CHANGES IN MILK 365 
Milk Proteins. According to the table from Bosworth and Van 
Slyke cow’s milk has a larger content of protein than goat’s or human 
milk. Data from other sources confirm this fact. The following 
proteins are found in milk: 
Casein 
Lact-albumin 
Lact-globulin 
CHANGES In MiLxK 
Milk is subject to many types of changes. One of the most common 
and one which occupied the attention of chemists for a long time is the 
coagulation or clotting of the casein. The natural state of this milk 
constituent may be changed either by acid or the presence of a specific 
enzyme. Other proteins may be coagulated in the same way by enzymes. 
This change in the state of the casein is the foundation of the whole 
cheese industry. 
Souring of Milk. This usually refers to the normal changes in milk 
which are brought about by the continued growth of lactic acid bacteria. 
The lactic acid which is thus formed removes the calcium from the 
calcium caseinate. The casein thus formed is insoluble in the milk 
serum and is precipitated. 
The chemical changes which take place in the soluble and insoluble 
constituents of milk have been studied by Van Slyke and Bosworth 
(1916). The milk was inoculated with a culture containing Bacterium 
lactis acidi and B. lactis aerogenes and allowed to stand at room tempera- 
ture for sixty hours. At the end of this time it was filtered through a 
porcelain filter and subjected to analysis. A blank was made by pre- 
serving milk with chloroform and separating the serum in the same way. 
The changes which took place in sixty hours are given in Table XX XVIII. 
Coagulation by Rennin. It was believed for a long time that the 
coagulation of milk was due only to acid. Fremy, in 1839, one of the 
earliest workers on the subject, supposed that a special enzyme was 
present in the lining of calves’ stomachs which formed acid from the 
lactose. Hammarsten (1872) proved that the production of acid from 
the lactose was not the cause of coagulation by rennin but that it was 
brought about by a special enzyme, which acted directly on the casein. 
The phenomenon involved in the action of rennin is the change of 
casein in milk into soluble paracasein along with another compound. 
However, before actual clotting takes place an insoluble calcium para- 
caseinate must be formed from calcium salts in the milk. Van Slyke 
