CHARACTERIZATION OF EUROPEAN OAKWOOD CONSTITUENTS ACTING IN THE BROWN DISCOLORATION DURING KILN DRYING

The brown discolouration of oakwood, during kiln drying, causes substantial financial loss to industrial driers. In this work we have studied the oak extractives from discoloured and non coloured areas. Analytical HPLC has shown that ellagitannins quantities and more particularly vescalagin and castalagin are lower in coloured areas compared to non coloured areas. After ultrafiltration, we have observed that the concentration of high molecular weight compounds was five times greater in coloured areas than in non coloured areas. Treatment of aqueous extracts of crude and purified ellagitannins between 5 and 55 degrees C has shown that increasing temperature induces hydrolysis of ellagitannins which produces ellagic acid and a yellowing of the solution. We propose that the brown discolouration is induced by hydrolysis and oxidative transformation of ellagitannins.