ved (one teaspoonful per gallon), and then separating it
again.[48]

The treatment of abnormal conditions due to bacteria has been given
already under the respective sources of infection, and is also still
further amplified in following chapter.

~Aeration.~ It is a common belief that aeration is of great aid in
improving the quality of milk, yet, when closely studied, no material
improvement can be determined, either where the milk is made into butter
or sold as milk. Dean in Canada and Storch in Denmark have both
experimented on the influence of aeration in butter making, but with
negative results. Marshall and Doane failed to observe any material
improvement in keeping quality, but it is true that odors are eliminated
from the milk during aeration. The infection of the milk during aeration
often more than counterbalances the reputed advantage. Especially is
this so if the aeration is carried out in an atmosphere that is not
perfectly clean and pure.

In practice aeration differs greatly. In some cases, air is forced into
the milk; in others, the milk is allowed to distribute itself in a thin
sheet over a broad surface and fall some distance so that it is brought
intimately in contact with the air. This latter process is certainly
much more effective if carried out under conditions which preclude
infection. It must be remembered that aeration is frequently combined
with cooling, in which case the reputed advantages may not be entirely
attributable to the process of aeration.

~Infection of milk in the factory.~ The problem of proper handling of milk
is not entirely solved when the milk is delivered to the factory or
creamery, although it might be said that the danger of infection is much
greater while the milk is on the farm.

In the factory, infection can be minimized because effective measures of
cleanliness can be more easily applied. Steam is available in most
cases, so that all vats, cans, churns and pails can be thoroughly
scalded. Special emphasis should be given to the matter of cleaning
pumps and pipes. The difficulty of keeping these utensils clean often
leads to neglect and subsequent infection. In Swiss cheese factories,
the custom of using home-made rennet solutions is responsible for
considerable factory infection. Natural rennets are soaked in whey which
is kept warm in order to extract the rennet ferment. This solution when
used for curdling the milk often adds undesirable yeasts and other
gas-ge