ferment the lactose in the milk, forming lactic acid which reduces the pH from 6.3 to 4.6. At this point the casein
coagulates into a gel which traps all the other milk components. Streptococci grow first, but as the pH falls, the lactobacilli dominate. Compounds produced by one type of bacteria also stimulate the growth of the other. The final acid content is about 0.9-0.95%.
Acetaldehyde produced during fermentation gives yoghurt its characteristic flavour.