rms. These organisms develop in milk with great rapidity, and the
decomposition changes that are noted in souring are due to the
by-products of their development.

The milk-sugar undergoes fermentation, the chief product being lactic
acid, although various other by-products, as other organic acids
(acetic, formic and succinic), different alcohols and gaseous products,
as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.

In this fermentation, the acidity begins to be evident to the taste when
it reaches about 0.3 per cent., calculated as lactic acid. As the
formation of acid goes on, the casein is precipitated and incipient
curdling or lobbering of the milk occurs. This begins to be apparent
when the acidity is about 0.4 per cent., but the curd becomes more solid
with increasing acidity. The rapidity of curdling is also dependent upon
the temperature of the milk. Thus milk which at ordinary temperatures
might remain fluid often curdles when heated. The growth of the bacteria
is continued until about 0.8 to 1.0 per cent. acid is formed, although
the maximum amount fluctuates considerably with different lactic acid
species. Further formation then ceases even though all of the milk-sugar
is not used up, because of the inability of the lactic bacteria to
continue their growth in such acid solutions.

As this acidity is really in the milk serum, cream never develops so
much acid as milk, because a larger proportion of its volume is made up
of butter-fat globules. This fact must be considered in the ripening of
cream in butter-making where the per cent. of fat is subject to wide
fluctuations.

The formation of lactic acid is a characteristic that is possessed by a
large number of bacteria, micrococci as well as bacilli being numerously
represented. Still the preponderance of evidence is in favor of the view
that a few types are responsible for most of these changes. The most
common type found in spontaneously soured milk changes the milk-sugar
into lactic acid without the production of any gas. This type has been
described by various workers on European as well as American milks, and
is designated by Conn as the _Bact. lactis acidi_ type.[51] It is
subject to considerable variation under different conditions.

Curiously enough if milk which has been drawn with special care is
examined immediately after milking, the lactic organisms are not usually
found. They are incapable of development in the udder itself