Characteristics of degraded hemicellulosic polymers obtained from steam exploded wheat straw

The fractionation of wheat straw was studied using a two-stage process based on a steam explosion pre-treatment followed by alkaline peroxide post-treatment. Straw was steamed at temperatures comprised between 200 °C for 10 and 33 min and 220 °C for 3, 5, and 8 min. The steamed straw was washed with water to yield a solution rich in hemicelluloses-derived mono- and oligosaccharides (20.5-28.5%) together with small amounts of degraded hemicellulosic polymers (2.4-6.2%) and minor quantities of lignin (1.9-2.1%). The washed fibre was post-treated by 2% alkaline peroxide (pH 11.5) at 50 °C for 5 h. The alkali-soluble lignin (13.7-15.0%) and hemicelluloses (8.4-13.3%) were recovered by precipitation. The post-treatment was sufficient to remove lignin and survived hemicellulosic polymers and increase the brightness of the exploded straw significantly. The two-stage treatments together degraded 77.0-87.6 of the total original hemicelluloses and 92.3-99.4% of the total original lignin. This paper examined these two groups of six hemicellulosic polymer preparations degraded during the various steam explosion pre-treatments (H1a-6a) and released during the sequential alkaline peroxide post-treatment (H1b-6b). The effect of the most important variables in steam explosion pre-treatment (temperature, residence time, and water to straw ratio) on physico-chemical properties and structural features of the degraded hemicellulosic polymers were investigated. Thermoanalysis of these degraded hemicellulosic polymer fractions indicated that hemicellulose degradation products were also condensed to form more thermostable substrates during the steam explosion.