206 MILK AND THE PUBLIC HEALTH CHAP. 



milk, all of which depend upon ascertaining the presence of 

 certain milk enzymes. There are a number of enzymes in 

 milk (see Chapter I.), but their thermal death points all appear 

 to lie between 65 C. and 80 C. Tests for the presence of 

 peroxidases are used to determine whether the milk has been 

 heated or not. The tests depend upon the fact that these 

 enzymes present in raw milk induce the oxidation of various 

 leuco compounds by hydrogen peroxide. A number of tests 

 may be used for this purpose, of which the following may be 

 specially mentioned. 



Guaiacum Test. Add a few drops of tincture of guaiacum 

 to half a test tubeful of milk. The formation of a green-blue 

 zone where the two liquids join, indicates a positive reaction. 

 The guaiacum tincture should be fairly recently prepared. 

 According to Arnold and Menzel, the tincture is best prepared 

 by dissolving the guaiacum resin in acetone. 



Starch's Test. To about 5 c.c. of milk in a test tube add 

 a drop of weak (0*2 per cent) solution of hydrogen peroxide 

 containing O'l per cent sulphuric acid, and 2 drops of a 2 per 

 cent aqueous solution of paraphenylendiamine hydrochloride. 

 Mix. A positive reaction consists in the rapid production of 

 a blue or dark-violet colour. The paraphenylendiamine hydro- 

 chloride does not keep well, and should be recently prepared. 

 If the milk is sour it must first be made alkaline with lime 

 water. 



Potassium Iodide Starch Test. To half a test tubeful of 

 milk add a few drops of a 5 per cent watery solution of 

 potassium iodide, a little starch solution, and a few drops of 

 dilute hydrogen peroxide. A positive reaction is shown by 

 the development of a blue colour. 



Ortol Test. To 5 to 10 c.c. of milk add a drop of 0'5 

 per cent ortol solution and 2 drops of dilute hydrogen peroxide 

 solution (0*2 per cent). A positive reaction is shown by the 

 presence of a mauve-red colour. The ortol solution should be 

 recently prepared. 



Kastle and Porch l found that a dilute solution of trikresol 

 acts as a sensitising agent in the peroxidase reaction of milk. 

 Phenol, /3-naphthol, etc., act similarly. They mixed 5 c.c. of 

 milk with 1 c.c. of 1 per cent solution of trikresol, 0'3 c.c. 



1 Journ. Biological Chemistry, 1908, iv. p. 301. 



