TESTING 261 



the number of bacteria in dairy products, and thus of 

 their quahties. Often when there is a question whether 

 certain milk or cream should be accepted by a creamery, 

 the quick determination of the acidity will settle the prob- 

 lem. The acidity test in the manufacture of butter is 

 used only on milk and cream. 



234. Sampling. — The ordinary precautions in sampling 

 milk or cream should be observed in obtaining samples for 

 an acidity test. 



235. Testing. — Following are steps in testing milk or 

 cream for acidity. 



1. Measure a Babcock pipetteful of milk or cream into 

 a titration cup or Erlenmeyer flask, add about 15 or 20 

 c.c. of water. In case of cream, use water at a tem- 

 perature of 100° to 120° F. to make the cream more fluid. 



2. Add two or three drops of phenolphthalein. This is 

 an indicator colorless in an acid solution and pink when 

 the solution is alkaline. 



3. Titrate with N/lO alkaline solution until a pink 

 color remains. Care must be observed in titrating, for 

 it is easy to go past the end point. 



4. Read by dividing the number of cubic centimeters 

 of N/lO alkaline solution required to neutralize the acid 

 in the milk or cream by 2. Then each cubic centimeter of 

 the quotient is equal to .1 per cent. Very often a special 

 pipette is used which has a capacity of one-half of the Bab- 

 cock pipette. In such case the reading is obtained directly. 

 Each cubic centimeter is equal to .1 per cent acid. 



236. Notes on the chemistry of acidity. — The chemical 

 reactions of acidity are clearly explained by Ross : ^ "It 

 is a chemical fact that equal volumes of acids and alkalies 

 of the same chemical strength will exactly neutralize one 



' Ross, H. E., A Dairy Laboratory Guide, p. 44, 1914. 



