Generally, in shallow setting, whether in large or small pans, cooling
the milk in air is depended upon. Formerly, an underground room, or
one in a shady place, was the only appliance usually resorted to for
cooling. But, of late years, some method of artificially cooling the
air by the use of ice is generally adopted. In some cases, the milk
room is made small, with low ceiling and double walls, so that a cake
of ice near the ceiling does the cooling. Usually, however, some sort
of refrigerator construction is resorted to, so that cool air from the
ice-house, or ice placed above the milk room, is introduced to regulate
the temperature and keep it steady. We prefer cooling in air, though
it may take a little more space and time. By this method, extremely
rapid cooling and low temperature are avoided, and no violence is done
to the milk or cream. Deep setting, it is true, exposes less surface
to the air; but if the milk is not submerged, the surface is likely to
be cooler than the air above, and to condense the vapor in it, which
falls with all its impurities on the surface of the cream. Any foulness
or bad odors are thus absorbed and go into the butter product. While
submerging obviates this objection and keeps out all impurities from
the air, it also prevents all escape of bad odors by evaporation.
Whatever that is objectionable may be in the milk is retained there. By
setting in open air, which should of course always be pure and sweet,
the air, being cooled down and used as a medium for cooling the milk,
takes up the exhalations of moisture and odor from the milk, and thus
purifies it. The colder medium is always the condenser and absorbent,
and it is only when the milk gets colder than the air above it that it
condenses the moisture in the air and absorbs its odor. This will never
occur where cold air is the cooling medium. The milk theoretically can
never get cooler than the air, while practically it remains a degree or
two warmer than the air.