CHEMISTRY OF WOOD DISCOLORATION - COMPARATIVE-STUDY OF POLYPHENOLIC COMPONENTS IN FRESH AND DISCOLORED OAK

The oak discoloration constitutes an undesirable phenomenon which leads to a considerable decrease of wood quality. The mechanisms of oxydation involved in the discoloration and not yet known, concern the polyphenolic components, above all gallotannins and ellagitannins in the oakwood. We have carried out several extractions and gel permeation fractionation, from fresh oak sawdust (CS) and discolored oak (CC) of dried industrial veeners. The chromatographic technics and chemical analyses have allowed to reveal qualitative and quantative differences even with polyphenolic components of CS and CC fractions. The discolored oak sawdust contains colored polyphenolic polymers and complex esters, hexahydroxydiphenoylesters, in higher quantity than in the fresh oak sawdust. Gallic acid is likely responsable of brown discoloration, which would result from oxidative coupling reactions between galloylunits of galloylglucose structures.