Ultraviolet spectroscopy and chemometrics for the identification of vegetable tannins

Abstract

In this study six different commercial vegetable tannins were analysed by ultraviolet spectroscopy and multivariate analysis. In addition, also it was determinated a specific absorptivity and quantified the total polyphenols and tannin content of each extract by filter and Folin Ciocalteu methods, respectively. The lowest values of specific absorptivity were for extracts of condensed tannins and the highest for the hydrolysable tannins. At analysis of total polyphenols, the condensed tannins showed the highest percentage, 90% and 80% for the quebracho and mimosa, respectively. Hydrolysable tannins were in the range 60⿿67%. In the evaluation of tanning percentage, the quebracho, chestnut and mimosa presented a percentage around 80%, followed by valonea, tara and myrobalan, 74, 56 and 41%, respectively. At multivariate analysis, a well-defined separation can be seen through both principal component analysis (PCA) and hierarchical cluster analysis (HCA), between condensed (quebracho and mimosa) and hydrolysable (valonea, chestnut, myrobalan, and tara) tannins. In hydrolysable tannins, it was also possible to observe the formation of two different subgroups between samples of chestnut and valonea and between samples of tara and myrobalan. Of all samples analysed, the chestnut and valonea showed the greatest similarity, indicating that these extracts contain equivalent chemical compositions and structure and, therefore, have similar properties.