METHODS OF EXAMINING MILK 299 



(176 F.), a blue ring is formed at the point of contact 

 of the two fluids. If the milk has been heated to 80 C. 

 ( 176 F. ) or above, no color change occurs. The guaiac- 

 wood tincture is more reliable than the guaiac-resin tinc- 

 ture of the U. S. Pharmacopoeia. The addition of a few 

 drops of a weak aqueous solution of hydrogen peroxide 

 after the tincture of guaiac has been run into the test 

 tube increases the reliability and sharpness of the test 

 (Zinc), although if the guaiac tincture is "ripe," a 

 change which takes place with age, the hydrogen peroxide 

 is not necessary. When the test is made with the tinc- 

 ture alone, the activity of the latter should be tested 

 against known raw milk. If the hydrogen peroxide is 

 added to the milk before the guaiac tincture, or if too 

 much is added, the reaction will be retarded or suppressed. 

 When milk is strongly acid it will not give the color 

 reaction at all, or the reaction will be faint or delayed. 

 Lime water should be added to such milk and to butter- 

 milk before testing. Cold milk may not react at all 

 or very slowly. An excess of hydrogen peroxide will pre- 

 vent the reaction. Heated milk to which formalin has 

 been added will give a color reaction to the Storch test 

 like raw milk, but formalin has no influence on the guaiac 

 test when it is present in the quantity used to preserve 

 milk. The guaiac test is also not affected by sodium 

 bicarbonate, borax, boric acid, and salicylic acid, but the 

 presence of potassium bichromate causes heated milk to 

 react like raw milk and intensifies the color reaction of 

 raw milk. 



EXAMINATION FOR DIRT 



The milk is filtered through small cotton discs and 

 the larger particles of dirt present are collected on the 

 surface of the disc. There are several different forms of 



