Evaluation of Tannins Content and Characteristics of some Indigenous and Exotic Woody Species

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El Mahdi Ahmed, Haroun
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Thirty four plant species (indigenous and exotic) were studied with the objective of evaluating the quantity and quality of their tannins. The tannins were first extracted from the species with cold water using Alca- Palsy apparatus. The presence of tannins in the extract was detected by the gelatine- salt test while its type was determined by the iron - alum and formaldehyde-HCl tests. Seventeen of the species studied had the condensed (catechol) type of tannin while the remaining species contained a mixture of hydrolyzable - condensed (gallo-catechol) tannins. From the 34 species studied only 19 contained more than 10% tannins (on oven-dry extracted part) and could by of commercial interest. The tannins purity ( the ratio of tannin - to - soluble solids) was equal to or higher than 0.6 only for 17 of the species studied, while the ratio of tannin –to-non tannin ( extraction ratio ) was acceptable for 15 species out of the 17 species. Different parts of the same species (bark, pods and leaves) contained the same type of tannin but in different amounts. Usually the content was higher in the deseeded pods (A albida. A nilotica) and the leaves (Anogeissus leiocarpus) The paper and thin-layer chromatography on different adsorbents and in different solvent systems gave further evidence of the presence of catechin ,gallic acid and also revealed the presence of tannic acid, fisetin, epicatechin and some unidentified phenolics, and confirmed the absence of dihydrofisetin and robinetin. Sugars present in the tannin hydrolyzates were detected by means of paper chromatography for A. nilotica and A. mearnsii only. Glucose was present in the hydrolyzates of the A. nilotica subspecies ( bark and pods) confirming that tannins of A nilotica subspecies were a mixture of condensed and hydrolyzable tannin types, since they contain gallic acid and glucose as well as catechin. No other types of sugars were identified in the hydrolyzates of these three subspecies: of A nilotica. The hide-powder method for tannin evaluation was compared with a spectrophotometric method. The first method's values were much higher and different in catechin content. This is expected since the reference in the spectrophotomic method used was tannic acid. The phenolic content determined using the Hagerman method was approximately half the Folin-Denis assay. However the correlation between the two assays was high. Further a combined method of analysis was used as a simpler and quicker modification of the hide-powder method. The data obtained with the official hide -powder method were slightly higher than the combined method values. Further analyses were carried out to estimate the relative astringency of the indigenous species, and were compared with that of mimosa. The highest astringency values obtained nearest to that of mimosa (0.16) were for three species: A. sengal (0.50), A. mellifera (0.33) and Azadirachta indica (0.18) A.senegal and A .mellifera were excluded since they contained less than 10% tannin To overcome the deficiency of the low astringency of A. nilotica, (0.11) which is available in large areas in Sudan, different mixtures of highly and weakly astringent extracts had been prepared in different proportions and their protein precipitation profiles studied. The blend of tannin extracts from A .nilotica ssp tomentosa (pods) and Azadirachta indica (bark) gave a precipitation profile that was closest to the one observed for wattle (mimosa) bark extract. This suggests further possibility of producing "a wattle substitute" from indigenous species by mixing extracts with different astringency profiles.
Evaluation of Tannins Content and Characteristics of some Indigenous and Exotic Woody Species