256 REPORT OF THE CHEMIST OF THE 
(3) The experiment was repeated, except that we used, only 
4 cc, of —— hydrochleric acid for one gram of casein. The fil- 
trate on titration showed no acid, indicating that the entire 4 ce. 
of =~ hydrochloric acid had been held by the casein. 
These results indicate to us that one gram of base-free casein 
combines with about .5 ec. of re hydrochloric acid, to form a 
compound of casein and hydrochloric acid, and that the disappear- 
ance of acid is due to chemical union with proteid and not merely 
to mechanical mixture or adhesion. 
The question has probably come to mind before this as to why 
free casein was not sooner isolated in the normal souring of milk 
or in the treatment of milk by direct addition of acid. It can 
now readily be seen why this was so. Since it requires so little 
acid to change the free casein into its acid combination, the point 
is quickly passed when we have any considerable proportion of 
free casein in the milk. In the normal souring of milk we were 
able to catch the change at a point when 65 per ct. of the casein 
of the milk was in the form of free casein, as shown in Bulletin 
No. 245, p. 12. When we use an acid to precipitate casein from 
milk, an excess is added, so that we get, not the free casein, but 
the compound formed by its combination with acid. 
Summary of action of acids on milk-casein (calcium casein) .— 
When the calcium casein of cows’ milk is treated with an acid, 
the first reaction that takes place is a union_between the acid and 
the calcium combined with the casein, resulting in the formation 
of base-free casein, a compound insoluble in water but soluble in 
warm 5 per ct. solution of sodium chloride and in hot 50 per ct. 
alcohol; a compound which also possesses the property, when 
warmed, of being very plastic and capable of being drawn out 
into long, fine, silky threads. This base-free casein is identical 
with the compound which we formerly regarded as being a casein 
mono-salt of the acid used as precipitant. 
When one gram of this base-free casein is treated with an 
amount of acid equivalent to about .5 cc. in the case of * hy- 
drochlorie acid, the properties of the casein are changed, so that 
it is no longer soluble in 5 per ct. salt solution and only. slightly 
soluble in hot 50 per ct. alcohol; and, in addition, it has lost. 
entirely its plastic properties and the power of being drawn out 
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