22 FRUITS AND FRUIT PRODUCTS. 



DETERMINATION OF TANNIN AND COLORING MATTER. 



Tannin and coloring matter are estimated in fruits and fruit products 

 by the permanganate or Neubauer-Loewenthal method, as used in 

 wine analysis. a The following reagents are used: 



(1) A solution of potassium permanganate containing about 1^ grams 

 of the salt to the liter. 



(2) A decinormal oxalic acid solution for determining the strength 

 of the permanganate solution; 1 cc equals 0.004157 gram of tannin. 



(3) An indigo-carmin solution to be used as an indicator and con- 

 taining 6 grams of indigo carmin (this must be pure and free from 

 indigo blue) and 50 cc of sulphuric acid to the liter. Sodium sulphin- 

 digotate may be used instead of indigo carmin. Dissolve 6 grams of 

 the salt in 500 cc of water by aid of heat, cool, add 50 cc of sulphuric 

 acid, make up the solution to 1 liter, and filter. 



(4) Pure boneblack which has been treated with hydrochloric acid 

 and washed with water until free from acid. This is kept covered 

 with water. 



The method of procedure is as follows: 



A large porcelain evaporating dish is used for the titration. About 

 750 cc of water are placed in it and from 25 to 50 cc of fruit juice 

 added, then 20 cc of the indigo solution, and the whole is well mixed. 

 The permanganate solution is then run in very slowly, with constant 

 stirring, and at the last added only a drop or two at a time, allowing 

 the reaction considerable time. The end point is the golden yellow. 

 To another portion boneblack is added and allowed to stand fifteen min- 

 utes. It is then filtered and carefully washed with water. This filtrate 

 is treated in the same manner as the original portion, and the differ- 

 ence between the number of cubic centimeters of permanganate used 

 the first time and that used the second time is the amount used in the 

 oxidation of the tannin and coloring matter. 



DETECTION OF PRESERVATIVES. 



Dissolve about 25 grams of the sample in water, acidify with dilute 

 sulphuric acid (1 to 3), and extract with ether. Remove the ether 

 layer and allow to evaporate spontaneously. Take up the residue, 

 which may contain salicylic acid, benzoic acid, and saccharin, with a 

 few cubic centimeters of water, and apply the tests given below for 

 these substances. If it is desired to make tests for other preservatives, 

 consult Bulletin 65. 



SALICYLIC ACID. 



To a few drops of the extract add 1 or 2 drops of a 0.5 per cent 

 solution of ferric chlorid. A purple coloration indicates the presence 

 of salicylic acid. 



11 Wiley, Principles and Practice of Agricultural Analysis, Vol. Ill, p. 593; U. S. De- 

 partment of Agr., Bureau of Chem. Bui. 65, p. 86. 



