102 Chapter 5
The condensed tannins consist of flavinoid groups (Figure 6) containing
various combinations of phenolic hydroxyl groups linked together to form di-,
tri- or larger polymers (Figure 7). Unlike the hydrolysable tannins, which are
broken down by hydrolysis, the condensed tannins polymerise further to form
high molecular weight, insoluble complexes.
The reaction between vegetable tanning materials and collagen is mainly
due to the large number of phenolic hydroxyl groups on the tannin molecules.
These form hydrogen bonds with the electronegative centres on the protein
chain, such as the carboxy- and imino groups. There is also the possibility of
reaction between the polyphenols and the amino or carboxylic side-chains
depending on pH.
Syntans. With the development of an understanding of the polyphenolic
nature of vegetable tannins, and the increasing shortage of these naturally-
occurring materials at the beginning of the twentieth century, efforts were
made to synthesise products with related chemical structures in the hope that
HOHOCO COOOOHOH HOHOHOOHOHOHCOOHHOOCFigure 4Ellagic acid
COOHO
OHOH OHOHOOCHOOCFigure 5Chebulic acid
OFigure 6The flavanoid ring structure on which condensed tannins are based