A critical analysis of techniques for measuring tannins in ecological studies

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Abstract
A series of seventeen taxonomically diverse plant extracts rich in phenolic materials, including condensed and hydrolysable tannins, have been subjected to a series of biochemical analyses in an attempt to gather ecologically significant information about their interaction with proteins and amino acids. Methods employed were (i) protein-precipitation, using bovine serum albumin as substrate, followed by computation of specific activities of the tannins present in the extracts, and (ii) the inhibition of cellulase activity by tannin extracts bound to the cellulose substrate and free in solution. Both techniques revealed that all extracts contained tannin material. However, attempts to relate the results of the two procedures and in turn to relate them to information reported previously on the chemical properties of these extracts revealed that there was little correlation between any of the chemical or biochemical properties examined. From this analysis it would seem that whilst the analytical procedures available for studying tannins may generate ecologically useful information it is at present impossible, at least where plant material that is taxonomically diverse is being examined, to extrapolate from one type of measure to anticipate what would be observed from another type of measure. In addition to the above three other observations arose from this study. First, it appears to be generally true that there is not an absolute positive correlation between the level of protein precipitation and the incorporation of tannin in the tannin-protein precipitate. As relative protein concentration increases the proportion of tannin bound in the precipitate decreases, leading to less stable precipitates. Second, it is confirmed that some basic amino acids will precipitate with tannins, a phenomenon that could potentially influence amino acid balance in the diet. Third, complexation between tannin and protein absorbed on a cellulose substrate is able to interfere with the digestion of that cellulose by cellulase enzymes. Cellulose masking of this type may potentially effect the efficiency of cellulolytic activity in the rumen and if so suggests another subtle variation in the potential antifeedant properties of tannins.