On Food and Cooking

causes the casein proteins to gather together
in semisolid curds (p. 20) and thicken the
milk.
There are two major groups of lactic acid
bacteria. The small genus Lactococcus (a
combination of the Latin for “milk” and
“sphere”) is found primarily on plants (but
it’s a close relative of Streptococcus, whose
members live mainly on animals and cause a
number of human diseases!). The 50-odd
members of the genus Lactobacillus (“milk”
and “rod”) are more widespread in nature.
They’re found both on plants and in animals,
including the stomach of milk-fed calves and
the human mouth, digestive tract, and vagina;
and their clean living generally benefits our
insides (see box, p. 47).
The bacteria responsible for the major
fermented products were identified around
1900, and pure cultures of individual strains
became available then. Nowadays, few dairies
leave their fermentations to chance. Where