654 ANNUAL, REPORT OF THE Off. Doc. 



such a waj that a certain amount of it equals one per cent, of lactic 

 acid. The two commo« forms of acid test are those devised by 

 Mann and by Farrington. 



(2.) Mann's Acid Test. — Measure exactly 50 cubic centimeters of 

 the cream or milk into a clean porcelain cup or a glass. Add a few 

 drops of phenolphthalein and then let in some of the "neutralizer" 

 (Mann's name for the alkaline solution), from a burette, previously 

 filled to the zero point. A pink color appears, but disappears on 

 stirring. Continue to add the alkali carefully, stirring the cream 

 or milk all the while. It will be noticed, sooner or later, that the 

 pink color disappears more slowly after each addition of alkali. 

 Finally, a point is reached when the pink color does not disappear 

 even after considerable stirring. Add no more alkali. Then look at 

 the burette and see how many cubic centimeters of alkali have been 

 used. Suppose 30 cubic centimeters of alkali have been required 

 to use up or equal the lactic acid in the cream, then multiply 30 by 

 .018 and the result is 0.54, which is the per cent, of lactic acid in 

 the cream or milk used. (See Fig. 23.) 



(3.) Farrington's Alkaline Tablet Test.— In this case, the alkali 

 and phenolphthalein are mixed together in the form of solid tablets. 

 In using this method, one first puts five tablets into a graduated 

 100 cubic centimeter cylinder and tills this up with water to the 

 97 c. c. mark with clean soft water, distilled water if possible. The 

 cylinder is then tightly corked and laid on its side until the tablets 

 dissolve. The cylinder must be kept tightly corked, so that none 

 of the solution can be lost while the tablets are dissolving. The 

 solution will be good to use for twenty-four hours after being pre- 

 pared. Solutions more tha« a day old should, therefore, not be used. 

 The solid tablets will not change if kept dry. (See Fig. 24.) 



In making the test, the cream or milk to be tested is thoroughly 

 mixed and then measured into a porcelain cup with a 17.6 c. c. 

 pipette. This pipette is rinsed once with water, and the rinsings 

 are added to the cream in the cup. Then a few c. c. of the tablet solu- 

 tion prepared as above directed are poured from the cylinder into the 

 cream and thoroughly mixed with it. The tablet solution is added 

 in small quantities until the pink color in the cream or milk lasts 

 for some time. Now, look at the cylinder and see how many c. c. 

 of solution have been used. One c. c. of tablet solution stands for 

 .01 per cent, of lactic acid. Thus, if 20 c. c. of solution are used, 

 there is .20 per cent, of lactic acid; if 50 c. c. are used, the lar-tic 

 acid is .50 per cent. 



