Published results on the proteolytic activity of yeast isolated from dairy products are mainly based upon the ability to produce a clearing zone in casein and skim milk agar. It has been demonstrated (Hansen and Jakobsen 2001; Larsen et al.
1998) that the agar diffusion tests may not offer the sensitivities required to detect proteolytic activity. Methods such as gelelectrophoresis, high performance liquid chroma-tography (HPLC), or capillary electrophoresis (CE) give a different and more detailed picture, which also demonstrates that the proteolytic activity of yeast is strain specific (Hansen and Jakobsen 2001; van den Tempel 2000). Several studies of strains of D. hansenii have indicated that the examined strains could not degrade casein (Hansen and Jakobsen 1997). However, low proteolytic activity was observed in D. hansenii isolated from Picante cheese (Freitas et al. 1999) and blue veined cheese (van den Tempel 2000). Similar variability in the proteolytic activity for strains of K. lactis has been observed (Grieve et al. 1983; Hansen and Jakobsen 1997; Roostita and Fleet 1996a). However, Grieve et al. (1983) did show that a strain of K. lactis was able to hydrolyze a-, b-, and k-casein, which indicates that some strains are able to contribute to the maturation of cheese.
Gueguen and Lenoir (1975) examined 30 strains of G. geotrichum and found that they produced extracellular and intracellular proteinases and peptidases. Extracellular activity was present in 66% of the strains, which could be divided into two groups of 25% showing high proteolytic activity and 75% having low proteolytic activity. Optimum pH for the proteolytic activity was close to 5.5. Other investigations of commercial strains of G. geotrichum used in cheese production showed that the proteolytic activity was weak when present.
Yarrowia lipolytica produces several proteinases and many of these are extracellular. The secretion of both alkaline and acid proteinases has been detected (Ogrydziak et al. 1982; Vasileva-Tonkova et al. 1996). The activity of the extracellular acid proteinase at pH 5.2 was optimal at 15°C but was significantly reduced at 8°C. Many investigations of Y. lipolytica have shown that different strains of Y. lipolytica have proteolytic activity and are able to breakdown all the casein components. (Freitas et al. 1999; van den Tempel and Jakobsen 2000; Wyder and Puhan 1999) and seems to be at the same level as a strong proteolytic strains of P. roqueforti (van den Tempel 2000).
Candida catenulata seems to have a high proteolytic activity similar to the level of Y. lipolytica (Roostita and Fleet 1996a), but detailed studies have not been reported.
Saccharomyces cerevisiae can be proteolytic but the majority of strains do not show proteolytic activity (Hansen and Jakobsen 1997; Hansen and Jakobsen 2001; Roostita and Fleet 1996a). The proteolytic activity was studied by Hansen and Jakobsen (2001) using CE for five selected Saccharo-myces spp. being a commercial starter cultures in blue veined cheese, an isolats from blue veined cheeses, a type strains of Saccharomyces spp. and two starter cultures of Saccharo-myces spp. used in other food fermentation. The CE results for the five strains showed that only the commercial blue cheese strain of S. cerevisiae could break down casein. The CE-profiles showed that S. cerevisiae was able to hydrolyze some of the as1-and b-casein. Differences in the CE-profiles indicated a synergistic effect in the degradation of casein by
S. cerevisiae and Penicillium roqueforti, seen as higher number and different grouping of peptides when grown together compared to growth of the individual culture.
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