THE TESTING OF MILK AND CREAM 147 



tion of the ether, the benzoic acid is left behind in almost 

 pure state, the only impurities being small quantities of fat 

 or ash. 



The benzoic acid which is obtained is dissolved in a small 

 quantity of warm water, a drop of sodium acetate and neutral 

 ferric chloride added, and the red precipitate of benzoate of 

 iron indicates the presence of the acid. (Milk and Dairy 

 Products, Barthel; translated by Goodwin, p. 121.) 

 Detection of heated milk — Storch's method. 



Five c.c. of milk are poured into a test tube ; a drop of weak 

 solution of hydrogen dioxide (about 0.2 per cent), which con- 

 tains about 0.1 per cent sulfuric acid, is added, and two drops 

 of a 2 per cent solution of paraphenylendiamin (solution should 

 be renewed quite often), then the fluid is shaken. If the milk 

 or the cream becomes, at once, indigo blue, or the whey violet 

 or reddish brown, then this has not been heated or, at all events, 

 it has not been heated higher than 78° C. (172.5° F.) ; if the 

 milk becomes a light bluish gray immediately or in the course 

 of half a minute, then it has been heated to 79° to 80° C. (174.2° 

 to 176° F.). If the color remains white, the milk has been 

 heated at least to 80° C. (176° F.). In the examination of sour 

 milk or sour buttermilk, limewater must be added, as the 

 color reaction is not shown in acid solution. 

 Arnold's guaiac method. 



A little milk is poured into a test tube and a little tincture of 

 guaiac is added, drop by drop. If the milk has not been heated 

 to 80° C. (176° F.), a blue zone is formed between the two 

 fluids ; heated milk gives no reaction, but remains white. The 

 guaiac tincture should not be used perfectly fresh, but should 

 have stood a few days and its potency have been determined. 

 Thereafter it can be used indefinitely. These tests for heated 

 milk are only active in the case of milks which have been heated 

 to 176° F. or 80° C. (Jensen's Milk Hygiene, Pearson's transla- 

 tion, p. 192.) 



