. The continued increase in acid is due
to the growth of long rods of the _Bacillus Bulgaricus_ type,
which apparently enter the milk with the fecal matter. The nature of
the change produced by them in milk is very similar to that caused
by _Bact. lactis acidi_ in that lactic acid is the chief product; no
gas is produced and hence the curd is uniform in appearance.
Temperatures from 100 deg. to 110 deg. F. favor their development.
Organisms belonging to this group are used in the preparation of the
fermented milks now so widely sold in the cities.

These desirable, acid-forming bacteria are of the greatest service
in every branch of the dairy industry, whether in butter or in
cheese making, or in the sale of milk in the city. The dairy
industry is dependent upon fermentative activity, as much as the
manufacture of beer or wine, and the main basis of this is the acid
fermentation of the milk by these desirable types of bacteria.

Although milk contains a large amount of nitrogenous substances
(casein and albumen), it does not undergo putrid decomposition, as
do meat and eggs, not because it is not fitted for the growth of the
bacteria causing that type of change, but because the acid formed in
it stops the growth of the putrefactive bacteria. If a sample of
milk is placed in a stoppered bottle, it will have much the same
taste and odor at the end of several months as at the end of a few
days. The acid acts as a preservative, like the vinegar in pickles,
or the acid in silage and in sauerkraut. Meat placed in a stoppered
bottle which is then filled with milk will be preserved.

The products formed in the decomposition of meat and eggs are not
only offensive but may also be injurious to the health of the
consumer. Milk that has been fermented by the desirable kinds of
acid-forming bacteria is not harmful. It is consumed in a variety of
forms (buttermilk, cottage cheese) as a common article of food and
its use is rapidly increasing. The preparation of the pure culture
buttermilks or artificially soured milks that are now so frequently
recommended for digestive troubles rests upon an acid fermentation
of this type.

=Undesirable acid-forming bacteria.= Other types of bacteria capable
of forming substances that impart to milk an offensive odor and a
disagreeable taste not infrequently appear instead of the desirable
group. Instead of producing from the sugar of milk large quantities
of lactic acid, these types generate o