n this substance is
added to fresh milk, it increases the acidity of milk without affecting
its taste. As normal milk tastes sour when it contains about 0.3 per
cent lactic acid, a milk that tests as much or more than this without
tasting sour has been probably treated with this antiseptic agent.

~Physical methods of preservation.~ Methods based upon the application of
physical forces are less likely to injure the nutritive value of milk,
and are consequently more effective, if of any value whatever. A number
of methods have been tried more or less thoroughly in an experimental
way that have not yet been reduced to a practical basis, as electricity,
use of a vacuum, and increased pressure.[129] Condensation has long been
used with great success, but in this process the nature of the milk is
materially changed. The keeping quality in condensed milk often depends
upon the action of another principle, viz., the inhibition of bacterial
growth by reason of the concentration of the medium. This condition is
reached either by adding sugar and so increasing the soluble solids, or
by driving off the water by evaporation, preferably in a vacuum pan.
Temperature changes are, however, of the most value in preserving milk,
for by a variation in temperature all bacterial growth can be brought to
a standstill, and under proper conditions thoroughly destroyed.

~Use of low temperatures.~ The effect of chilling or rapid cooling on the
keeping quality of milk is well known. When the temperature of milk is
lowered to the neighborhood of 45 deg. F., the development of bacterial
life is so slow as to materially increase the period that milk remains
sweet. Within recent years, attempts have been made to preserve milk so
that it could be shipped long distances by freezing the product, which
in the form of milk-ice could be held for an indefinite period without
change.[130] A modification of this process known as Casse's system has
been in use more or less extensively in Copenhagen and in several places
in Germany. This consists of adding a small block of milk-ice (frozen
milk) to large cans of milk (one part to about fifty of milk) which may
or may not be pasteurized.[131] This reduces the temperature so that the
milk remains sweet considerably longer. Such a process might permit of
the shipment of milk for long distances with safety but as a matter of
fact, the system has not met with especial favor.

[Illustration: FIG. 22. Microscopic ap