ther acids, such as acetic and
formic, which impart a sharp taste to the milk. Besides the acids
the bacteria of this group form gases from the sugar of the milk.
Some produce small amounts of gas; others so much that the curd will
be spongy and will float on the surface of the whey. The
fermentation caused by them is often called a "gassy fermentation"
and is dreaded by butter and cheese makers since the gas is
indicative of bad flavors that will appear in the product. Gas may
also be produced in other types of fermentations to be discussed
later.

This class of bacteria enters the milk with the dust, dirt, and
manure, in which materials they are especially abundant. No spores
are formed; hence they are easily killed by heating the milk. They
grow both in the presence and in the absence of free oxygen. High
temperatures favor their growth, most rapid development taking
place at 100 deg. to 103 deg. F.

=Spontaneous fermentation of milk.= The normal souring of milk is due
to a mixture of these two groups of bacteria. The relative
proportions existing between the two in any sample of milk is
dependent on a number of factors, most important of which is the
degree of cleanliness exercised in the production of the milk. Where
careless conditions obtain under which dust and manure particles
find their way into milk, it becomes more abundantly seeded with
gas-generating bacteria, and consequently, the type of fermentation
is undesirable. If, however, the milk is drawn into clean utensils
and care is taken to exclude dirt, the pure lactic acid types are
able to control the character of the changes produced, and a clean,
pleasant tasting liquid results. It will be seen that things are
well arranged by nature; one of the most important food products
undergoes a type of decomposition that is not offensive and when
produced under clean conditions, the sour milk is as healthful a
food as is the fresh product. Thus there is every reason for
cleanliness in the production of milk, for cleanliness' sake and
because clean milk means better products, and greater returns to
everyone, producer and dealer.

There are other kinds of acid-forming bacteria in milk but they are
of small importance compared with those just discussed. Some of the
bacteria derived from the inside of the udder of the cow form acid,
but these forms grow very slowly in milk at ordinary temperatures,
and have no influence on the keeping quality.

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