Characterization of soluble lignin released from alfalfa by sheep digestion

A faecal soluble lignin fraction (FSL) extracted with 90% dioxane from the faeces of sheep fed on alfalfa hay was characterized by chemical analysis, nitrobenzene oxidation, fourier transform infrared spectroscopy and gel permeation chromatography, and compared with milled wood lignin (MWL) isolated from the alfalfa hay. The amount of FSL in the faeces was low, accounting for only 1% of the lignin present in the alfalfa hay. FSL and MWL consisted mostly of lignin components and contained a small amount of carbohydrate. FSL had a much higher proportion of syringylpropane units than MWL and showed a wide molecular size distribution. The results indicate the selective and limited solubilization of syringyl-rich lignin from alfalfa by sheep digestion.