566 MANUAL OF MILK PRODUCTS 



two minutes or longer, depending upon the condition of the 

 reagent. This test is very delicate, and in experienced hands 

 will reveal the presence of 1 per cent of raw milk in heated 

 milk. It is, however, open to errors and objections which 

 should be considered. The paraphenylendiamine is difficult 

 to procure, the solution spoils rapidly, and, when decomposed, 

 will give a color in heated milk much resembling that given in 

 raw milk. Again, the test cannot be used for sour milk or 

 buttermilk unless the acidity is neutralized to a point equal to 

 that of fresh milk. In the acid milk no color develops in the 

 raw nor in the heated milk. 



Potassium iodide-starch test. — The test recommended for 

 general use is the potassium iodide-starch test. The hydrogen 

 peroxide as previously described can be used, or it can be em- 

 ployed in the form purchased on the market (3 per cent solu- 

 tion). To prepare the potassium iodide-starch reagent two to 

 three parts of wheat starch are mixed in a little cold water; 

 100 parts of boiling hot water are then poured over the starch, 

 and stirred well. To this solution of starch is then added two 

 to three parts of potassium-iodide, dissolved in a little water. 

 The test can be carried out by placing in a tea cup 20 to 30 c.c. 

 of the milk to be tested. Fifteen to 20 drops of the starch 

 solution are added, and mixed with the milk. After the milk 

 has become quiet, add one drop of the commercial hydrogen 

 peroxide, or six to eight drops of the diluted. In raw milk 

 a color will develop at once in the neighborhood of the drop. 

 The color will usually be blue. Sometimes a greenish color 

 will develop, due to the lack of sufficient potassium iodide-starch 

 solution. The development of any color is to be looked upon 

 as a positive test for raw milk. If the milk has been heated 

 to 80° C. (176° F.), or above, no color will be noted on the 

 addition of the hydrogen peroxide. The test should be read 

 at once, and no attention paid to the development of color 

 later, either with this test or with the paraphenylendiamine. 



