Chemical characteristics of surface colour of birch veneer (Betula pendula)

Abstract

This thesis investigates the discoloration of veneer produced from silver birch (Betula pendula) logs by means of biochemical and chemical analyses on the veneer and sap obtained from wood which was soaked at varying temperatures (20-70 ˚C). Drastic colour development was confirmed from the sap squeezed from green birch wood. The results from biochemical tests on the sap by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) showed the presence of polyphenol oxidases (PPO) which might form quinonoid intermediates in sap. Further chemical analysis by gas chromatography coupled with mass spectrometry revealed that the major phenol, which acts as a substrate for PPO, was epicatechin (EC). Control of the discoloration by either sodium sulphite (Na2SO3), or ethylenediaminetetraacetic acid (EDTA) was demonstrated. This thesis also investigates the chemical impact of elevated soaking temperature on the birch log being soaked. The results from spectrophotometric analysis on the veneer showed a significant increase in lightness (L*) and a decrease in yellowness (b*) in the veneer produced from logs soaked at 70 ˚C. Conversely, the effect of long-term storage resulted in a significant reduction in L* and an increase in b*. The results from the chemical analysis of squeezed sap revealed that the physical interaction between the core part of log and soaking water is limited when soaking at elevated temperatures. Assessment of the soaking water quality showed an increase of organic loading in water as soaking temperature increases. Bark seemed to be the major source of the organic substances. The results from the analysis of the squeezed sap from the soaked log showed that glucose and fructose were dominant free monosaccharides and their concentration was increased during soaking. The concentration of carboxylates, sulphite and sulphate had a positive correlation with soaking time at 70 ˚C. EC in the sap was diminished under soaking. In addition, the proanthocyanidins (PAs) contents had a moderate negative correlation between the contents and the soaking temperature. It was observed that phenolic compounds including EC and PAs played a major role in the colour change of the veneer, and their concentration was altered by elevated temperature.