80 



A MANUAL OF BACTERIOLOGY 



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 2 O 2 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 () 

 refrigerator, (b) 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. Continue the determi- 

 nations until there is no further increase in acidity. 



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

 acidity, (/>) increase in acid-forming bacteria, (c) 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. 



