Improved Mäule color reaction provides more detailed information on syringyl lignin distribution in hardwood

Abstract

The traditional Maule color reaction was improved by use of tris(hydroxymethyl) aminomethane-hydrochloride acid (Tris–HCl) buffer coupled with a fluorescence microscope. The purple-red color of Maule-treated hardwood was more stable with the novel treatment than with traditional treatment. In addition, Maule-treated wood samples had a characteristic fluorescence when excited with blue (460–495 nm) light. Examination of white birch (Betula platyphylla) showed that syringyl-rich and guaiacyl-rich cell walls emitted red and yellow fluorescence, respectively. Measuring the fluorescence spectra of hardwood and softwood powders after treatment with the new Maule reagents showed that hardwood powder containing syringyl lignin emitted a red fluorescence, whereas softwood powder containing guaiacyl lignin emitted a green fluorescence when excited with blue light. In conclusion, this improved technique has many advantages compared with the traditional Maule color test.