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ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS, 89 
are more concentrated than in milk; in ordinary cream the two phases 
oil-casein and casein-water tend to progressive separation up to a limit. 
Milk also contains about 0:8 per cent. of a coagulable albumen which is 
stated to be identical with serum albumen. 
Milk has an amphoteric reaction, 7.e., when fresh, it reacts feebly 
both acid and alkaline according to the indicator used; on keeping, it 
develops lactic acid by bacterial action, and at a certain acidity the whole 
solidifies to a gel, which after a time or with pressure separates into a 
solid ‘ curd ’ (casein or cheese) and a liquid ‘ whey.’ 
With hydrochloric acid the amount required to produce clotting was 
20 c.c. of N/10 HCl for 20 c.c. of milk. 
An investigation has been made into the influence of the hydrogen 
ion in milk clotting by C. Allemann (* Biochem. Zeitschr.’ 1912, 45, 346- 
858), who found that when different acids were added to milk together 
with rennet the time of clotting depended, not upon the acidity, but 
upon the hydrogen ion concentration. The clotting time diminishing 
up to a concentration of H ions of 1:3 x 10-> the point at which acid 
alone caused coagulation. The effect of salts, e.g., mixtures of phosphates 
and of sodium acetate and acetic acid, was also studied. It was found 
that the clotting points of whole milk, skimmed milk, and caseinogenate 
solutions showed certain differences. 
When the precipitation of casein is brought about by acid in presence 
of a protective colloid, e.g., gelatin or isinglass, the coagulum is much 
more diffuse, not separating in curds; hence these additions are useful 
for milk foods. The coagulum is also more finely divided when produced 
by kephir, koumiss, or youghout (.e., ferments producing lactic acid), 
the proteids being probably to some extent peptonised. ; 
When albumin is added to milk and the mixture heated, the albumin 
and casein separate together as a complex in the form of a gel. 
Commercial casein is prepared from skimmed milk by addition of 
rennet or acids (acetic, hydrochloric), in the case of rennet a certain amount 
of peptone may be formed by the digestive ferment. There is a difference 
in the behaviour of caseins according to the method of preparation. They 
contain about 11 per cent. of water and 7 per cent. of mineral matter, 
mostly calcium phosphate ; usually also a small quantity of fat is present 
which has an influence on its properties. 
On treatment with water, casein swells to a certain extent but does 
not dissolve ; with NH,HO there is a considerable increase in the swelling, 
but the mixture is heterogeneous ; with caustic alkalies, alkaline carbonates, 
alkaline earths, and alkaline salts, e.g., borax, sodium phosphate, &c., 
it forms various states of solution, those with alkalies showing the least 
viscosity. On heating with alkali there is a slight decomposition, ammonia 
_ being evolved and the liquid becoming extremely thin. Casein is also 
soluble in hydrochloric acid. 
_ The amount of hydrochloric acid required for solution of 1 gram of 
casein at 1°25 per cent. concentration was found to be 32 x 10> equiva- 
lent gram mols of HCl, and the amount of caustic soda at 2 per cent. 
concentration 11-4 x 10~* equivalent gram mols of NaHO (J. B. Robertson, 
* Jour. Phys. Chem.’ 1909, 18, 469-489) ; casein is dissolved much Tess 
readily by solutions of alkaline earths (J. B. Robertson, ‘Jour. Phys. 
Chem.’ 1910, 14, 377-392). On boiling casein with acids the rate of 
solution is directly in proportion to the strength and concentration of 
