154 MODERN METHODS OF TESTING MILK 



in flakes of casein, the size of the flakes depending on 

 the acidity. There is no close relation between this 

 test and real acidity of milk, though commonly milk 

 containing more than 0.21 per cent, of acid gives a 

 marked precipitation with alcohol. 



Alizarol test for acidity. This is a modification of 

 the alcohol test but is more useful. The solution used 

 is prepared by adding to the alcohol small amounts 

 of alizarine (di-oxy-anthraquinone), the mixture being 

 shaken repeatedly until a perfectly clear solution of 

 dark reddish-brown color is formed. The solution is 

 of right strength when 2 cc., added to an equal 

 amount of fresh normal milk, gives an intense reddish 

 color, similar to lilac or red-clover blossoms. This 

 solution is very sensitive to acids, the color depend- 

 ing on the degree of acidity, as shown by the follow- 

 ing statement of Morres : With fresh, normal milk 

 having 0.16 per cent, of acidity, the color is lilac-red; 

 with o.i 8, pale-red; with 0.20, brownish-red; with 

 0.22, reddish-brown; with 0.25, brown; with 0.27, 

 yellowish-brown; with 0.31, brownish-yellow; with 

 0.36 or over, yellow. 



The degree of acidity is, to some extent, an indica- 

 tion of the age of milk, the temperature at which it 

 has been kept, the condition of care exercised in keep- 

 ing it clean, and of the abundance of lactic acid 

 bacteria. 



The acidity of market milk should usually be under 

 0.15 per cent, when it reaches the consumer, and 

 should not exceed 0.2. The same may be said of 

 milk that is intended for the purpose of cheese- 

 making. 



