ON MILK AND MILK-TESTING 85 



Prepare an acid solution of mercuric nitrate by- 

 dissolving mercury in twice its weight of nitric acid 

 of 1.42 specific gravity, and diluting the solution to 

 25 times its bulk with water. To 10 c. c. of the 

 cream to be examined, add an equal volume of 

 acid mercuric nitrate solution, shake the mixture, 

 add 20 c. c. of water, shake again, allow to stand 

 five minutes, and filter. If much gelatin is present 

 the filtrate will be opalescent and cannot be ob- 

 tained very clear. To a portion of the filtrate con- 

 tained in a test tube add an equal volume of a 

 saturated aqueous solution of picric acid. A yellow 

 precipitate will be produced in the presence of any 

 considerable amount of gelatin, while smaller 

 amounts will be indicated by a cloudiness. In the 

 absence of gelatin the filtrate obtained will be per- 

 fectly clear. The test will work equally well for 

 determining the presence of gelatin in milk. 



How may the presence of starch in cream be de- 

 tected? 

 By adding a small amount of iodine solution as 

 in the test for starch in milk. A slightly larger 

 quantity of the iodine solution should be added, as 

 the greater amount of fat in cream will absorb 

 more of the iodine. 



What substances are often used as cream thicken- 

 ers? 



1. Sucrate of lime (viscogen). 



2. Condensed milk or condensed skim milk. 



3. Gelatin. 



4. Starch. 



