MILK 243 



taining the indicator is added to the sour cream it shows no color 

 until the point of neutrality is reached. At this point color gradually 

 appears and becomes permanent. In other forms of the test it is 

 necessary to add the coloring matter or indicator to the cream before 

 beginning the test, three or four drops being sufficient to give the 

 proper color when the cream becomes alkaline. 



Farrington's Alkaline Test. The two most common forms of 

 the acid test are known as Farrington's alkaline test and Mann's acid 

 test. In the former the alkaline substance is put up in the form 

 of tablets of a uniform strength ; the indicator is added to the tablet 

 and gives it its pink color. The best method of using these tablets is 

 described as follows : 



The apparatus required is a 100 cc. cylinder, a 17.6 cc. pipette, 

 and a white porcelain cup. Clean, soft water to the amount of 97 cc. 

 is placed in the cylinder and five tablets are dissolved in it. The 

 cylinder should be tightly corked and laid away until the tablets 

 are entirely dissolved. If tablets and water are put in the cylinder in 

 the evening they should be ready for use the next morning. In mak- 

 ing the test, take the 17.6 cc. pipette, measure that quantity of cream 

 and place it in the clean cup. Rinse out the pipette with a small 

 quantity of clean water and add the rinsings to the cream in the cup. 

 Pour a few cubic centimeters of the tablet solution into this cream, 

 giving the cup a rotary motion with the hand so as to thoroughly 

 mix the solution with the cream. Add a few drops at a time until 

 a slight pinkish color appears and remains permanent. The amount 

 of 'alkali used can then be read off on the cylinder. The number so 

 obtained represents the per cent of acid actually in the cream. 



In making tests of this kind, in order to have results that will 

 be uniform, exactly the same shade of pink, as near as it is possible 

 to determine with the eye, should be observed each time. There are 

 various other methods for using the tablet test, but the one given is 

 the simplest and gives a direct result which needs no further compu- 

 tation. 



Mann's Acid Test. In using Mann's acid test the alkaline solu- 

 tion is procured in the form of a solution of known strength. This 

 solution, added to the cream by means of a burette, determines very 

 readily the acidity of the cream. In using this test a few drops of 

 indicator w r ill have to be added to the cream before the alkali solu- 

 tion is added. The amount of solution is read directly from the 

 scale on the burette. This does not give the percentage of acid in the 

 cream, as in the case of the tablet test, but gives a relative factor, 

 which has to be worked out by the following formula in order to re- 

 duce to percentage: Multiply the number of cubic centimeters of 

 alkaline solution required to produce the pink color by 0.9 and divide 

 the result by the number of cubic centimeters of cream used in mak- 

 ing the test. The result is the per cent of acid in the cream. For 

 the mathematical explanation of this formula and a more extended 

 description of the acid test, the reader is referred to publications on 

 the subject of testing. 



