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648 MILK.
NaOH (Long l
). The question whether the casein from different kinds
of milk is identical or whether there are several caseins cannot be decided
by the elementary analysis. According to Tangl and Csokas, 2
mare’s
and ass’s casein seem to be somewhat richer in nitrogen (16.44 and 16.28
per cent, respectively) but poorer in sulphur (0.528 and 0.588 per cent)
and carbon (52.36 and 52.27 per cent) than the casein from cud chewers.
The ass’s casein was richer in phosphorus (1.057 per cent) than the mare’s
or cow’s casein (both with 0.887 per cent).
Casein when dry appears like a fine white powder, which has no
measurable solubility in pure water (Laqueur and Sackur). Casein
is only very slightly soluble in the ordinary neutral-salt solutions. Accord-
ing to Arthus it dissolves rather easily in a 1-per cent solution of sodium
fluoride, ammonium or potassium oxalate. Robertson thinks that it
is more soluble in potassium cyanide and the alkali salts of certain vola-
tile fatty acids such as butyric acid and valeric acid, than in solutions
of the ordinary neutral salts. It is at least a tetrabasic acid, whose
equivalent weight is 1135, according to Laqueur and Sackur, and 1250
according to Robertson. The statements as to the molecular weight
are disputed (Laqueur and Sackur, L. and D. van Slyke 3
).
It dissolves readily in water with the aid of alkali or alkaline earths,
also calcium carbonate, from which it expels carbon dioxide ana it
thus forms caseinates of variable composition. If casein is dissolved
in lime-water and the solution carefully treated with very dilute phos-
phoric acid until it is neutral in reaction (to litmus), the casein appears
to remain in solution, but is probably only swollen as in milk, and the
liquid contains at the same time a large quantity of calcium phosphate
without any precipitate or any suspended particles being visible. The
casein solutions containing lime are opalescent, and have on warming
the appearance of milk deficient in fat (which is also true for the salts
of casein with the alkaline earths). Therefore it is not impossible that
the white color of the milk is due partly to the casein and calcium phos-
phate. Soldner and others have prepared two calcium compounds
of casein with 1.5 p. c. CaO (the neutral caseinate according to Soldner)
and 2.4 p. c. CaO (the basic caseinate). The first is neutral to litmus
while the other is neutral to phenolphthalein.
According to Robertson 4
the alkali equivalent of casein at neutrality
toward litmus =53 XlO -5 equivalent-grm.-mol. per gram and at neutrality toward
1
Hoppe-Seyler, Handb. d. physiol. u. pathol. chem. Analyse, 8. Aufl., 489; Long,
Journ. Amer. Chem. Soc, 27.
2
Pfluger’a An.-h., 121.
3
Laqueur and Sackur, Hofmeister’s Beitrage, 3; M. Arthus, Theses presentees
a la faculty des sciences de Paris, 1893; Robertson, Journ. of biol. Chem., 2; L. an 1
D. van Slyke, Amer. Chem. Journ., 38.
4
See Ergebnis. d. Physiol. 10 and Journ. of physical Chem., 13.
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