282 GENERAL MICROBIOLOGY 



may be easily recognized, and insert the rack in the water- 

 bath. 



6. Pour water into the water-bath until the height of 

 the water corresponds to the height of the milk in the 

 tubes. 



7. Put aside two tubes of milk or cream from each 

 sample, one to be employed for comparative check-obser- 

 vation, and the other for check-plating against those which 

 will be subjected to pasteurization. 



8. Apply heat to the water-bath. 



9. At 50, 60, 70, 80, 90 and 100 C., remove two 

 tubes of each sample of milk or cream undergoing pasteuri- 

 zation and place in cold water. 



10. Employ one of the tubes thus removed for plating 

 and the other place at a temperature of 25 to 28 C. along 

 with the previous check-observation tube (7). 



11. Make two plates in litmus lactose agar from the tube 

 held for check-plating (7) and from one of the two tubes 

 removed at each of the temperatures designated above. 

 The remaining tube is to be left undisturbed and placed 

 at 25 C., to observe macroscopical changes. 



Dilutions for plating: 



Fresh milk, unpasteurized, 1 : 10 and 1 : 100. 



Milk twenty-four hours old, but sweet, unpasteurized, 

 1 : 10,000 and I : 1,000,000. 



Milk with an acidity of 22, unpasteurized, 1 : 100,000 

 and 1 : 10,000,000. 



Milk pasteurized at 50 C. (fresh) 1 : 10 and 1 : 100. 



Milk pasteurized at 50 C. (old) 1 : 10,000 and 

 1 : 1,000,000. 



Milk pasteurized at 60 C., 1 : 10 and 1 : 100. 



Milk pasteurized above 60 C., 1 : 10. 



12. Keep the plates at 25 C. for seven days, counting 

 colonies at the end of this time. 



13. Determine the character of the microorganisms 

 left after pasteurization with those before pasteurization 



