Studies on Tannins of Fruits and Vegetables

Part V. The colour development of tannin by iron ion

Abstract

The effects of the iron salts, oxidation and reduction, temperature, pH, and organic and inorganic acids on the colour development of various tannins and low molecular polyphenoles by iron ion were studied. The colour developed by ferric ion and was reduced by ascorbic acid. With the ascent of pH value, optical density increased and the λ max. shifted to short wave length side. The increase of the colour intensity was observed at high temperature. Polybasic acids, such as citric, tartaric, malic, and phosphoric acid which have the chelating action to iron, prevented the formation of phenol-iron complex and several foods containing these acids showed little discoloration by iron. Chelating action of polyphosphoric acids, phytic acid, and EDTA was remarkedly affected by pH and differed on the same pH. It is obvious from these results that the discoloration of food caused by iron ion is prevented by the acidification with polybasic acid, the addition of ascorbic acid, and the use of chelating substance which acts at the pH of foods.