Starter culture

The starter culture for most yogurt production in North America is a symbiotic blend of Streptococcus thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB). Although they can grow independently, the rate of acid production is much higher when used together than either of the two organisms grown individually. ST grows faster and produces both acid and carbon dioxide. The formate and carbon dioxide produced stimulates LB growth. On the other hand, the proteolytic activity of LB produces stimulatory peptides and amino acids for use by ST. These microorganisms are ultimately responsible for the formation of typical yogurt flavour and texture. The yogurt mixture coagulates during fermentation due to the drop in pH. The streptococci are responsible for the initial pH drop of the yogurt mix to approximately 5.0. The lactobacilli are responsible for a further decrease to pH 4.5. The following fermentation products contribute to flavour:

  • lactic acid
  • acetaldehyde
  • acetic acid
  • diacetyl