i MILK 97 



(1) Those based upon the changes which the protein under- 

 goes when milk is heated. 



(2) Those which start from the presence of oxidising enzymes 

 in the milk. 



It has been proved that only those reactions which belong 

 to the second category are applicable in practice. 



(a) Arnold's Gruaiacum Test. 



Arnold l was the first to utilise the oxidising power which 

 milk possesses to prove whether the milk had been heated or 

 not. For this purpose Arnold used tincture of guaiacum. If 

 the resin of the guaiac tree (Guaiacum officinale) is dissolved 

 in an organic solvent, or if the wood of the tree is extracted, 

 then either of these solutions becomes of a dark blue colour 

 under the action of oxidising materials. As was seen in the 

 Introduction (p. 12), cow's milk contains several ferments or 

 enzymes, and it is one of these peroxidase which causes 

 ordinary milk to turn blue on the addition of tincture of 

 guaiacum. If the milk, however, has been heated from 78 to 

 80 C., the enzyme is destroyed, and there is no coloration 

 with tincture of guaiacum. 



The test is applied as follows : A small quantity of milk is 

 put into a test-tube, and about ten drops of a 5 to 10 per cent, 

 tincture of guaiacum poured into it. Arnold and Menzel 2 say 

 the tincture of guaiacum is best prepared by dissolving the 

 resin in acetone. In a few minutes after the guaiacum has 

 been added to the milk a blue ring makes its appearance at 

 the junction of the two liquids, but if the milk has been heated 

 to 80 C. there is no reaction. Before making a test the solution 

 of guaiacum must always be tested to see if it reacts with 

 unheated milk, for the purchased tinctures often behave 

 irregularly. 



The peroxidase only oxidises in the presence of peroxides, and 

 the oxygen which is liberated from the peroxide by means of 

 the enzyme is able to oxidise any easily oxidisable substance, 

 In some tinctures of guaiacum organic peroxides are found as 

 a result of auto-oxidation, and such solutions cause the blue 

 colour to appear in milk. In other solutions again, particularly 



1 Jahresbericht der Koniglidien Tierdrztlichen Hochschule, Hannover, 

 18801882, p. 161. 

 - Mi/ch-Zeitnng, 1902, p. 247. 



H 



