268 PRACTICAL DAIRY BACTERIOLOGY 



Place in some dish in which the mixture may be boiled, and 

 carefully weigh the dish and its contents. Dissolve the mixture 

 at a temperature of about 150 F. (60 C), after which weigh 

 again and restore the evaporated water by bringing the weight 

 to its original mark. 



Bacteria culture media need to have a proper grade of acidity. 

 If the amount of acid is either above or below the desired point 

 the bacteria either do not grow at all or not very readily. The 

 method of bringing the bouillon to the desired point is some- 

 what complicated and must be described in detail. 



a. Measure out five c.c. of the bouillon, place it in an evap- 

 orating dish with ten times its bulk of water, and bring nearly 

 to a boil. While still hot add to it a few drops of phenolthalein. 1 

 This is a solution which is colorless so long as it is acid, but 

 turns red as soon as it becomes alkaline. The bouillon is always 

 acid and consequently the phenolthalein will make no change in 

 its appearance. 



b. Place the evaporating dish under a burette containing i/io 

 normal NaOH. Take the reading of this solution carefully 

 in the burette. Allow the solution to drop from the burette 

 into the evaporating dish, drop by drop, stirring carefully. (See 

 Fig. 60.) As long as the material remains acid no color will 

 appear. Allow the solution to drop into the dish until a faint 

 red color makes its appearance and does not disappear upon stir- 

 ring. When this point has been reached the contents of the 

 evaporating dish are at a neutral point. 



c. Taking the reading for the burette, determine how many 

 cubic centimeters of the solution have been used to neutralize 

 the 5 c.c. of bouillon. Divide by 5 to determine the amount 

 necessary to neutralize I c.c. and multiply by 995 to determine 

 the amount necessary to neutralize the rest of the bouillon. 



d. Add to the entire quantity of bouillon enough NaOH to 

 neutralize it, but in doing so use normal NaOH instead of i/io 



1 8 per cent, dry phenolthalein in 50 per cent, alcohol. 



