176 NORMAL MILK : ITS ADULTERATIONS, ETC. 



The colour of the milk itself is no guide, as it is frequently 

 artificially coloured to give it an appearance of richness. Annatto 

 was the colouring-matter chiefly used, but this is now somewhat 

 largely replaced by coal-tar colours, especially the sodium salt 

 of di-methyl-amino-azo-benzene sulphonic acid or methyl orange. 

 Artificial colouring-matters generally may be detected by pre- 

 cipitating the casein with acetic acid, washing well with water, 

 and digesting with strong alcohol ; the casein carries down the 

 colouring-matter and gives it up to the alcohol ; on evaporating 

 this, and taking up with a little water, the colour can be detected. 

 Annatto is unchanged by mineral acids, while many of the coal- 

 tar colours turn pink. 



The following method for the detection of colouring-matters 

 in milk is based on a scheme devised by M. Wynter Blyth : — 



Preliminarij Tests. — (1) Allow a portion of the milk to stand 

 in a cool place till the cream rises ; if the skim milk is more 

 highly coloured than the cream the milk is artificially coloured. 



(2) Add a drop or two of hydrochloric acid to a little milk ; 

 a pink colour indicates the presence of an azo colour, of which 

 the following, among others, may occur in milk : — 



Aniline yellow. Amino-azo-benzene. 



Butter-yeUow. Chrysoidine. Di-methyl-amino-azo-benzene. 

 Acid yellow. Salts of amino-azo-benzene sulphonic acid. 

 Methyl-orange. Salts of di-methyl-amino-azo-benzene sulphonic acid. 

 Orange IV. Diphenylamine-yellow. Salts of di-j)henylamine-azo- 

 benzene sulphonic acid. 



(3) Make the milk alkaline with sodium bicarbonate, and 

 immerse a strip of filter paper therein for at least twelve hours. 

 A reddish-yellow stain indicates annatto. 



These tests may fail to show artificial colouring-matters, because 

 (1) aniline-yellow and butter-yellow are soluble in fat, and may 

 rise with the cream ; (2) azo-compounds are reduced in stale milk 

 to colourless compounds ; and (3) a colour such as phosphine 

 (di-amino-phenyl-acridine usually mixed with di-amino-toluyl- 

 acridine) or caramel has been used. 



It is better, therefore, to use the general method : — Take 

 50 c.c. (or more) of milk, make just alkaline to litmus, and evapor- 

 ate to a paste, and thoroughly extract the fat with ether. Evapor- 

 ate the ethereal solution, and shake up the fat with a little 

 hot distilled water, separate the water, and evaporate to dryness 

 in a small porcelain dish. Pure milk gives no coloured residue ; 

 if the residue is coloured, this will be due to a reduction product 

 of an azo colouring-matter, or to the unreduced colouring-matter. 

 Next, extract the fat-free residue with absolute alcohol, filter 

 the alcoholic extract, and evaporate to dryness in three or four 



