ON MILK AND MILK-TESTING 83 



All free ammonia should be driven off, otherv/ise 

 ferric hydrate would be formed. 



How may the presence of salicylic acid in milk be 

 detected? 



The acid is seldom used as a preservative in milk. 

 If its presence is suspected, proceed exactly as in 

 testing for benzoic acid. On applying the ferric 

 chlorid to the solution after the evaporation of the 

 ammonia a violet color indicates the presence of 

 salicylic acid. 



How can the presence of starch in milk be de- 

 tected? 



To 10 or 15 c. c. of milk in a test tube or vial add 

 a few drops of an iodine solution. If starch is 

 present it will be colored blue by the iodine. 



How may milk that has been heated to 175° F. be 

 detected? 



1. To 15 or 20 c. c. of milk in a small bottle or 

 test tube add i c. c. of a concentrated starch solu- 

 tion and 6 or 8 drops of a 10 per cent solution of 

 potassium iodid. Next add 4 or 5 drops of a 2 per 

 cent solution of hydrogen peroxid. Upon shak- 

 ing the mixture it will turn to a dark blue color if 

 the milk has not been heated to 175° F. 



2. In the same manner as above, add a quantity of 

 paraphenylenediamin hydrochlorid about the size 

 of a pea and 4 or 5 drops of a 2 per cent solution 

 of hydrogen peroxid. The mixture turns blue on 

 shaking if the milk has not been heated to 175° F. 



