No abstract

The proteolytic systems of lactic acid bacteria are important as a means of making protein and peptide N available for growth and as part of the curing or maturation processes which give foods their characteristic rheological and organoleptic properties. The proteolytic systems of lactic acid bacteria are described in relation to their growth and their functions in protein-rich foods. Their role in the manufacture of milk products is discussed.

No abstract

B. A. LAW lipases (including phospholipases) which, because they are produced extracellularly, can act directly on micellar casein and on the fat globules in milk. They can act in raw milk while it is being stored and the psychrotrophs are multiplying, but they may also continue to act in heat-treated liquid milk and other products. PROTEINASES OF PSYCHROTROPHIC BACTERIA Proteinase-producing groups Some thermoduric bacteria are well known for their ability to produce extracellular proteinases (e.g. Priest, 1977; Law, 1979) and psychrotrophic thermoduric sporeformers have been isolated from milk (see Introduction). However, there have been no definitive reports linking proteinases produced by these organisms with changes in milk or milk products. Proteinase-producing psychrotrophic Gramnegative bacteria are often isolated from milk and references to the identification of such isolates are listed in Table 1. The best represented genus is Pseudomonas, with Ps. fluorescens as the most common species. However, the apparent high incidence of the latter may be due to the ease with which it may be tentatively identified, rather than to its true distribution. The proteinases of several psychrotrophic pseudomonads have been isolated and partly characterized. For example, a, Ps. fluorescens enzyme which gels UHT-sterilized milk (Law, Andrews & Sharpe, 1977) appears to be a thiol proteinase, insensitive to EDTA and inhibited by Co 2+ and Zn 2+ (Alichanidis & Andrews, 1977); it has a nearneutral pH optimum with casein as substrate and its mol. wt is 38400. On the other hand, Barach, Adams & Speck (1976a) isolated an EDTA-sensitive, Zn 2+-containing neutral proteinase (mol. wt 48000) from another Pseudomonas sp. Other psychrophilic Pseudomonas isolates from soil and cold-stored meat produce similar EDTA-sensitive neutral metallo-proteinases (Law, 1979). Protein breakdown in liquid milk Most studies of the degradation of proteins in milk and milk products have concerned the caseins; whey proteins have received little attention. Psychrotroph proteinases may act first during refrigerated handling of raw milk before heat treatment and Law et al. (1977, 1979a) have shown that strains of Pseudomonas and Acinetobacter spp. growing to approx. 10 7 colony-forming units (cfu/ml) and above produce sufficient proteinase to degrade /?-and K-casein to an extent detectable by polyacrylamide-gel electrophoresis (PAGE) and starch-gel electrophoresis (SGE). When milks in which the Pseudomonas sp. had grown were subsequently UHTsterilized and stored at 20 °C, those which had contained 5 x 10 7 and 8 x 10 6 cfu/ml before sterilization gelled after 10-14 d and 8-10 weeks respectively (Law et al. 1977). Uninoculated low-count milks, or milks containing only 8 x 10 5 cfu/ml of the pseudomonad remained liquid for at least 20 weeks after sterilization, although a sediment gradually formed in the latter. The /?-and /e-caseins were extensively degraded in gelled UHT milks and some loss of a sl-casein was seen. Whey proteins were not detectably degraded....

No abstract