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656 MILK.
The mineral bodies of milk will be treated in connection with its quan-
titative composition.
The methods for the quantitative analysis of milk are very numerous,
and a* all cannot be treated here, we will give the principal points of a
few of the methods considered most trustworthy and most frequently
employed.
In determining the solids a carefully weighed quantity of milk is mixed with
an equal weight of heated quartz sand, fine glass powder, or asbestos. The
evaporation is first done on the water-bath and finished in a current of carbon
dioxide or hydrogen not above 100° C.
The mineral bodies are determined by incinerating the milk, using the pre-
cautions mentioned in the text-books. The results obtained for the phosphoric
acid are incorrect on account of the burning of phosphorized bodies, such as
casein and lecithin. We must, therefore, according to Soldner, subtract in round
numbers 25 per cent from the total phosphoric acid found in the milk. The
quantity of sulphate in the ash also depends on the combustion of the proteins.
In the determination of the total amount of proteins Ritthausen’s method
is employed, namely, the precipitation of the milk with copper sulphate according
to the modification suggested by Munk. 1
He precipitates all the proteins by
means of cupric l)3
r
droxide at boiling heat, and determines the nitrogen in the
precipitate by means of Kjeldahl’s method. This modification gives more
exact results.
According to Sebelien’s method, three to four grams of milk are diluted
with an equal volume of water, a little common-salt solution added, and the
proteins precipitated with an excess of tannic acid. The precipitate is washed
with cold water, and then the quantity of nitrogen determined by Kjeldahl’s
method. The total nitrogen found when multiplied by 6.37 (casein and lactal-
bumin contain both 15.7 per cent nitrogen) gives the total quantity of proteins.
This method, which is readily performed, gives very good results. I. Munk
used this method in the analysis of woman’s milk. In this case the quantity
of nitrogen found must be multiplied by 6.34. G. Simon 2
found that the
precipitation with tannic acid, also with phosphotungstic acid, is the simplest
and most accurate. The objection to this and other methods in which the pro-
teins are precipitated is that perhaps other bodies (extractives) may be carried
down at the same time (Camerer and Soldner 3
). It is not known to what
extent this takes place.
A part of the nitrogen in the milk exists as extractives, and this nitrogen is
calculated as the difference between the total nitrogen and the protein nitrogen.
According to Mink’s analyses about r$ of the total nitrogen belongs to the extract-
ives in cow’s milk. Camerer and Soldner determine the nitrogen in the filtrate
from the tannic-acid precipitate by Kjeldahl’s method, and also according to
Hufner’s method (hypobromite). In this way they found 18 milligrams of
nitrogen according to Hufner (urea, etc.) in 100 grams of cow’s milk.
To determine the casein and albumin separately we may make use of the
method first suggested by Hoppe-Seyler and Tolmatscheff, 4
in which the casein
is precipitated by magnesium sulphate. According to Sebelien the milk is diluted
with its own volume of a saturated magnesium-sulphate solution, then saturated
with the salt in substance, and the precipitate then filtered and washed with a
saturated magnesium-sulphate solution. The nitrogen is determined in the pre-
1
Ritthausen, Journ. f. prakt. Chem. (N. F.), 15; I. Munk, Virchow’s Arch., 134.
’ Sebelien, Zeitschr. f. physiol, Chem., 13; Simon, ibid., 33.
1
Zeitschr. f. Biologie, 33 and 36.
4
Hoppe-Seyler, Med. chem. Untersuch., 272.
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