Studies on Tannins of Fruits and Vegetables

Part XII. A new simple method for determination of tannin in Kakishibu (commercial astringent extract of persimmon fruits)

Abstract

There is a need for a rapid, simple and practical method for determing tannin in Kaki-shibu. As the result of analysis on non-tannin components, the existence of the considerable amount of D-mannitol was proved. The exact tannin content was determined by a hide powder method resulting in higher content than those obtained with Folin-Denis method or Lowenthal method. It was also observed that the adsorption of tannin to hide powder decreased according to the enlargement of molecular of tannin.
The method developed is based on the principle that tannin content is in propotion to the difference of refractive indexes between original Kaki-shibu and tannin removed one. As the tannin removing reagent, 7% polyethyleneglycol solution, which consists of equal amount of MW 1, 000 and 2, 000 of polyethyleneglycol is used. The refractive index is indicated as 0Bx by the use of a hand refractometer. This method therefore is called Brix method.
The procedure of Brix method is as follows; To 5 ml of Kaki-shibu is added 2ml of water and ⁰Bx of this mixture is A. Then 2ml of 7% polyethyleneglycol solution is added to 5ml of Kaki-shibu and mixed vigorusly. After standing for about 10min., it is centrifuged and ⁰Bx of the supernatant is B. The tannin content is given by 0.93 (A-B)%. This method is also applied to the rapid determination of other tannin extracts.