Flavor lipolysis

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. 

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). 

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). 

Some Effects of Lipolysis. The most serious effect of lipolysis is the appearance of the so-called rancid flavor which becomes detectable in milk when the ADV exceeds 1.2-1.5 mEq/liter (Brathen 1980). The fatty acids and their soaps, which are thought to be implicated in the rancid flavor, have been studied in an effort to assess the role of the individual acids in the overall rancid flavor picture. Scanlan et al (1965) reported that only the even-numbered fatty acids from C4 to Cl2 account for the contribution of fatty acids to the flavor, but that no single acid exerts a predominating influence. Another study has implicated the sodium and/or calcium salts of capric and lauric acids as major contributors to the rancid flavor (Al-Shabibi, et al. 1964). Butyric acid, assumed to be the compound most intimately associated with the flavor, was not singled out in either study as being especially involved. 

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-... 

Pillay, V. T., Myhr, A. N. and Gray, J. L. 1980. Lipolysis in milk. I. Determination of free fatty acid and threshold value for lipolyzed flavor detection. J. Dairy Sci. 63, 1213-1218. 

Kristoffersen, T. 1985. Development of flavor in cheese. Milchwissensch 40, 197-199. Kuzdzal-Savoie, S. 1980. Determination of free fatty acids in milk and milk products. In Flavor Impairment of Milk and Milk Products due to Lipolysis. J. H. Moore (Editor). Int. Dairy Fed. Annu. Bull. Doc. No. 118. 

Lipolysis by P. roqueforti is necessary for flavor development in blue-vein cheese. P. roqueforti produces intracellular and extracellular li-... 

Lipolysis is considered to be an important biochemical event during cheese ripening and the current knowledge have been discussed in detail (Collins et al., 2003, 2004 McSweeney and Sousa, 2000). The formation of short-chain FFAs by the lipolysis of milk fat by lipases is a desirable reaction in many cheese types (e.g., mold-ripened cheeses). The catabolism of FFAs, which is a secondary event in the ripening process, leads to the formation of volatile flavor compounds such as lactones, thioesters, ethyl esters, alkanols, and hydroxyl fatty acids. The contributions of lipolysis to the flavor of bacterially ripened cheeses are limited. 

Contribution of Lipolysis and Catabolism of Free Fatty Acids (FFA) to Cheese Flavor... 

The role of milk-fat in the development of flavor in cheese during ripening will be discussed below although it should not be forgotten that lipolysis and the metabolism of fatty acids do not occur in isolation from other important biochemical events during ripening. 

In addition to positive aspects, numerous flavor and textural defects may be associated with the fat phase of ice cream. Such flavor defects are usually related to either autoxidation of the fat, resulting in oxidized flavors (cardboardy, painty, metallic) or, especially in the case of milk-fat, lipolysis of free fatty acids from triglycerides by the action of lipases (referred to as hydrolytic rancidity). A significant content of free butyric acid gives rise to very undesirable rancid flavors. These defects tend to be present in the raw ingredients used in ice cream manufacture, rather than promoted by the ice cream manufacturing process itself. However, processing... 

Lipolysis, the enzymic hydrolysis of milk lipids to free fatty acids and partial glycerides, is a constant concern to the dairy industry because of the detrimental effects it can have on the flavor and other properties of milk and milk products. However, free fatty acids also contribute to the desirable flavor of milk and milk products when present at low concentrations and, in some cheeses, when present at high concentrations. 

There were several new developments during the 1970s. Of particular importance was the purification and characterization of a lipoprotein lipase (LPL) and the acceptance of the postulate that this was the major, if not the only, lipase in cows milk (Olivecrona, 1980). Similarly, the elucidation of the lipase system in human milk as consisting of an LPL and a bile salt-stimulated lipase, and the possible role of the latter in infant nutrition, were noteworthy (Fredrikzon et al, 1978). Also, microbial lipolysis assumed substantial significance with the widespread use of low-temperature storage of raw milk and the recognition that heat-stable lipases produced by psychrotrophic bacteria were a major cause of flavor problems in stored dairy products (Law, 1979). 

Raw cows milk contains a relatively large amount of lipase activity, but seldom undergoes sufficient lipolysis to cause an olf-flavor. Under optimal conditions, the lipase (milk LPL) can catalyze the hydrolysis of up to ca. 

In milk with a normal pH of 6.7, most of the acids are in the salt form and have much less flavor than if they were completely in the acid form (Kuzdzal-Savoie, 1980). In fact, acidification of milk greatly enhances the sensitivity of organoleptic detection of lipolysis in milk (Tuckey and Stad-houders, 1967). The detection of rancidity is reduced by the association of the FFAs with certain proteins in milk (Parks and Allen, 1979 Keenan et al., 1982) and by heating of milk (Kintner and Day, 1965). The phase in which the fatty acids are soluble also influences their flavor threshold, since the short-chain acids have much lower thresholds in fat than in water, while the opposite applies to the long-chain acids (Patton, 1964). For example, butyric acid (C o) has a flavor threshold of 7 mg/kg in water, but only of 0.6 mg/kg in oil (Delahunty and Piggott, 1995). 

The difficulty in relating rancid flavors in butter to FFA content arises because the short-chain acids, C4 o and C6 o, which are the most flavorsome (McDaniel et al., 1969), are water-soluble and hence are mostly lost in the buttermilk and wash water during the manufacture of butter. For this reason, even butter made from cream with an ADV as high as 2.4 meq/lOOg fat may show little defect while on the other hand, butter with quite a low ADV can be rancid, particularly if lipolysis occurs after manufacture (Deeth et al., 1979 Woo and Lindsay, 1980). 

The flavor thresholds for the individual fatty acids are quite different in butter and in milk. Whereas in milk, Cio o and Ci2 o acids are most significant to rancid flavor, in butter C4 o and C o are of most interest since they have much lower flavor thresholds in fat than do the longer-chain acids (Patton, 1964). The reported thresholds of the C4 o to Ci2 o acids added singly or in pairs to butter are shown in Table 15.3, together with the theoretical amounts for an increase in ADV of 0.1 meq/100 g fat. From these data, it is evident that a low level of lipolysis in butter produces sufficient butyric acid to exceed its flavor threshold and to impart a rancid flavor. Thus, measurement of C o, C(, o and Cg o gives the best indication of hydrolytic rancidity in butter (McNeill et al., 1986). 

The typical flavor of aged cheese is due to the combination of a variety of flavor compounds, including FFAs (Law, 1984). When excessive lipolysis... 

Levels of individual short-chain FFAs or combinations of these have been suggested as superior to total FFA or ADV as indicators of the desirable/undesirable lipolysis status and flavor potential of various cheeses, in particular butyric acid (C o) and total short-chain FFAs (Czm) + C o + C8.0) (Woo et al., 1984 Arbige et al., 1986 Lin and Jeon,... 

While flavor defects are the most likely result of lipolysis in dairy products, several other practical problems may arise from an elevated level of FFAs. The most common of these is lack of foaming of pasteurised milk for cappuccino-style coffee (IDF, 1987). Reduced efficiency of skimming of raw milk and reduced churning efficiency in cream may be associated with lipolysis, especially where excessive agitation or pumping causes damage to the milk fat globule membrane. 

Lipolysis plays an important role in providing the characteristic flavor of many milk products. In particular, the ripening of most cheese varieties is accompanied by lipolysis due to microorganisms or to added enzyme preparations, and, in raw milk cheese, to the milk LPL. Lipolysis is not extensive, but is more pronounced in some cheeses (e.g., blue-veined and hard Italian varieties), than in others. Excessive lipolysis renders the cheese unacceptable (Fox and Law, 1991 Gripon et al., 1991). 

Flavor preparations typical of particular varieties of cheese can be produced with the aid of lipases of appropriate specificities (Kilara, 1985). Such flavors are used in processed cheeses, dips and spreads (Jolly and Kosikowski, 1975b). Controlled lipolysis of milk fat is also used to produce creamy and buttery flavors for bakery and cereal products, confectionery (milk chocolate, fudge), coffee whiteners, and other imitation dairy products (Arnold et al., 1975 Fox, 1980 Kilara, 1985). 

Krukovsky, V.N., Herrington, B.L. 1942b. Studies of lipase action. VI. The effect of lipolysis upon flavor score of milk. J. Dairy Sci. 25, 237-239. 

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