A lot of strains of Lactococcus are able to produce flavor compounds, which can be divided in two categories:
- 1) Compounds present in fermented milks
- 2) Compounds present in ripened cheese
In fermented milks, there are mainly organic acids such as lactic and acetic acid, produced by Lactococcus lactis ssp. lactis, Lactococcus lactis ssp. cremoris. Lactococcus lactis ssp. lactis biovar diacetylactis and Lactococcus lactis ssp. cremoris produce acetaldehyde, diacetyl, acetoin and 2-3 butylene-glycole from citrate in milk. It has been suggested that these compounds are produced to avoid pyruvate accumulation in the cell.
Lactic acid bacteria are mildly proteolytic compared to other bacteria like Bacillus spp and Pseudomonas spp. Their proteolytic system is constituted by:
- • Proteinases
- • Peptidades
Lactococci and Lactobacillus helveticus are able to attack as casein and partly p-casein, while Lactobacillus bulgaricus degrades all the major caseins, p-casein being the most susceptible.
The proteolytic activity of Streptococcus thermophilus is lower than for lactobacilli and lactococci.
The proteinases of lactococci involved in casein breakdown are high-molecular- weight proteins located in the cell wall. Their optimal pH is around 5.5 to 6.5 and their isoletric point is between 4.4 and 4.55. They can be activated or stabilized by Ca+ ions.
Some proteinases are, by the way, involved in the bitterness of cheese. There is an active role of the lactococcal proteinases in producing bitter peptides of medium size, during cheese ripening.
Casein degradation can continue with peptidades, which produce smaller peptides and amino-acids. There are a lot of peptidades such as: aminopeptidades, di- and tripeptidases, arylpeptidyl-amidase, aminopeptidase P, prolinase, prolidase, X-proly- dipepdyl aminopeptidase, endopeptidases, and carboxypeptidase.
Lactobacilli have a wide range of peptidase activity, from low to high—an example of high activity is in Lactobacillus helveticus. Several peptidases have been isolated from Lactobacilus casei NCDO 15 and also in some strains belonging to Lactobacillusplan- tarum. These species have a relevant role on proteolysis, texture, and taste of cheese. The specific localization of all known peptidades is not assigned yet.