90 EXPERIMENTAL DAIRY BACTERIOLOGY 



rapidly to the surface. If the clusters are broken up and each 

 globule is separated from every other, the creaming is much 

 less rapid. Heating under constant agitation breaks up the 

 aggregates of fat globules and injures the creaming power of 

 the milk. If the individual fat globules are broken into minute 

 fat droplets, as in the case of the so-called "homogeneous 

 milk," the fat cannot be removed, even by passing the milk 

 through a cream separator. The viscosity of cream is dimin- 

 ished when heated on account of the breaking up of the 

 aggregations of fat globules. 



Milk contains a substance whose nature is unknown, 

 which has the power of decomposing hydrogen peroxide. 

 If the milk is heated to temperatures above 80 C., it loses 

 this power. This property is made use of in the Storch test 

 for heated milk. 



In commercial practice two methods are used for the 

 pasteurization of milk. They do not differ in principle, but 

 simply in the time and temperature at which the milk is 

 heated. In the intermittent or discontinuous method the 

 milk is heated at temperatures from 60 to 70 C. for twenty 

 or thirty minutes, either in bottles or in a container, so ar- 

 ranged that the milk can be heated rapidly and stirred dur- 

 ing the process. In the continuous method the milk is heated 

 almost instantaneously to 70-90 C., maintained there a few 

 seconds, and rapidly cooled. Many types of apparatus have 

 been devised for this purpose, none of which, however, are 

 wholly satisfactory. 



Exercise. EXAMINATION OF MILK PASTEURIZED IN A DISCONTIN- 

 UOUS MACHINE. Each student will procure in a sterile container a 

 sample of raw milk and also a sample of the same milk after it has 

 been pasteurized by the intermittent method at 65 C., and subse- 

 quently cooled. Quantitative lactose-agar plates should be made 



