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 C^H^On +H 2 O. 

 When a perfect decomposition of milk-sugar into lactic acid 

 takes place, the following equation would represent the 

 change : 



(Milk-sugar) (Lactic acid) 



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 

 (C0 2 ); marsh gas (CH 4 ); hydrogen (H); and nitrogen (N). A 



* Thesis I. S. C., Ames, la. 



