To focus on changes in the lignin structure, the aromatic
composition of lignin was examined by the alkaline nitrobenzene
oxidation procedure.15 The benzaldehyde derivatives
were analyzed as trimethylsilyl derivatives using a
Shimadzu gas chromatograph GC-1700 with a Neutrabond-
1 capillary column. The (syringaldehyde and syringic acid)/
(vanillin and vanillic acid) mole ratio was defined as S/V
ratio and was used to investigate the structure of lignin.