Characterization of Natural Tanning Extracts by FTIR and Multivariate Analysis

The tannins are polyphenolic compounds with complex structures formed by the secondary metabolism in various plants. Among the oldest applications of tannins is leather tanning. Tanning properties are due to the ability of these polyphenols to form complexes with proteins. In this study were analysed six different types of commercially polyphenolic extracts by Fourier transform infrared spectroscopy (FTIR) and multivariate analyses. By FTIR was possible to differentiate condensed (quebracho and mimosa) and hydrolysable tannins (valonea, chestnut, myrobalan and tara). The hydrolysable have in the region between 1734-1707 cm, the C=O stretching of esters, while the condensed tannins have at 1285-1283 cm C-O stretching of flavonoid pyran. These are the two main signals to identify these extracts. In chemometric evaluation, both the principal component analysis (PCA) and by hierarchical cluster analysis (HCA), it can be observed a well-defined separation between condensed and hydrolysable tannins. It was also possible to observe for hydrolysable tannins the formation of two different subgroups among chestnut and valonea and among tara and myrobalan samples. Among all analysed samples, chestnut and valonea showed the greatest similarity, indicating that these extracts have chemical composition and structural equivalent and therefore similar properties. The FTIR technique associated with multivariate analysis were effective in identifying the nature of these polyphenolic extracts.