Studies on tannins in plants

Abstract

Tannins have been defined as phenolic hydroxyl compounds of relatively high molecular weight which precipitate proteins. Chemically, they can be divided into two classes (hydrolysable and condensed tannins), depending on their reaction with hot mineral acid. The structure of both classes has been discussed and a number of theories enumerated regarding possible linkages involved in the polymerisation of condensed tannins. The biosynthesis of tannins has been discussed. The widespread distribution of tannins in the plant kingdom and the simultaneous occurrence of leuco-anthocyanins and "botanical tannins" is mentioned. The loss of both astringency and tannins in fruits on ripening is discussed. The effect of mineral nutrition, light and season on the concentration of tannins in plants is referred to. Chemical and physical methods for the analysis of tannins have been discussed, and five analytical methods have been investigated for characterising the phenolic nuclei and applied to model compounds. These methods have been extended and used to study the changes in tannins in bananas, peaches and plums on ripening, with particular reference to the changes in the leuco-anthocyanins. The results with the banana confirm the hypothesis of Swain (1956) that the reduction in astringency on ripening is due to polymerisation of the tannins; somewhat ambiguous results were obtained with the juicy fruits. In order to develop a different method for measuring astringency, the inhibition of enzymes by tannins was investigated. A number of reagents were shown which could reverse this inhibition, and the use of some of these reactivating agents, both for extracting tannins and for protecting enzymes from inhibition, is suggested. Lastly, the synthesis of leuco-anthocyanins in sycamore tissue culture was studied, and it was found that both restricted aeration and "strain" differences affect the production of these compounds.