216

QUANTITATIVE AGRICULTURAL ANALYSIS

in commercial milk is usually forbidden, as is that of most other
preservatives.
Test for Formaldehyde.—In a porcelain dish mix 5 cc of milk with 10
cc of concentrated hydrochloric acid, containing 0.2 gm of ferric chloride
per liter. Heat cautiously nearly to boiling and keep near the boiling point
for 1 minute. A violet color indicates the presence of formaldehyde.
Test for Berates.—Make 25 cc of milk slightly basic with lime water and
evaporate to dryness in a porcelain dish over the steam bath. Char the
residue in the dish and when cool add 15 cc of hot water, then boil. Add
hydrochloric acid drop by drop until neutral to litmus, then add an excess
of about ten drops. Filter and evaporate to dryness on a steam bath.
Immerse turmeric paper in the solution while the evaporation is taking
place. If borax or boric acid is present, the turmeric paper will turn cherry
red when dry and it will change to a bluish green when moistened with
ammonium hydroxide.
Cane Sugar.—Cane sugar is occasionally present in milk
which has been thickened with calcium saccharate or which has
been mixed with sweetened condensed milk.
Test for Cane Sugar.—Mix 10 cc of milk in a test-tube with 0.5 gm of
ammonium molybdate and 10 cc of 3-per cent hydrochloric acid. Make
a blank test using milk of known purity. Place the tubes in a water bath
and gradually raise the temperature to 80°. A blue color will develop
in normal milk but if sucrose is present the milk remains unchanged in
color. The test is quite delicate as even 1 gm in a liter may be detected by
the reaction.
Heated Milk.—One method for the detection of heated
milk is based upon the presence in raw milk of an enzyme which,
in the presence of hydrogen peroxide, is capable of producing
color changes with certain organic substances, the enzyme
probably acting as a catalyzer.
Bacteria in normal fresh milk will change methylene blue to a
colorless compound in about 20 minutes. But when milk has
been heated these bacteria are present in greatly diminished
numbers and are no longer capable of decolorizing the solution.
This furnishes another test for heated milk, since by this reaction
one may detect milk which has been pasteurized at 65° and held
at that temperature for 30 minutes, or at 70° for 10 minutes.
Test for Heated Milk.—(a) To 10 cc of milk add about three drops of a
freshly prepared 2-per cent solution of £>-phenylenediamine hydrochloride
and a few drops of hydrogen peroxide. Shake well. Milk which has not
been heated will give a blue color.