TESTS USED IN CHEESE-MAKING 427 



position of the sugar in the milk. It is the amount of 

 •acid thus formed which we usually desire to determine. 



The method of ascertaining the acidity of milk is 

 based upon the chemical action taking place between 

 acids and alkalis. Acids and alkalis neutralize each 

 other and form compounds called salts, which are 

 neutral (neither acid nor alkaHne). A substance used 

 in showing whether a solution is acid, alkaline or 

 neutral is called an indicator. The one in most com- 

 mon use is a compound called phenolphthalein, which 

 turns pink in alkali solutions and colorless in acid or 

 neutral solutions. Only a few drops need be used in 

 making one test. There are several different methods 

 for testing acidity, but all are alike in principle. 



Publow's acid test. — (i) The apparatus (p. 422) 

 consists of (i) a plain 5-pint bottle with an opening 

 in the bottom, through which a brass pipe is connected 

 so securely as to prevent leakage. (2) A small 2-ounce 

 wash-bottle, fastened to the neck of the large bottle 

 by a copper band and connected by means of rubber 

 corks and glass tubing. (3) A plain 10 cc. burette 

 graduated in tenths and a simple wire burette-holder. 

 (4) A straight, non-bulbous, 9-gram pipette, which 

 can be easily cleaned. (5) A simple rubber-stoppered 

 dropping-bottle. (6) A plain white cup and stirring- 

 rod. (7) A small bottle containing 50 cc. of a solu- 

 tion of caustic soda (equal to 9.2 grams of purest 

 caustic soda), which, when added to 2,250 cc. of water, 

 makes 2,300 cc. of a tenth-normal alkali solution. The 

 large bottle is marked to show the level of 2,300 cc. in 

 order to save time in measuring. (8) A small bottle 

 of phenolphthalein indicator. 



(2) Preparing alkali solution. — As the most ac- 

 curate, convenient and economical method of prepar- 



