By Harold McGee
By contrast to the complex and variable flora of traditional yogurts, the industrial version is reduced to the essentials. Standard yogurt contains just two kinds of bacteria, Lactobacillus delbrueckii subspecies bulgaricus, and Streptococcus salivarius subspecies thermophilus. Each bacterium stimulates the growth of the other, and the combination acidifies the milk more rapidly than either partner on its own. Initially the streptococci are most active. Then as the acidity exceeds 0.5%, the acid-sensitive streptococci slow down, and the hardier lactobacilli take over and bring the final acidity to 1% or more. The flavor compounds produced by the bacteria are dominated by acetaldehyde, which provides the characteristic refreshing impression of green apples.