Effect of lignin on the molecular weight analysis of pressurized hot-water extracted hemicelluloses

Pressurized hot-water extraction is a method where plain water is utilized to isolate hemicelluloses from lignocellulosic materials, such as spruce and birch. Utilization of hemicelluloses as high added value materials has been restricted due to the difficulty to isolate high molecular weight (Mw) fractions. Therefore, monitoring the changes in Mw during the isolation processes of hemicelluloses is very important. However, lignin is always present in the isolated hemicellulose fractions either alone or linked to carbohydrates. The presence of lignin causes problems in the Mw analysis applying universal calibration with light scattering-based detection due to the autofluorescence of lignin or its residues. Average Mw of pressurized hot-water extracted spruce galactoglucomannans and birch glucuronoxylans were analyzed and compared with different size exclusion chromatography (SEC) configurations. Peracetic acid treatments with gradient dosages and ethanol precipitation were carried out to both extracts to explre the effect of the lignin content on the determination of Mw. All SEC systems included refractive index (RI) detector as the concentration detector. Additionally, multi-angle laser light scattering (MALLS) detector or right angle light scattering (RALS) with viscometric and ultraviolet detectors were used. Eluent was aqueous or organic depending on the applied column. Both universal calibration and conventional calibration with pullulan and dextran standards were used for Mw calculations. In measurements of both spruce and birch samples, the Mw values were different from RI and LS detectors. The difference in the Mw values of birch samples was relatively larger. In comparison with samples after precipitation by ethanol, a larger difference in the Mw values between RI and LS detectors was obtained for the samples before the precipitation. As the ethanol precipitation removes partially lignin-rich compounds, particularly after peracetic treatment, the above larger difference in the Mw values before ethanol precipitation could be ascribed to higher lignin content in those samples.