594 TESTING MII,K AND CREAM FOR ACID 



which is so prepared that one liter shall contain the hydrogen equiv- 

 alent of the active reagent weighed in grams (Sutton). 1 



In other words, a normal solution of an acid is a solution 

 which contains in one liter as much of the active reagent, the acid, 

 as is represented by the molecular weight of the acid in grams. In 

 the case of lactic acid (C 3 H 6 O 3 ) the molecular weight is 90. 



A normal solution of an alkali is a solution which contains in 

 one liter as much of the active reagent, the alkali, as is represented 

 by the molecular weight of the alkali in grams. In the case of 

 caustic soda or sodium hydroxide (NaOH) the molecular weight is 

 40. Since equal amounts of normal solutions of acids and alkalies 

 neutralize each other, 1 c.c. of N Na OH (1 c.c. of a normal solu- 

 tion of caustic soda or sodium hydroxide) neutralizes 1 c.c. of N C 3 

 H 6 O 3 (1 c.c. of a normal solution of lactic acid). 



The acid contained in milk, cream and starter is largely lactic acid 

 and is always calculated as such. The alkali used for determining 

 the per cent acid is sodium hydroxide (NaOH), commonly known 

 as caustic soda. In order to know when a liquid is alkaline and when 

 it is acid, or when enough alkali has been added to reduce all the 

 acid and render the liquid neutral, a color indicator is used that shows 

 different color reaction in acids and in alkalies. Of the several indi- 

 cators available, that known as phenolphthalein is most commonly 

 and most suitably used in acid tests of milk and cream. In acids, 

 such as sour milk, sour cream and starter, phenolphthalein is color- 

 less similar as water. In alkaline solutions, such as sodium hydrox- 

 ide, phenolphthalein turns to a deep pink color. Hence, when 

 enough sodium hydroxide has been added to milk, cream or starter, 

 to neutralize all the acid, these products turn pink. The moment a 

 permanent faint pink color appears, the test is completed and the 

 number of cubic centimeters of sodium hydroxide needed to produce 

 this permanent faintly pink tint, serves as the basis upon which the 

 per cent acid is calculated. 



In order to augment the sensitiveness and accuracy of the test, 

 alkaline solutions, much weaker than a normal solution, are used. 

 In most tests the alkaline solution is a tenth normal solution of 



N 

 sodium hydroxide (^- NaOH). 



The alkaline solution must be of standard strength, it must be 

 accurate, otherwise the test cannot be dependable in its results. It 



1 Farrington & Woll, Testing Milk and Its Products. 



