y-product of this group of bacteria is lactic acid; small
amounts of acetic acid and alcohol, with traces of other compounds,
are also formed. The agreeable odor and to some extent the flavor of
milk fermented by these bacteria is due to other by-products than
lactic acid, for this has no odor and only a sour taste. The acid
fermentation of milk is often called the lactic acid fermentation.
In reality only the fermentation produced by the desirable group in
which lactic acid is the most evident by-product should be thus
called.

[Illustration: Fig. 20.--Different Types of Curds.

On the left a solid, homogeneous curd produced by desirable
bacteria; on the right, the curd produced by harmful bacteria. Note
the gas holes and free whey.]

The bacteria of this group may enter the milk from the dust coming
from the coat of the cow. They are also found in the barn dust and
on cultivated plants. Under ordinary farm conditions, the larger
part of those found in milk come directly from the utensils. If the
milk is drawn under extremely clean conditions and care is taken to
sterilize the utensils, but few acid-forming bacteria of any kind
will enter the milk; under such conditions most of the acid-forming
bacteria will belong to the group in question. They find, however,
such favorable conditions for growth in milk that they develop more
rapidly than most other types with which milk becomes seeded;
consequently under normal conditions, they gain the ascendency and
so control the type of fermentation.

The desirable type of acid-forming bacteria do not form spores;
hence, are easily killed by heating the milk. They can grow in the
presence or in the absence of free oxygen. In the bottom of a can of
milk or in the middle of a cheese, there is no air, yet these
bacteria grow as well under these conditions, as in milk exposed to
the air. The range of temperature for growth varies from 50 deg. to 100
deg. F. but development is most rapid at 90 deg. to 95 deg. F. and about
1 per cent of acid is formed.

Another group of bacteria which may be classed among the desirable
acid-forming organisms is constantly found in milk. They have little
to do with the ordinary acid fermentation as they grow very slowly
at ordinary temperatures. If a sample of raw milk is placed at the
temperature of the animal body, the acidity will reach 1 per cent in
a few hours. Thereafter the acidity will increase slowly and may
reach three per cent or above