Characterization and molecular weight distribution of carbohydrates isolated from the autohydrolysis extract of mixed southern hardwoods

In order to utilize hemicelluloses from biomass as a feedstock for production of higher value added products such as biofuels and bio-based chemicals, it is essential to assess the chemical composition of the liquid phase and molecular weight distribution of isolated carbohydrates. The association of hemicelluloses with lignin also plays an important role for utilization of isolated hemicelluloses. In the present study carbohydrates were first extracted from a mixture of southern hardwoods by autohydrolysis at different temperatures for 100 min, and then precipitated after mixing with four volumes of ethanol. Depending on the temperature, all (100% at 130 °C) or a small part (5% at 170 °C) of the oligomeric hemicelluloses present in the extract precipitated together with some lignin and oligo-glucose (glucans). The classical iodine test confirmed that most of the glucans are actually starch. Oligo-glucose is the major component in the precipitate from extracts produced at 150 °C and below, while xylan is the main component at 160 °C. Almost all lignin in the precipitate is in the form of lignin–carbohydrate complexes. The average Mw of the carbohydrates isolated from the extract decreases with increasing autohydrolysis temperature. The average DP of hemicelluloses isolated from the extract decreases from about 71 to 36 over the temperature range from 130 to 170 °C during 100 min autohydrolysis.