16 Testing Milk and Its Products. 



the milk sugar as they grow, and decomposing it into 

 lactic acid. When this change alone occurs, there is 

 not necessarily a loss in the nutritive value of the milk, 

 since milk sugar breaks up directly into lactic acid. 

 This is shown by the following chemical formula : 



CiaH^OnHaO (lactose) =4 C 3 H 6 3 (lactic acid)* 



Ordinarily the souring of milk is, however, more 

 complicated, and other organic bodies, like butyric acid, 

 alcohol, etc., and gases like carbonic acid 'are formed, 

 resulting in a loss in the feeding value of the milk. 

 While sour milk may therefore contain a somewhat 

 smaller proportion of food elements than sweet milk, 

 it will generally produce better results when fed to 

 farm animals, especially pigs, than is obtained in feed- 

 ing similar milk in a sweet condition. The cause of this 

 may lie in the stimulating effect of the lactic acid of 

 sour milk on the appetites of the animals, or in its aid- 

 ing digestion by increasing the acidity of the stomach 

 juices. 



That the souring of milk is due to the activities of 

 bacteria present therein is shown clearly by the fact 

 that sterile milk, i. e., milk in which all germ life has 

 been killed, will remain sweet for any length of time 

 when kept free from infection. 



The amount of milk sugar found in normal cow's 

 milk varies from 3.5 to 6 per cent., the average content 

 being about 5 per cent.; in sour milk this content is 

 decreased to toward 4 per cent. 



1 One molecule of milk sugar is composed of 12 atoms of carbon (C), 

 22 atoms of hydrogen (H), 11 atoms of oxygen (O), and one molecule 

 of water (H 2 O). In the same way, the lactic acid molecule consists of 3 

 atoms of carbon, 6 atoms of hydrogen, and 3 atoms of oxygen. 



