l6o MODERN METHODS OF TESTING MILK 



100 to 104 F. This method is more sensitive, de- 

 colorization taking place more quickly. Milk in 

 which the blue color disappears in 15 minutes is re- 

 garded as unfit for use. The samples of milk are al- 

 lowed to remain at the same temperature for the 

 fermentation test and, at the end of 24 hours, the con- 

 ditions are observed as in connection with the Wiscon- 

 sin fermentation test (p. 155). This method gives use- 

 ful evidence in regard to the abundance of living 

 organisms present as well as the kinds of organisms 

 present. 



Catalase test. Normal fresh milk has the property 

 of decomposing hydrogen peroxide into free oxygen 

 gas and water. This is believed to be due in part to 

 the presence of white blood corpuscles (leucocytes), 

 which are always present in milk, and in part to the 

 action of micro-organisms. Substances in milk having 

 the power to decompose hydrogen peroxide are known 

 under the general name of catalase. It increases with 

 the age of milk and is present more abundantly in milk 

 of diseased cows. 



The amount of catalase in milk can be estimated 

 by treating a certain amount of milk with a certain 

 amount of hydrogen peroxide under given conditions 

 and measuring the free oxygen that is given off. Sev- 

 eral forms of apparatus have been devised for the pur- 

 pose. 



The test is performed as follows: In a small fer- 

 mentation tube or other suitable apparatus, we mix 

 15 cc. of milk and 5 cc. of i per cent, hydrogen per- 

 oxide; then place the apparatus at a temperature of 

 77 to 86 F. The amount of gas formed at the end 



