128 Milk and Its Products 



Place in the agate cooking -pan and weigh. Dissolve 

 carefully at a temperature not over 150 F. (60 C.) 



It has been found that organisms develop best at 

 a reaction slightly but distinctly acid. That is, the 

 media should be -\- 1 per cent to phenol phthalein. In 

 order to standardize the media to this degree of 

 acidity, take 5 c. c. of bouillon, and place in a por- 

 celain evaporating dish or white cup with about 45 c. c. 

 of warm water. Add a few drops of phenol phthalein 

 (8 per cent dry in 50 per cent alcohol). This is a 

 solution which is colorless when acid, but tinted pink 

 when alkaline. Stir and add to the solution in the 

 cup enough of the twentieth normal sodium hydroxid 

 solution from a burette to give it a clear, bright pink 

 color. This is the amount needed to neutralize 5 c. c. 

 of the bouillon. In order to bring the entire amount 

 to +1 per cent, subtract 1 from the amount of twen- 

 tieth normal sodium hydroxide used; multiply this by 

 the number of cubic centimeters of bouillon, and 

 divide by 100. This result represents the amount of 

 normal sodium hydroxide to be added. After adding 

 the normal hydroxide,' test the reaction again. It 

 should take 1 c. c. of twentieth normal sodium hydrox- 

 ide to bring a pink color with 5 c. c. of bouillon. 

 This process is called titration. 



After obtaining the desired acidity, boil briskly for 

 twenty minutes, restore weight, with distilled water, 

 cool and filter through filter paper into sterile flasks. 

 The bouillon is now ready to be made into either gela- 

 tine or agar, or it can be sterilized and set aside 

 until needed. (See directions for sterilizing media.) 



