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CASEIN. 649
phenolphthalein = S0xl0 -5
equivalent-gnu. -mol. per gram. On BaturatioD (with
monacidic liases) the alkali equivalent is = llxi0~ 5
grm.-mol. per gram. Od
saturating (with monobasic acids) the acid equivalent is=32X10 -5
grm.-mol.
per gram.
Besides the rather earlier investigations on the salts of casein by Sold-
ner, Courant, Rohmanx. Laqieur, Raudnitz !
and others we have
the recent observations and theoretical discussion of Robertson 2
on the
composition, nature and dissociation of the caseinates. We can here
only refer to this and the earlier investigations.
Casein solutions do not coagulate on boiling, but solutions of casein-
lime are covered, like milk, with a pellicle. They are precipitated by
very little acid, but the presence of neutral salts retards the precipitation.
A casein solution containing salt or ordinary milk requires, therefore,
more acid for precipitation than a salt-free solution of casein of the same
concentration. The precipitated casein dissolves very easily again in
a small excess of hydrochloric acid, but less readily in an excess of acetic
acid. The combination between casein and acid, like other protein
and acid compounds, is precipitated by neutral salts. These acid solu-
tions are precipitated by mineral acids in excess.3
Casein is precipitated
from neutral solutions or from milk by common salt containing calcium,
or magnesium sulphate in substance, without changing its properties.4
Metallic salts, such as alum, zinc sulphate and copper sulphate, com-
ply precipitate the casein from neutral solutions.
On drying at 100° C, casein, according to Laqieur and Sackur, decomposes
and splits into two bodies. One of these, called caseid. is insoluble in dilute alkalies,
while the other, the isocasein, is soluble therein. The isocasein is a stronger acid
and has other precipitation limits and a rather lower equivalent weight than the
casein.
The property which is the most characteristic of casein is that it
coagulates with rennet in the presence of a sufficiently large amount
of lime-salts. In solutions free from lime-salts the casein does not coagu-
late with rennet, but it is changed so that the solution (even if the enzymes
are destroyed by heating) yields a coagulated mass, ha ring the properties
of a curd, if lime-salts are added. The rennet enzyme, rennin, has there
1
Soldner, Die Salze der Milch, etc., and Maly’s Jahresber., 25; Courant, 1. c.
;
Rohmann, Berlin, klin. Wochenschr., 1895; Laqueur, L c; and Hofmeister’s Beitrage,
7; Raudnitz, Ergebn. d. Physiol., 2, Abt. 1.
2
Journ. of physical Chem., 11 and 12; Journ. of biol. Chem., 5.
3
In regard to the acid combinations of casein and the ability to take up acid, see
Laxa. Milch win hsoh. Centralbl., 1905; Long, Journ. Amer. Chem. Soc, 29; L. and
D. van Slyke, Amer. Chem. Journ., 38; Robertson, Journ. of biol. Chem., 4.
4
See the works of Hammarsten and Schmidt-Nielsen, Hammarsten’s Festschrift,
1906.
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