DETECTION OF ADDED COLORING MATTER 85 



tion of ferric chloride, a solution of salicylic acid produces a 

 characteristic violet colour, the intensity of which is somewhat 

 proportional to the amount of salicylic acid present. 



Hydrogen Peroxide. As hydrogen peroxide decomposes 

 into free oxygen and water soon after its addition to milk, it is 

 impossible to detect this substance by means of the usual 

 reagents. The oxygen liberated, however, considerably mod- 

 ifies the enzymes present, and it is upon this fact that several 

 inferential tests for detecting hydrogen peroxide are based. 

 The immediate reductase reaction (see p. 89) is destroyed by 

 hydrogen peroxide, and the catalase (see p. 91) destroyed in 

 proportion to the amount added. 



Before the hydrogen peroxide has decomposed it may be 

 detected by the peroxidase reaction (see p. 91). 



Hjrpochlorites. Although hypochlorites have been sug- 

 gested as milk preservatives they have not been extensively 

 used as the amoimt required to produce any appreciable effect 

 also adversely affects the taste and odour. Milk containing 

 hypochlorites does not give the usual starch-iodide reaction 

 even with as large a quantity as 50 parts of available chlorine 

 per 100,000. 



Detection of Added Colouring Matter. The following are 

 the provisionally official methods of the American Association 

 of Official Agricultural Chemists. 



Warm about 150 c.cms. of milk in a basin over a flame and 

 add about 5 c.cms. of acetic acid, after which slowly continue 

 the heating almost to the boiling point whilst stirring. Gather 

 the curd, when possible, into one mass by means of the stirring 

 rod, and pour off the whey. If the curd breaks up into small 

 flecks, separate from the whey by straining through a sieve or 

 muslin. Press the curd free from adhering liquid, transfer to a 

 small flask, and macerate for several hours (preferably over- 

 night) in about 50 c.cms. of ether, the flask being tightly corked 

 and shaken at intervals. The ether is finally decanted from the 

 curd and is examined for annatto, the curd being reserved for 

 the detection of aniline orange and caramel. 



