202

QUANTITATIVE AGRICULTURAL ANALYSIS

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It may sometimes be impossible to begin the analysis before
bacterial action begins. In such a case add formaldehyde at
the rate of 1 cc of the 40-per cent solution to 2 liters of milk.
Specific Gravity.—This determination is usually made with a
lactometer, which is a hydrometer of a special form. However,
it can be determined also by a Westphal balance or a picnom-
eter. For a discussion of the use of these instruments, see
pages 96 to 100, Part II.
Added Water.—As a means for detecting adulteration the
specific gravity determination alone is of little value. The spe-
cific gravity of butter fat is about 0.93 and of milk solids other
than fat is 1.5, while that of whole milk is 1.030 to 1.034. If
water is added the specific gravity is lowered but if milk is
skimmed the specific gravity is raised because the lighter
portion has been removed. Therefore fat could be removed
and water added in such a way as to keep the specific gravity
unchanged.
A more certain method for the detection of added water is in
the examination of milk serum, from which all of the fat and
proteins have been removed.
Determination of Specific Gravity.—A sample of fresh milk is thoroughly
mixed by pouring from one vessel to another several times, avoiding violent
agitation. Determine the specific gravity at 20° within 2 minutes after
mixing.
Detection of Added Water.—The ash and milk sugar are the least variable
constituents of milk and they afford a suitable basis for the detection of
adulteration. A clear serum may be obtained by precipitating the proteins
with acetic acid or copper sulphate or by spontaneous souring, and filtering.
Examine the filtrate for ash, also for other dissolved solids by means of a
dipping refractometer. This instrument is described on page 118, Part II.
Examination of Acetic Serum: (a) Zeiss Dipping (Immersion) Refractom-
eter Reading.—To 100 cc of milk at a temperature of 20° add 2 cc of 25-per
cent acetic acid (specific gravity 1.035) in a beaker and heat the mixture,
covered by a watch glass, by immersing in a water bath at 70° for 20 minutes.
Place the beaker in ice water for 10 minutes and separate the curd by filtering
through a 12.5-cm folded filter. Transfer about 35 cc of the serum to one
of the beakers that accompanies the temperature control bath used in
connection with the Zeiss dipping refractometer or fill the metal cup that is
attachable to the instrument; take the refractometer reading at 20°, using
a thermometer graduated to tenths of degrees. A reading below 39 indi-
cates added water. If the reading is between 39 and 40 the addition of water
is not certain but is to be suspected.