would
have been the case when the butter was fresh. At the temperature of
an ordinary refrigerator the changes go on much more rapidly. This
fact has often been looked on as indicating that the factors causing
the changes are biological ones. The influence of temperature in
accelerating the changes would be the same if no biological factor
were active.

That biological factors are of importance is indicated by the fact
that the keeping quality of the product is profoundly affected by
the quality of the cream. Butter made from sweet, fresh cream, that
has been thoroughly pasteurized, has the best keeping quality, while
butter made from such cream, but not pasteurized, has the poorest
keeping quality, especially when no salt is added. Every process by
which the desirable lactic bacteria are increased in proportion to
other kinds has a marked effect in enhancing the keeping quality of
the butter. Thus, the use of pure cultures in raw cream, and
pasteurization together with the pure cultures, have a marked
beneficial effect.

The addition of preservatives exerts an effect on keeping quality.
Borax is the chemical most frequently employed for this purpose. Its
use is allowed in Australia and New Zealand in butter that is
shipped to England, but the use of all preservatives is forbidden in
the United States.

The size of the package also has an effect on the keeping quality;
the smaller the package, the greater is the surface exposed to the
air and the more rapidly the butter deteriorates. Butter used in the
United States Navy is packed in hermetically sealed cans so as to
exclude the air as far as possible.

From the fact that any condition which restrains or inhibits the
growth of micro-organisms has a tendency to improve the keeping
quality of butter, it would appear that the detrimental changes in
the quality of butter are due to biological causes. The most common
defect known is that usually referred to as rancidity. There are,
however, different types of changes that are probably included under
this head and it is very probable that different causes are
operative in their production. True rancidity is probably due to
biological causes; the so-called tallowy change, in which the butter
acquires the odor of tallow is probably due to the combined action
of light and air on the fat.

=Bacterial defects in butter.= There are a number of defects in butter
that are positively known to be due to the growth of bacteria i