Yogurt: Role of Starter Culture

Yogurt should contain living strains of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Lactose is fermented homofermentatively by the mixed strain culture. Only the glucose moiety of lactose is fermented and galactose accumulates in the medium. Both species in the culture are thermophilic and show synergistic interactions (protocooperation). Both species produce acetaldehyde, the most typical flavor component in yogurt. Some strains of both species produce exopolysaccharides, acting as biothickeners in the product. The bacterial growth may be inhibited by bacteriophages, antibiotics and sanitizing agents. A serious imbalance between the two species may then occur.