"94 Dairy Bacteriology. 



as the curdling of milk by rennet is dependent upon the 

 presence of these salts, their absence in heated milks retards 

 greatly the rennet action . The character of the coagulum 

 is also considerably different from what it is with raw 

 milk, being much softer and less tenacious. 



5. Hydrogen peroxid test (Storch's test). When a 

 small quantity of potassium iodid and starch is added 

 to milk and the same then treated with a few drops of 

 dilute hydrogen peroxid, normal milk will turn blue, 

 while that heated to 175 F. or above, will not change. 

 This reaction is due to the fact that the inherent enzymes 

 normally found in milk are destroyed by this degree of 

 heat, and therefore the hydrogen peroxid is not decom- 

 posed 1 . 



93. Sterilization. To thoroughly sterilize milk, it is 

 necessary to render it germ free. So far as the vegetating 

 bacteria are concerned, this can be done by an applica- 

 tion of heat such as is used in pasteurizing, but on ac- 

 count of the presence of spore-bearing forms, a much 

 higher temperature is necessary. To render milk per- 

 fectly sterile, it is necessary to heat it for about two 

 hours at 250 F., or thirty minutes at 270 F. At the 

 temperature of boiling- water, milk often withstands, es- 

 pecially during the summer months, a prolonged steam- 

 ing for five to six hours. 



Another method of sterilizing is that used in what is 

 k:nown as intermittent sterilization, a method employed 

 by Dahl, of Norway. This consists in heating the milk 

 to a temperature fatal to the contained bacteria, 158 F. 

 or above, for one-half hour in order to kill the vegetating 

 bacteria. The milk is then placed under conditions that 

 will cause the latent spores to germinate. After this oc- 



1 V. Storch, 4.0th Kept. Danish Expt. Stat., Copenhagen, 1898. 

 Babcock and Russell, 15th Kept. Wis. Expt. Station, p. 84, 1898. 



