246 Testing Milk and Its Products. 



impart stiffness and an appearance of richness to the 

 cream. To detect the gelatine, a quantity of the sus- 

 pected cream is mixed with warm water, and acetic acid 

 is added to precipitate the casein and fat (1.5 cc. of 10 

 per cent.-acetic acid per 10 cc. of cream is sufficient). 

 The precipitate is filtered off, and a few drops of a 

 strong tannin solution are added to the clear filtrate. 

 Pure cream will give a slight precipitate, while in the 

 presence of gelatine a copious precipitate will come 

 down. 



The picric-acid method has also been proposed for 

 the detection of small quantities of gelatine in cream. 1 



297. Starch in cream. Starch is mentioned in the 

 dairy literature as an adulterant of milk and cream. It 

 is doubtful, however, if it is ever used for this purpose 

 at the present time. In the case of ice-cream, on the 

 other hand, a small quantity of corn starch is often 

 added to thicken the milk used. It may in such a case 

 be readily detected by means of the iodin reaction. A 

 solution of iodin will produce a deep blue color in the 

 presence of starch ; a small amount of iodin is taken up 

 by the cream before the blue coloration appears. 



298. Macroscopic impurities (particles of hay, litter, 

 woolen or cotton fibres, dung, etc.). These impurities 

 may be separated by repeated dilution of the milk with 

 pure distilled water, leaving the mixture undisturbed 

 for a couple of hours each time before the liquid is 

 syphoned off. When the milk has been entirely re- 

 moved in this manner, the residue is filtered off, dried 



1 The Analyst, 1897, p. 820. 



