I $8 LABORATORY GUIDE IN BACTERIOLOGY 



SECTION 6 



A COMPARATIVE STUDY OF THE EFFECTS OF PAS- 

 TEURIZATION AND SO-CALLED STERILIZA- 

 TION OF MILK 1 



. i. Mix three quarts of raw milk and pour plates 

 to determine the number of bacteria and the number 

 of acid-forming bacteria. 



2. Divide the mixed milk into three parts. Heat 

 one-third for 20 minutes at 65 C., boil one- third for 

 several minutes, and place all parts in an ice chest. 



3. As soon as cooled, prepare plates of the milk 

 heated to 65 C. (pasteurized) and of the boiled milk. 

 Note the "cooked" taste and odor of the boiled milk. 



4. Check the keeping qualities of the three kinds of 

 milk by preparing plates on three successive days, 

 and after that every other day for six days. 



It is of importance to make differential counts and 

 determine the ratio of acid-forming and non-acid- 

 forming bacteria in all plates. 



SECTION 7 



A QUALITATIVE AND QUANTITATIVE STUDY OF 

 ANAEROBES IN MILK 



1. Secure a sample of raw milk, one of pasteurized 

 milk, and one of certified milk. 



2. Prepare dilutions of the raw milk i : 10, i : 100, and 



1 The term " sterilized milk " is commonly used for boiled milk. 

 In a bacteriological sense boiled milk is not always sterile, some 

 of the spores being able to survive boiling. 



