238 MILK 



incubated at 37° C. for one hour. If the milk was heated to 

 62° C. for one minute or more a small amoimt of oxygen will 

 collect in the closed arm, but if heated to 62° C. for one hour, 

 or to 66° C. for thirty minutes, or to 68° C. for one minute, no 

 oxygen is liberated. The test is vitiated if heated milk is inocu- 

 lated with bacteria or if a small amount of raw milk has been 

 added. 



The following tests also are designed to indicate heated milk: 



Starch's Test. — 20 c.c. of milk are placed in a container and 

 10 drops 3 per cent, hydrogen peroxid and 1 c.c. paraphenylen- 

 diamin-chlorid solution (2 per cent, aqueous) added. The reac- 

 tion is negative if milk has been heated to 80° C. or above for a 

 short time. Below this temperature a blue color develops unless 

 the milk has been heated for five minutes at 75° C, or for fifty 

 minutes at 70° C. Preservatives in milk vitiate the reaction. 



Arnold's Reaction. — Guaiac resin is dissolved in alcohol or 

 acetone to make a 5 or 10 per cent, solution, and 0.5 c.c. of the 

 solution mixed with 5 c.c. of the milk to be tested. If the mUk 

 has been heated to 72° C. for fifteen minutes the reaction is nega- 

 tive. Heated to 75° C. for five minutes and at 76° C. for one min- 

 ute the reaction is also negative. A positive reaction is indicated 

 by formation of a blue color. 



Instead of a solution of guaiac resin an aqueous solution of 

 crystallized guaiacol (1 per cent.) can be used. 



Rothenfusser Reaction. — One gram paraphenylendiamin-chlorid 

 is dissolved in 15 c.c. of water. Two grams guaiacol are dissolved 

 in 135 c.c. of alcohol. The two solutions should be preserved 

 separately and mixed immediately before using. To 100 c.c. of 

 milk are added 6 c.c. of a solution of subacetate of lead, and the 

 mixture shaken and filtered. To the filtrate are added 2 drops 

 hydrogen peroxid (3 per cent.) and 2 drops of the reagent. Raw 

 milk turns violet, but if heated above 80° C. remains white. The 

 color should appear in one to two minutes. This test is more 

 delicate than Storch's test and is not influenced by formalin, boric 

 acid, and salicylic acid when one of these preservatives has been 

 used. If boiled milk has been contaminated the reaction remains 

 negative. 



Benzidin Test. — In presence of hydrogen peroxid benzidin is 

 changed by peroxydase action into an azo-compound and a blue 

 color is produced. Execution of the test: Prepare an alcoholic 

 solution of benzidin, 4 parts benzidin in 100 parts alcohol; add 

 2 c.c: of this solution to 10 c.c. of the milk, and then 2 to 3 drops 

 of 30 per cent, acetic acid. Then add carefully without shaking 

 a few drops hydrogen peroxid. If the milk was heated above 

 79° C. no color appears. 



