ent and on the composition of the milk.
Certain bacteria will not produce enough acid to cause the curdling
of the milk; still others will form 2 or even 3 per cent. These
types, however, do not play any important part in the spontaneous
souring of milk.

In milk the acid first formed combines with the ash constituents and
the casein to form salts which do not seriously affect the growth of
the bacteria. Ultimately, the limit of the ash and casein to take up
acid is reached, and free lactic acid which is harmful to bacterial
growth appears. If the content of casein and ash constituents is
high, a higher degree of acidity will be reached than in a milk with
a lower content. If a large part of the volume of the milk is made
up of a compound that has no role whatever in the acid fermentation,
such as the butter fat in cream, the amount of acid formed per unit
volume of milk will be reduced, since in determining the acidity, a
definite volume of milk is taken, and the acidity is expressed, as
such a per cent of this amount.

=Types of acid-forming bacteria.= When substances undergo
decomposition, it is a common belief that compounds offensive to the
odor and taste are formed; but such is not necessarily the case. The
products of the decomposition may be as agreeable and as harmless as
the compounds decomposed. Whether the decomposition products of any
substance are offensive or not is dependent on the kinds of
micro-organisms acting on it. There are forms of acid-producing
bacteria that change milk in odor, taste, and appearance, yet the
sour milk is not offensive in any sense of the word. Other bacteria
also sour the milk, but produce offensive odors and a disagreeable
taste. Thus, the acid-forming bacteria may be divided into two main
groups, which may be designated as desirable and undesirable. This
division is of importance to the butter and cheese maker and to the
consumer of milk.

=Desirable acid-forming bacteria.= If milk is produced under clean
conditions, it is not likely to have a disagreeable odor or taste at
any time, even when it is sour; rather the taste is agreeable like
that of good butter milk. The curd is perfectly homogeneous, showing
no holes or rents, due to the development of gas, and there is but
little tendency for the whey to be expressed from the curd. This
type of fermentation is largely produced by the group of bacteria to
which has been given the name, _Bacillus lactis acidi_.

The main b