DAIRY PRODUCTS

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(6) Ash.—-Transfer 25 cc of the serum to a fiat bottomed platinum dish
and evaporate to dryness on a steam bath, then heat over a low flame until
the solids are thoroughly charred. Place the dish in a muffle furnace
and ignite to a white ash at a temperature not higher than 500°, cool and
weigh. Express the results as grams per 100 .cc. Multiply by the factor
1.02 to correct for the dilution by addition of acetic acid. The result
is the ash on the undiluted sour serum. An ash content below 0.715 gm
per 100 cc indicates added water.

Examination of Sour Serum: (a) Zeiss Dipping Refractometer Reading.—
Allow the milk to sour spontaneously, filter and determine the dipping
refractometer reading of the clear serum at 20°. A reading below 38.3
indicates added water.

(6) Ash.—Determine the ash in 25 cc of the sour serum, using the method
as directed for ash of acetic serum. Ash lower than 0.730 gm per 100 cc
indicates added water.

Examination of Copper Serum: Zeiss Dipping Refractometer Reading.—
Use a solution of copper sulphate containing 72.5 gm per liter, adjusted if
necessary to read 36 at 20° on the scale of the dipping refractometer. To
one volume of this solution add four volumes of milk. Shake well and filter.
^Determine the refractometer reading of the clear serum at 20°. A reading
below 36 indicates added water.

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Acidity of Milk. — Acidity of milk is due to acid phosphates
and lactic acid, the latter being produced by bacterial action
upon milk sugar. Tfcds is the "souring" of milk.

Determination of Acidity. — Place 20 cc of milk of known specific gravity
in a 100-cc porcelain casserole and add tenth-normal (to phenolphthalein)
sodium hydroxide from a burette, using phenolphthalein as an indicator,
until a pink color appears and remains for 1 minute. Calculate the per cent
of lactic acid, HCaHaOa, in the milk.

Total Solids. — In order to dry the solids rapidly without
decomposing them it is desirable to use a weighed flat porcelain
or aluminium dish in which has been placed enough sand or
asbestos fiber to cover the bottom. The sand or asbestos
increases the drying surface and hinders the formation of a scum,
which would interfere with the evaporation of the liquid beneath.
The solid thus formed should be nearly white except as it may
be colored by sand. If there is any considerable browning or
blackening it is probable that the milk sugar has been partly
caramelized and the resulting loss would therefore not indicate
correctly the evaporated water.