80 



A MANUAL OF BACTERIOLOGY 



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2. Mix in the apparatus 15 cc. of milk and 5 cc. of dilute 

 hydrogen peroxide. The dilute hydrogen peroxide should con- 

 tain one part H^O^ in 100 parts of 

 water. Shake the apparatus to insure 

 thorough mixing, and place it in a 

 water bath having a temperature of 

 22° C. After two to twelve hours 

 read off the amount of gas evolved. 



3. Repeat, using separator slime, 

 colostral milk, and boiled milk. 



Exercise 116. Relation of Bacteria to the 

 Normal Souring of Milk 



1. Secure six samples of milk, three 

 of raw milk and three of pasteurized 

 milk. Place a sample of each in (a) 

 refrigerator, (5) laboratory cupboard 

 (near the floor if possible), and (c) in- 

 cubator (temperature 37° C). 



2. On the day of installation and on 

 each succeeding day pour litmus-lactose- 

 agar plates from each of the samples. 

 Record differential counts of both acid- 

 forming and non-acid-forming colonies 

 as far as possible. 



3. As the number of bacteria increases make determinations of 

 the increasing acidity of the milk. Remove 5 cc. of milk from 

 each sample with a sterile pipette. Add a few drops of phenol- 

 phthalein and titrate with N/20 NaOH. Contiuue the determi- 

 nations until there is no further increase in acidity. 



4. Express results by plotting curves to show (a) increase in 

 acidity, (6) increase in acid-forming bacteria, (<?) total increase 

 in bacteria, for each kind of milk used. 



The percentage of acidity may be computed by the following 

 formula : 



Fig. 34. Apparatus to dem- 

 onstrate catalase 



The corked test tube, containing 



milk and hydrogen peroxide, 



stands in water 



Per cent acidity = 



cc. alkali used x .0045 

 cc. of milk tested 



X 100. 



