It may not always be convenient to make a test immediately when the sample is taken. In the creamery where the milk is paid for according to its fat contents, samples are taken every day from every patron's milk and it would take too much time for the butter maker to test all of these samples before they would spoil. A preservative, corrosive sublimate,—poisonous but all right for the purpose,—is therefore added and all the samples of one farmer's milk for several days or a whole week are put together in one glass to be kept and tested at one time. This is called a composite test and has proven entirely satisfactory.
The Lactometer has been used to discover adulterations,
depending upon the difference in specific gravity
of the various constituents. The specific gravity of
whole milk is about 1.032 which means that, if a certain
volume of water weighs 1.000 weight units, the same
volume of whole milk weighs 1.032, the same volume
of butter-fat weighs, say, .900, or of cream about 1.000,
and of skim milk 1.036 units. If the Lactometer shows
a sample of milk to have a higher specific gravity than
1.032 it may therefore be suspected of having been
skimmed. But it will readily be seen that by removing
from whole milk some of the cream and adding water,
the specific gravity can easily be brought back to normal
for whole milk. This test is therefore unreliable and has
been discarded with the advent of the Babcock.
- [Footnote: fat in the milk. The bottle with contents should be warm—about 140°—when
the measure is taken.
For testing skim milk and cream special forms of test bottles are used—which are described in the circulars coming with the testers and students who desire fuller information are referred to Farrington and Woll's "Testing Milk and its Products," published by the Mendota Book Co., Madison, Wis.]
