l62 MODERN METHODS OF TESTING MILK 



two drops of a 2 per cent, solution of para-phenylene- 

 diamine hydrochloride. The mixture -is well shaken. 

 If the milk has not been heated at all or to a tempera- 

 ture not above 172 F., an intensely blue color appears. 

 If, however, after half a minute a clear, grayish-blue 

 color appears, the milk has been heated up to 175 F. or 

 higher. The reaction is of sufficient delicacy to detect 

 10 per cent, of milk heated below 172 F. in a lot of 

 milk pasteurized above this temperature. 



The solution of para-phenylene-diamine hydrochlor- 

 ide does not keep longer than 2 months and a new pre- 

 paration .should be made. 



The test can be applied to cream or to butter. Melt 

 about 10 grams of butter in a cup or beaker, heating 

 to 105 to 115 F., add 10 cc. of warm water, 2 drops 

 of 0.3 per cent, hydrogen peroxide solution and sev- 

 eral drops of para-phenylene-diamine solution. Pour 

 the mixture into a cream-testing bottle, shake vigorous- 

 ly and whirl in centrifuge I minute. In the watery 

 solution that separates from the fat, a blue color ap- 

 pears if the cream from which the butter was made 

 was not pasteurized. 



Guaiac or Arnold test for heated mik. To a small 

 amount of milk in a cup or test-tube are added 5 or 

 10 drops of guaiac tincture (preferably a 5 or 10 per 

 cent, solution prepared by dissolving guaiac resin in 

 acetone) and the mixture shaken. In case of milk 

 heated to less than 176 F., a blue color appears after 

 some minutes ; if it has been heated above 176 F., 

 no coloration appears. The precaution should always 

 be taken to test the guaiac solution with unheated milk, 

 because some solutions produce no coloration even in 



