COMPOSITION OF MILK. 17 
quickly and easily decomposes. This decomposition is caused 
by micro-organisms. If these could be entirely excluded 
from the milk, it would keep for an almost indefinite length 
of time. As it is impossible under practicable conditions to 
entirely exclude organisms from the milk, the only way in 
which the growth of germs can be retarded: and prevented, 
and thereby prevent the changing of the sugar into other 
products, is to cool the milk to a low temperature (50° F.), 
or to heat the milk to a sufficiently high temperature (180° F.) 
to destroy most of the germs. According to Van Slyke and 
Hart, the decomposition of the caseous matter produces free 
casein. When about .5% acid has developed in the milk, 
the free casein combines with the acid and forms casein 
lactate. 
The chemical composition of milk-sugar is C12H2201; +H20. 
When a perfect decomposition of milk-sugar into lactic acid 
takes place, the following equation would represent the 
change: 
(Milk-sugar) (Lactic acid) 
Ci2HesO12 a 4C3H,603. 
Such an ideal change, however, never takes place. In 
such a case, one gram of milk-sugar should produce one 
gram of lactic acid. In a number of experiments carried 
on by one of the authors of “ The Analysis of Cream During 
Different Ripening Stages,” * the highest amount of acid 
produced from one gram of milk-sugar was .8 of a gram. 
This indicates that there are always accompanying by-products 
produced, besides lactic acid, when milk-sugar is being decom- 
posed in cream or milk. The sourness of milk is due to this 
change. The by-products which accompany the production 
of lactic acid are many and various. The most important 
ones are gases of different kinds, such as carbonic acid gas 
(COz); marsh gas (CH4); hydrogen (H); and nitrogen (N). A 
* Thesis I. S. C., Ames, Ia, 
