228 Testing Milk and Its Products. 



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

 cipitate 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 gela- 

 tine 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 



288. 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. 



289. Microscopic 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 removed 

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

 weighed. A quart of milk or cream should not give any 

 visible sediment on standing for several hours. 



DETECTION OF PRESERVATIVES IN DAIRY PRODUCTS. 



290. a. Boracic acid (borax, borates, preservaline, etc.). 

 100 cc. of milk are made alkaline with a soda or potash 

 solution, and then evaporated to dryness and incinerated. 



1 The Analyst, 1897, p. 320. 



