A proposal by the European Commission for a council regulation laying down additional rules on the common organization of the market in milk and milk products for yoghurt and yoghurt-like products was presented recently (AGRI/38 743/2003rev3). Article 2 and Annex of the proposal establish that ‘yoghurt’ is a product obtained by the fermentation of milk with cultures of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus.
Lactobacillus delbrueckii ssp. bulgaricus
The lactic acid fermentation of Bulgarian yoghurt is a unique biological process in which symbiotic cultures of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus are involved. Both microorganisms cooperate with each other during fermentation and their associative growth results in accelerated acidification, but the exact mechanisms are only partially understood. Present day molecular biology and genomic studies indicate that Lactobacillus delbrueckii ssp. bulgaricus is in process of ongoing human-driven reductive evolution. Ages of human selection of strains adapted to the protein rich milk medium led to the establishment of stable natural symbiotic cultures in which Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus stimulate each other’s growth, cooperate and compensate one another metabolically. Nowadays several genetic and biochemical features are known to be responsible for the complex positive interaction between those two bacterial species:
- S. thermophilus degrades milk urea and consequently excretes CO2 and in this way stimulates the growth of lactobacilli, which is hampered from the low CO2 concentration in milk after heat treatment
- S. thermophilus S. thermophilus stimulates the growth of L. bulgaricus through formic acid formation, which is limiting step in the purine biosynthesis.
- Mixed yoghurt cultures may stimulate the production of some metabolites such as acetaldehyde.
- L. bulgaricus, unlike S. thermophilus, possesses extracellular cell-wall bound protease and therefore can supply S. thermophilus with peptides and amino acids.
- S. thermophilus genome contains the genes responsible for p-aminobenzoic acid (PABA) biosynthesis and might supply L. bulgaricus with PABA which is necessary for the production of folate.
- Genomic analysis data indicate that S. thermophilus might provide L. bulgaricus with ornithine during the fermentation, which is decarboxylized to putrescine from L. bulgaricus, who, on its turn, provides with it S. thermophilus. In this way both bacteria benefit mutually to provide each other with polyamines, which are important for many cell functions and oxidative stress resistance.