Xylanase treatment and cellulosic ethanol fermentation of alkaline pretreated wheat straw

Abstract

Concerns about the sustainability of crude oil supply and the fossil fuel related climate change have led to an ever increasing interest in the production of classically oil derived fuels and chemicals from renewable resources such as biomass in recent years. In the present work a multiple product approach towards such a biorefining process is presented. Dried wheat straw was pretreated using low temperature (50°C) alkaline ethanol pretreatment, which selectively releases lignin from the biomass. Commercially available hemicellulase preparations were screened for their potential to release the hemicellulose fraction of the solid pretreatment residue. A selective hemicellulose extraction procedure was developed with the best suited enzyme and xylose, the main component of wheat straw hemicellulose, could be recovered with yields up to 85% of the theoretical at high initial solid loadings (up to 8% w/w), thereby creating a xylose rich hydrolyzate. Possible barriers to the quantitative recovery of wheat straw hemicellulose were further studied. It was found that unproductive adsorption of [beta]-xylosidase resulted in a more severe loss of enzyme activity than did product inhibition and thermal inactivation.<br />Hemicellulase treatment was also shown to decrease the degree of polymerization of wheat straw cellulose. Alkaline pretreated wheat straw (APWS) and hemicellulase treated APWS were furthermore tested as cellulosic ethanol (CE) substrates in Simultaneous Saccharification and Fermentation (SSF) experiments. The hemicellulase treated substrate cellulose could be converted to ethanol with a yield of 77.8% of the theoretical resulting in an ethanol concentration of 31.8 g/l at an initial solid loading of 10% (w/w). Compared to APWS without hemicellulase treatment, which could be converted with a yield of 64.7% of the theoretical resulting in an ethanol concentration of 21.1 g/l, hemicellulase treatment improved the convertibility of wheat straw cellulose. It is therefore concluded that alkaline ethanol pretreatment and hemicellulase treatment produce a substrate that can be used to feasibly produce CE in a multiple product biorefinery where lignin as well as hemicellulose are converted into value added products.