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.
Making Yogurt There are two basic stages in
yogurt making: preparing the milk by heating
and partly cooling it; and fermenting the
warm milk.
The Milk Yogurt is made from all sorts of
milk; sheep and goat were probably the first.