120 LABORATORY GUIDE IN BACTERIOLOGY 



comparatively safe and less subject to the ordinary 

 fermentative changes. 



1. Sterilization at ioo° for jo minutes. — Such milk, 

 if chilled and kept at a low temperature, will remain 

 unchanged for more than a week; but, by the heating, 

 certain alterations have been produced in its taste and 

 nutritive qualities which render it supposedly less fit for 

 food than when pasteurization is employed. 



2. Pasteurizing milk. — The changes occurring in 

 milk, as above mentioned, begin at about 82°. Sterili- 

 zation at a low temperature is accomplished by raising 

 the temperature to only 75° for a period of 20 minutes. 

 This has been shown to be sufficient to kill the germs of 

 typhoid, cholera, diphtheria, and pyogenic cocci. Spored 

 organisms are not killed. As shown by Theobald 

 Smith, tubercle bacilli, when suspended in distilled 

 water, physiological salt solution, broth, and milk, are 

 destroyed at 60° in 15-20 minutes; but, if milk contain- 

 ing tubercle bacilU has its surface exposed to the air when 

 heated to 60°, the pellicle which forms on its surface 

 may contain living tubercle bacilli after an exposure of 

 60 minutes. 



Study of the effect of the above two methods of steri- 

 lization as compared with each other and with unsteri- 

 lized milk: 



1. From the fresh milk provided make three agar- 

 plates, using i, 2, and 3 loopfuls, respectively. 



2. Fill about 10 c.c. into each of ten sterile culture- 

 tubes, and keep one at room temperature and one in the 

 thermostat. 



3. Treat four of these tubes in the following manner: 

 Place water in a saucepan sufficient to cover completely 



