36 



This simple formula can be used in cases not provided 

 for in the table. 



DETERMINATION OF ACIDITY OF MILK AND 

 CREAM. 



Acidity in milk is attributable to two causes, (1) the pres- 

 ence in milk of acid phosphates and perhaps of carbon 

 dioxid, and (2) lactic and other acids produced by the 

 decomposition of the milk sugar by bacterial action. 

 When freshly drawn milk is acid to phenolphthalein, this 

 acidity is from 0.07 per cent to 0.08 per cent and is owing 

 to causes given under (1). Lactic acid is not present in 

 freshly drawn milk; it develops only on standing. Milk 

 is not sour to the taste until it has a total acidity of at 

 least 0.3 per cent. 



For convenience the total acidity of milk is usually 

 calculated as lactic acid. The principle upon which the 

 determination of acidity is based is the well-known 

 chemical action of acids upon alkalies. To illustrate, the 

 action of hydrochloric (sometimes called muriatic) acid 

 on a solution of caustic soda may be taken. This acid 

 has a sharp and very sour taste, while caustic-soda solu- 

 tions have a soapy feel and a peculiar odor, and if suffi- 

 ciently strong will attack the skin. If the solution of 

 caustic soda is slowly added to the hydrochloric acid, the 

 sour taste will gradually disappear until the exact point of 

 neutrality is reached, when a new substance is produced 

 sodium chlorid, or common salt, which has neither the 

 acid properties of the one not the alkaline properties of 

 the other. The sense of taste, however, is not sufficiently 

 sensitive to determine when the exact point of neutrality 

 has been reached. Phenolphthalein is an organic com- 

 pound, having the property, when in solution, of turning 

 pink with alkalies and remaining colorless with acids. 

 Such a substance is called an indicator because it indicates 

 by a color change when a certain chemical reaction has 

 taken place. 



There are several so-called acid tests before the public. 

 The one known as Manna's acidity test is widely used and 

 is conducted as follows: 



