Mesophilic undefined starters with mixed species are used in the production of semihard cheese. This kind of starter is composed of several strains of lactic acid bacteria that give rise to the development of a dynamic population during the manufacture and ripening of cheeses. They are referred to as DL-starters because of their composition. The bacterial strains are of four different kinds; namely, the two main acid producers, Lactococcus lactis subsp cremoris (Lc. cremoris) and Lc. lactis subsp. lactis (citrate-) (Lc. lactis) and those metabolizing citrate, Lc. lactis subsp. lactis (citrate+) (Lc. diacetylactis) (D) and Leuconos-toc mesenteroides subsp. crermoris (Leu. cremoris) (L). The citrate-using bacteria (Lc. diacetylactis and Leu. cremoris) are responsible for eye formation by production of CO2 from citrate with simultanous production of flavor compounds such as diacetyl. The three different kinds of Lc. lactis strains have historically been considered to belong to different species (i.e., Streptococcus cremoris, S. lactis, and S. diacetylactis). Improved methods for genetic comparison have later shown that the bacteria are very closely related, that Lc. cremoris and Lc. lactis are of the same species, and that Lc. diacetylactis is a biovariant of Lc. lactis. The only difference between Lc. diacetylactis and Lc. lactis observed consistently is a plasmid in Lc. diacetylactis coding for citrate permease and lyase, making them able to transport citrate into the cell where it may be converted to diacetyl. Today Lc. lactis and Lc. diacetylactis are referred to as Lc. lactis subsp. lactis (citrate-) and Lc. lactis subsp. lactis (citrate+). Some important properties of the four kinds of starter bacteria are given in Table 2 (4).
Commonly Lc. cremoris and Lc. lactis make up about 80-90% of the bacteria found in the starter. The development of the different groups of bacteria depend on the manu-
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