86 DAIET CHEMISTRY 



decomposing products, the utmost cleanliness should 

 be practiced in order to prevent its formation. 



78. Fibrin in Milk. — Another nitrogenous com- 

 pound in milk is iibrin, which is also present in blood 

 and forms the " clot." The best proof of the presence 

 of fibrin in milk is the microscopic appearance of the 

 fat globules. As previously stated, the fat globules 

 always appear in little groups or colonies. It is sup- 

 posed that they are held together by bands or meshes 

 of fibrin which tend to prevent their coming to 

 the surface in the gravity creaming process. The 

 fibrin in milk is about the same in amount as the 

 urea. Fibrin is produced by the action of the fibrin 

 ferment. The chemical tests for its determination 

 in milk are unsatisfactory. 



79. Gases in Milk. — The gases which are dis- 

 solved in the milk as it comes from the cow are : 

 nitrogen, oxygen, and a small amount of carbon 

 dioxide. Nitrogen and oxygen are found in the 

 milk in about the same proportion as in pure spring 

 water. As milk gets older, the oxygen decreases 

 and the carbon dioxide increases. Carbon dioxide 

 is the gas which is given off in respired air ; in stale 

 milk the oxygen has been used up to form carbon 

 dioxide. At the end of four or five days, 90 per 

 cent of the gas in milk is carbon dioxide. There is 

 always more gas in old than in fresh milk. When 

 the milk becomes saturated with gas, the gas is 

 given off ; and as it escapes from the surface of cream 

 or thick milk, it leaves small holes on the surface. 



