Psychrotrophs, lipase-producing,

Many lipases produced by psychrotrophic bacteria retain activity after pasteurization and ultra-high-temperature (UHT) heat treatments (Cousin 1982 Adams and Brawly 1981). Butter made from cream which supported growth of lipase-producing psychrotrophs became rancid within two days (Kishonti and Sjostrom 1970). UHT milk processed from raw milk contaminated with lipase from a Pseudomo-... 

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). 

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). 

Andersson, R.E., Hedlund, C.B., Jonsson, U. 1979. Thermal inactiviation of a heat-resistant lipase produced by the psychrotrophic bacterium Pseudomonas fluorescens. J. Dairy Sci. 62, 361-367. 

The enzymes responsible for the detrimental effects of lipolysis are of two main types those indigenous to milk, and those of microbial origin. The major indigenous milk enzyme is lipoprotein lipase. It is active on the fat in natural milk fat globules only after their disruption by physical treatments or if certain blood serum lipoproteins are present. The major microbial lipases are produced by psychrotrophic bacteria. Many of these enzymes are heat stable and are particularly significant in stored products. 

Several psychrotrophic bacteria produce extracellular phospholipases, the most prevalent in milk being pseudomonads (particularly P. fluorescens), Alcaligenes, Acinetobacter, and Bacillus species (Fox et al., 1976 Owens, 1978a Phillips et al., 1981). Most of these produce phospholipase C, some produce phopholipase Ai and some produce both types (Deeth, 1983). Ser-ratia spp. have been shown to produce only phospholipase A (Deeth, 1983), while P. fragi has been reported not to produce phospholipases (Kwan and Skura, 1985). Phospholipase C from some pseudomonads has been purified and characterised (Doi and Nojima, 1971 Sonoki and Ikezawa, 1975 Stepa-niaketa/., 1987a Ivanov etal., 1996). Like the lipases, many of these enzymes have considerable heat stability and are not destroyed by pasteurization... 

O Donnell, E.T. 1978. Heat resistance of lipase enzymes produced by psychrotrophic bacteria. 

Enzymes are a group of proteins that have the ability to catalyze chemical reactions and the speed of such reactions. The action of enzymes is very specific. Milk contains both indigenous and exogenous enzymes. Exogenous enzymes mainly consist of heat-stable enzymes produced by psychrotrophic bacteria lipases, and proteinases. There are many indigenous enzymes that have been isolated from milk. The most significant group is the hydrolases ... 

Pseudomonads produce extracellular enzymes that breakdown proteins, glycoproteins, fats, glycolipids, and phospholipids in milk. The enzyme linked immunosorbent assay (ELISA) method can be used to measure levels of thermostable psychrotrophic proteases and lipases in milk as a determinant of its suitability for cheesemaking or ultrahigh-tempera-ture processing. 

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