8 April, 1908.] 



A Xezv Reaction for Milk. 



253 



allowing it to run down the side of the tube instead of mixing with the 

 milk. In the paraphen\ lendiamine test the colouration produced is more 

 or less dirty slate-blue. We have found that the depth of the blue 

 colouration produced by the Benzidin test in milk heated to any tempera- 

 ture up to 75 deg. C. is intense, and have made comparisons with other 

 methods including the paraphenylendiamine and guaiacum tests to deter- 

 mine at what temperature milk ceases to gi\e the reaction. The reactions 

 obtained by these substances with milk heated to various temperatures be- 

 tween 70 and 80 deg. C. are summarised below : — 



Thus it is seen that, as in the other methods, the reaction ceases to be 

 given by milk heated to above 78 deg. C. 



We also prepared mixtures of fresh milk and boiled milk and deter- 

 mined the proportion of raw milk detectable in boiled milk by the Benzidin 

 process and found that with 15 per cent, of raw milk in boiled milk the 

 colour reaction is quite positive. When the proportion is 10 per cent, 

 the reaction is less distinct and 5 per cent, of raw milk no longer gives 

 a colouration. 



The advantages of the Benzidin test are : — 



1. The reaction is more reliable than either the guaiacum or 



potassium-iodide test. 



2. It is more sensitive and more intense than the paraphenylendia- 



mine test. 



3. The cost of Benzidin is only one-eighth that of paraphenylendia- 



mine, and one-third that of potassium iodide the reagent 

 favoured by various authors as being cheaper than parapheny- 

 lendiamine. 



4. The alcoholic solution of Benzidin when stored in a stoppered 



bottle will keep for a long time. A solution prepared in 

 October, 1907, was still usable in March. 1908, after a period 

 of five months. 



The usefulness of Benzidin as a test for unboiled milk may be extended 

 by reversing the reaction and applying it as a very sensitive test for 

 hydrogen peroxide. Unboiled milk is treated with the Benzidin solution 

 and acetic acid and the solution to be tested for hydrogen peroxide added. 

 In presence of a trace of hydrogen peroxide an intense blue colouration is 

 immediately produced. 



We have used this method in the Government Laboratory of Victoria 

 for the past twelve months in the course of routine milk examinations. 



