion Methods for Determining Lipase Activity ). Milk fat ob-... 

Astrup, H. N. 1980. Effect on milk lipolysis of restricted feeding with and without supplementation with protected rape seed oil. J. Dairy Res. 47, 287-294. 

Astrup, H. N., Vik-Mo, L., Skrovseth, O. and Ekern, A. 1980. Milk lipolysis when feeding saturated fatty acids to the cow. Milchwissenschaft 35, 1-4. 

Fleming, M. G. 1980. Mechanical factors associated with milk lipolysis in bovine milk. Int. Dairy Fed. Doc. 118, 41-52. 

Kason, C. M., Pavamani, I. V. P. and Nakai, S. 1972. Simple test for milk lipolysis and changes in rancidity in refrigerated pasteurized milk. J. Dairy Sci. 55, 1420-1432. 

Salih, A. M. A. and Anderson, M. 1979A. Effect of diet and stage of lactation on bovine milk lipolysis. J. Dairy Res. 46, 623-631,... 

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). 

Barbano, D.M., Bynum, D.G., Senyk, G.F. 1983. Influence of reverse osmosis on milk lipolysis. 

Chazal, M.P., Chilliard, Y. 1985. The effect of animal factors on milk lipolysis. Dairy Sci. Abstr. 49, 364. 

Sundheim, G., Zimmer, T.-L., Astrup, H.N. 1983. Induction of milk lipolysis by lipoprotein components of bovine blood serum. J. Dairy Sci. 66, 400-406. 

Consumer acceptance of milk is strongly determined by its sensory characteristics. The development of off-flavor in milk as a result of lipolysis can reduce the quality of milk. The enzymatic release, by milk lipase, of free fatty acids (FFA) from triglycerides causes a flavor defect in milk described as rancid . Triglycerides in milk contain both long chain and short chain fatty acids, which are released at random by milk lipase. The short chains FFA, like butyric acid, are responsible for the off-flavor. 

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. 

Milk from cows affected with mastitis alters the sensory quality of raw milk and cheese (Munro el al., 1984). Sensory defects are reported as increased rancidity and bitterness, factors which are consistent with higher levels of lipolysis and proteolysis (Ma et al., 2000). 

Lipases catalyse the development of hydrolytic rancidity in milk, and, consequently, lipases and lipolysis in milk have been studied extensively. Milk contains three types of esterase ... 

Natural variations in the levels of free fatty acids in normal milk and the susceptibility of normal milks to lipolysis may be due to variations in the level of blood serum in milk. 

To increase the stability of milk products. Lipoprotein lipase is probably the most important in this regard as its activity leads to hydrolytic rancidity. It is extensively inactivated by HTST pasteurization but heating at 78°C x 10 s is required to prevent lipolysis. Plasmin activity is actually increased by HTST pasteurization due to inactivation of inhibitors of plasmin and/or of plasminogen activators. 

On the positive side, thermization and minimum pasteurization should not cause the formation of undesirable flavours and aromas and should, in fact, result in improved flavour by reducing bacterial growth and enzymatic activity, e.g. lipolysis. If accompanied by vacuum treatment (vacreation), pasteurization removes indigenous off-flavours, i.e. those arising from the cow s metabolism or from feed, thereby improving the organoleptic qualities of milk. 

Lipolysis. Some lipolysis occurs in all cheeses the resulting fatty acids contribute to cheese flavour. In most varieties, lipolysis is rather limited (Table 10.5) and is caused mainly by the limited lipolytic activity of the starter and non-starter lactic acid bacteria, perhaps with a contribution from indigenous milk lipase, especially in cheese made from raw milk. 

Extensive lipolysis occurs in two families of cheese in which fatty acids and/or their degradation products are major contributors to flavour, i.e. certain Italian varieties (e.g. Romano and Provolone) and the Blue cheeses. Rennet paste, which contains pre-gastric esterase (PGE) rather than rennet extract, is used in the manufacture of these Italian cheeses. PGE is highly specific for the fatty acids on the sn-3 position of glycerol, which, in the case of milk lipids, are predominantly highly flavoured short-chain fatty acids (butanoic to decanoic). These acids are principally responsible for the characteristic piquant flavour of these Italian cheeses. 

Blue cheeses undergo very extensive lipolysis during ripening up to 25% of all fatty acids may be released. The principal lipase in Blue cheese is that produced by Penicillium roqueforti, with minor contributions from indigenous milk lipase and the lipases of starter and non-starter lactic acid bacteria. The free fatty acids contribute directly to the flavour of Blue cheeses but, more importantly, they undergo partial /J-oxidation to alkan-2-ones (methyl O... 

The data in Table 4.2 are from analyses of pooled milk. As mentioned, TGs account for about 98% of the lipids the DGs, MGs, and free fatty acids (FFA) are mostly products of lipolysis, and the cholesterol and phospholipids are cellular membrane material which accompanies the fat globule during extrusion from the secreting cell. 

Timmen and Dimick (1972) characterized the major hydroxy compounds in milk lipids by first isolating the compounds as their pyruvic ester-2,.6-dinitrophenylhydrazones. Concentrations as weight percent of the compounds from bovine herd milk lipids were 1,2-DGs 1.43, hydroxyacylglycerols 0.61, and sterols 0.35. Lipolysis tripled the DG content. The usual milk fatty acids were observed, except that the DGs lacked 4 0 and 6 00, again indicating that these lipids were in part intermediates in milk lipid biosynthesis. With the large hydrazone group... 

The increased use of tanks for the storage of raw milk on the farm between pickups has introduced the danger of potential off-flavor development caused by lipases that are produced by certain microorganisms (psychrotrophs) at low temperatures. The exocellular lipases of psychrotrophic bacteria are extremely heat resistant, and although the microorganisms are killed, the enzymes survive pasteurization and sterilization temperatures. Rancidity may become noticeable when cell counts exceed 106 or 107/ml. Downey (1975) has summarized the potential contribution of enzymes to the lipolysis of milk (Table 5.1). 

Most, if not all, milks contain sufficient amounts of lipase to cause rancidity. However, in practice, lipolysis does not occur in milk because the substrate (triglycerides) and enzymes are well partitioned and a multiplicity of factors affect enzyme activity. Unlike most enzymatic reactions, lipolysis takes place at an oil-water interface. This rather unique situation gives rise to variables not ordinarily encountered in enzyme reactions. Factors such as the amount of surface area available, the permeability of the emulsion, the type of glyceride employed, the physical state of the substrate (complete solid, complete liquid, or liquid-solid), and the degree of agitation of the reaction medium must be taken into account for the results to be meaningful. Other variables common to all enzymatic reactions—such as pH, temperature, the presence of inhibitors and activators, the concentration of the enzyme and substrate, light, and the duration of the incubation period—will affect the activity and the subsequent interpretation of the results. 

Table 5.1. Contribution of Enzymes Present to Lipolysis of Milk.

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. 

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