xii CHEMICAL TESTS OF MILK 209 



acid. The red changes to a blue-green colour on the addition 

 of dilute alkali solution. 



Formaldehyde. — -Hehner's test is simple and reliable. To 

 about 10 c.c. of the milk in a test tube add about half its 

 volume of concentrated commercial sulphuric acid, pouring 

 the acid slowly down the side of the tube. A violet zone at 

 the junction of the two liquids shows the presence of formal- 

 dehyde. 



The Massachusetts^ test is also reliable. In this test 

 commercial hydrochloric acid (sp. gr. 1'2) containing 0"2 per 

 cent of 10 per cent ferric chloride solution is used. Equal 

 (10 c.c.) quantities of this solution and of the milk are mixed 

 in a porcelain dish and the mixture slowly heated nearly to 

 boiling. The presence of formaldehyde is shown by a violet 

 coloration, the degree of colour varying with the amount 

 present. This test is said to be able to detect 1 part of 

 formaldehyde in 250,000 parts of milk. 



Another useful test, said to be able to detect 1 in 200,000 

 parts of formaldehyde, is the gallic acid test. It is carried out 

 by adding 2 c.c. of normal sulphuric acid to 30 c.c. of milk and 

 distilling over 5 c.c. ; O'i c.c. of a saturated solution of gallic 

 acid in absolute alcohol is added, and 4 c.c. of concentrated 

 sulphuric acid is run into the mixture so as to form a separate 

 layer. The presence of formaldehyde is shown by a green zone, 

 gradually changing to blue, at the junction of the liquids. 



Benzoic Acid. — Leach ' gives the following method :- — 

 Shake 5 c.c. of hydrochloric acid with 50 c.c. of the milk in 

 a flask. Then add 150 c.c. of ether, cork the flask and shake 

 well. Break up the emulsion which forms by the aid of a 

 centrifuge, or in the absence of a centrifuge extract the curdled 

 milk by gently shaking with successive portions of ether, 

 avoiding the formation of an emulsion. A volume of ether 

 largely in excess over that of the curdled milk has been found 

 to be less apt to emulsionise (the ether may be readily recovered 

 by distillation). Transfer the ether extract to a separatory 

 funnel, and separate the benzoic acid from the fat by shaking 

 out with dilute ammonia, which takes out the former as 

 ammonium benzoate. Evaporate the ammonia solution in a 



1 Annual liejjort, Massachusetts State Board of SecUth. 1897, p. 558. 

 - Food I/ispcc/ion and Analysis, 1909. 



