Microbial lipolysis

Anderson, R. 1980. Microbial lipolysis at low temperatures. Appl. Environ. Microbiol 39, 36-40. 

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). 

Mastitis and microbial contamination can also contribute to hydrolytic rancidity. In general, lipolysis caused by indigenous milk lipase accounts for most of the rancidity in raw milk and cream microbial lipolysis is of minor practical importance as little if any lipolysis occurs before the bacterial population reaches 106—107 cfu/ml (Suhren and Reichmuth, 1990). However, in stored milk products, lipolysis by microbial lipases is of greatest significance. Short shelf-life products such as pasteurized milks may be affected by pre-pasteurization lipolysis caused by milk lipase but may be affected by bacterial lipolysis at the end of their shelf-life (Deeth et al., 2002). 

Bucky, A.R., Hayes, P.R., Robinson, D.S. 1987. A modified ultra-high temperature treatment for reducing microbial lipolysis in stored milk. 1. Dairy Res. 54, 275-282. 

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. 

Hygiene on the farm and in the factory is of paramount importance in controlling microbial growth and minimising lipolysis problems. Inadequately cleaned equipment can be a major source of lipolytic psychro-trophic contaminants (Drew and Manners, 1985 Stead, 1987). 

Alkanhal, H.A., Frank, J.F., Christen, G.L. 1985. Microbial protease and phospholipase C stimulate lipolysis of washed cream. J. Dairy Sci. 68, 3162-3170. 

Carlsson, A., Bjorck, L. 1992. Liquid chromatography verification of tetracycline residues in milk and influence of milk fat lipolysis on the detection of antibiotic residues by microbial assays and the Charm II test. J. Food Prot. 55, 374-378. 

An extension of this technology lead to the development of enzyme modified cheeses or EMC which are a very important product. The basic function of the process was to shorten the ripening time of a mature cheese without losing flavor. The potential financial gains are obvious. Young cheeses are subjected to a controlled lipolysis and proteolysis which is brought about by adding suitable microbial enzymes (8). After thermal inactivation of the enzymes, a pasty product is obtained which can have a flavor intensity of up to 20 times that of the mature cheese. 

According to several authors, cheese taste is mainly due to the compounds found in the cheese water-soluble extract (WSE) (1, 2). Thus, to study cheese taste, the focus is usually on the cheese WSE which contains small polar molecules such as minerals, acids, sugars, amino acids, peptides and some volatile compounds produced by different processes such as lipolysis, proteolysis microbial metabolism (3). These compounds are responsible for the individual taste sensations like sourness, bitterness and saltiness which are the main taste descriptors for cheese. However, in a complex mixture they also exert otiier taste sensations due to taste / taste interactions (4). Peptides are generally considered to be the main bitter stimuli in cheese (5). However, it was shown that in goat cheese, bitterness resulted mainly from die bitterness of calcium and magnesium chlorides, partially masked by sodium chloride (6). 

Karahdian et al. (1985a,b) demonstrated the production of l-octen-3-ol, 8-nonen-2-one, 3-octanone, 3-octanol and octanoic acid from linoleic acid and linolenic acid. The use of microbially produced fatty acids for characterization of mold fungi has been suggested (Blomquist et al., 1992). Lipolysis of triglycerides or amino acids may lead to the production of compounds such as 2-methylpropanoic acid, butanoic acid, 2-methylbu-tanoic acid, pentanoic acid, hexanoic acid and octanoic acid (Jolivet and Belin, 1993). The presence of different lipids and lipases in different fungi (Ha and Lindsay, 1993) and under different environmental conditions may explain a great deal of the variation in the volatile compounds produced. 

Fat is, on the one hand, a source of taste and aroma compounds and, on the other, a medium that regulates the distribution of these compounds between water, fat and vapour phases, which influence their perception by the sense organs. Some flavour compounds result from the decomposition of lipids by lipolysis, oxidation (section 3.3.4), and microbial or thermal degradation. These processes may produce free fatty acids, aldehydes, ketones, lactones and other volatile compounds (Figure 5.10). 

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