tter cheese.= In a previous chapter the bitter fermentation of milk
has been discussed. If milk containing large numbers of such
organisms is made into cheese, the bitterness is very likely to be
noted in it. Cheese made from milk containing few or no lactic
bacteria is likely to develop a bitter taste, due to the growth of
the digestive bacteria that are able to grow through the lack of
acid in the cheese.

If the milk contains considerable numbers of yeasts, a sweet or
fruity flavor is apt to develop, due to the products of the
fermentation of the sugar by the yeast. This flavor resembles that
of fermented fruit, or the bouquet of certain kinds of wine.

=Putrid cheese.= In the absence of acid-forming bacteria, the cheese
may develop a putrid or rotten odor, due to the growth of some types
of putrefactive or digesting bacteria. This trouble is very
infrequent in cheddar cheese, since this is made from ripened milk,
but occurs more frequently in those types in which no acid is
developed.

Bacteria develop in the cheese in colonies or masses, just as they
do in the plate cultures of the bacteriologist, made with
transparent media, such as gelatin. Cheese is opaque; therefore, the
growing colonies cannot be readily discovered, but when
pigment-forming bacteria grow in the cheese, their presence is
likely to be noted, because of the colored spots that are formed.

=Rusty spot.= The "rusty spot" that has been encountered in New York
and Canada is due to one of the colored bacteria which produces an
orange or yellowish-red pigment. Various other pigment-forming
organisms have been met in cheese, each producing its colored colony
which differentiates itself from the mass of the cheese. If the
pigment is produced in considerable quantities, and is soluble in
any of the constituents of the cheese, the color will not appear in
spots but will be more diffuse, or may impart a color to the entire
mass.

Cases of acute poisoning arising from the ingestion of cheese are
not infrequently reported; similar instances result from the use of
ice cream. In both cases it is believed that poisonous products have
been formed by bacteria, probably by some of the putrefactive forms.

From what has been said with reference to the abnormal fermentations
of cheese, it will be seen that they are always due to the lack of
acid-forming bacteria, or to their partial replacement by other
types. In order to prevent such troubles, it is necessar