f bacteria
possess this property of producing gas (H and CO_{2} are most common
although N and methane (CH_{4}) are sometimes produced). The more common
forms are those represented by _B. lactis aerogenes_ and the common
fecal type, _B. coli commune_. The ordinary habitat of this type is dirt
and intestinal filth. Hence careless methods of milk handling invite
this type of abnormal change in milk.

It is a wide-spread belief that thunder storms cause milk to sour
prematurely, but this idea has no scientific foundation. Experiments[54]
with the electric spark, ozone and loud detonations show no effect on
acid development, but the atmospheric conditions usually incident to a
thunder storm are such as permit of a more rapid growth of organisms.
There is no reason to believe but that the phenomenon of souring is
wholly related to the development of bacteria. Sterile milks are never
affected by the action of electric storms.

~"Gassy" milks.~ Where these gas bacteria abound, the amount of lactic
acid is generally reduced, due to the splitting up of some of the sugar
into gaseous products. This type of germ life does not seem to be able
to develop well in the presence of the typical lactic acid non
gas-forming bacteria.

[Illustration: FIG. 16. Cheese made from "gassy" milk.]

~"Sweet curdling" and digesting fermentations.~ Not infrequently milk,
instead of undergoing spontaneous souring, curdles in a weakly acid or
neutral condition, in which state it is said to have undergone "sweet
curdling." The coagulation of the milk is caused by the action of enzyms
of a rennet type that are formed by the growth of various species of
bacteria. Later the whey separates more or less perfectly from the curd,
producing a "wheyed off" condition. Generally the coagulum in these
cases is soft and somewhat slimy. The curd usually diminishes in bulk,
due to the gradual digestion or peptonization of the casein by
proteid-dissolving enzyms (tryptic type) that are also produced by the
bacteria causing the change.

A large number of bacteria possess the property of affecting milk in
this way. So far as known they are able to liquefy gelatin (also a
peptonizing process) and form spores. The Tyrothrix type of bacteria (so
named by Duclaux on account of the supposed relation to cheese ripening)
belongs to this class. The hay and potato forms are also digesters.
Organisms of this type are generally associated with filth and manure,
and find their