Evaluation of harvesting time effects on kenaf bast lignin by pyrolysis-gas chromatography

A Chinese kenaf (Hibiscus cannabinus L.), variety Sekko-ichi, was planted in May 1996, and harvested at 76, 116, 152, 185, and 226 days after planting. The hand-separated bast fibers were analyzed for lignin content and in situ lignin composition to clarify their variations with maturity. Bast fibers increased in lignin until 152 days after planting, and did not increase thereafter. Variations in in situ lignin compositions with maturity were monitored using pyrolysis-gas chromatography. The volatile pyrolysis products released at 500 °C for 4 s were identified by gas chromatography-mass spectrometry. Pyrolysis products derived from guaiacyl and syringyl lignin units were revealed. The yield of guaiacyl lignin-derived pyrolysis products decreased with maturity, while the yield of the syringyl analogous increased with maturity. The ratio of yield of syringyl lignin-derived products to that of guaiacyl ones (S/G) increased from 1.86 to 3.16 with maturity, but remained constant at 152 days after planting. High S/G ratio showed that mature bast lignin is of syringyl type. Quantitative pyrolysis results suggested that harvesting of kenaf at age of 5-months growth period provides a raw material with an ease of delignification.