Pretreatment and fractionation of wheat straw for production of fuel ethanol and value-added co-products in a biorefinery

Authors: ZHANG, XIU - Beifang University Of Nationalities; Nghiem, Nhuan

Submitted to: Mathematical Biosciences and Engineering (MBE) Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2014
Publication Date: 8/20/2014
Citation: Zhang, X., Nghiem, N.P. 2014. Pretreatment and fractionation of wheat straw for production of fuel ethanol and value-added co-products in a biorefinery. Mathematical Biosciences and Engineering (MBE) Journal. 1(1):40-52.

Interpretive Summary: The objective of this research is to develop an integrated process for production of fuel ethanol and value-added co-products from lignocellulosic biomass feedstock. Production of ethanol as the sole product using biomass feedstocks has two major issues, which are: 1. High economic risk due to high capital costs and small profit margin of ethanol; and 2. Low efficiency of microorganisms developed for conversion of both five-carbon and six-carbon sugars to ethanol. To overcome these problems we have developed an integrated process using wheat straw as a model biomass feedstock, which includes the following steps: 1. Pretreatment of wheat straw by soaking in aqueous ammonia (SAA), which allows high retention of carbohydrates in the recovered solids for subsequent bioconversion; 2. Use of a commercial enzyme product containing high hemicellulase activity to produce a solution rich in five-carbon sugars; 3. Use of the solution rich in five-carbon sugars for production of high value-added products such as astaxanthin and xylitol; 4. Recovery of the resulting cellulose-enriched residue for ethanol production in a simultaneous saccharification and fermentation process using a commercial enzyme product having high cellulase activity and the yeast Saccharomyces cerevisiae, which so far still is the most efficient and commercially proven ethanol-producing organism but only is capable of metabolizing glucose and none of the five-carbon sugars. In our research astaxanthin and xylitol were used to illustrate the feasibility of using the aforementioned solution rich in five-carbon sugars for production of value-added products. Other products of interest can be produced from this solution using suitable microorganisms. The developed integrated process also is expected to be applicable to other biomass feedstocks.

Technical Abstract: An integrated process has been developed for a wheat straw biorefinery. In this process wheat straw was pretreated by soaking in aqueous ammonia (SAA), which extensively removed lignin but preserved high percentages of the carbohydrate fractions for subsequent bioconversion. The pretreatment conditions included 15 wt o/o NH4OH, 1:10 solid:liquid ratio, 65 degree C and 15 hours. Under these conditions, 48 percent of the original lignin was removed, whereas 98 percent, 83 percent and 78 percent of the original glucan, xylan, and arabinan, respectively, were preserved. The pretreated material was subsequently hydrolyzed with a commercial hemicellulase to produce a solution rich in xylose and low in glucose plus a cellulose-enriched solid residue. The xylose-rich solution then was used for production of value-added products. Xylitol and astaxanthin were selected to demonstrate the fermentability of the xylose-rich hydrolysate. Candida mogii and Phaffa rhodozyma were used for xylitol and astaxanthin fermentatioin, respectively. The cellulose-enriched residue obtained after the enzymnatic hydrolysis of the pretreated straw was used for ethanol production in a fed-batch simultanious saccharification and fermentation (SSF) process. In this process, a commercial cellulase was used for hydrolysis of the glucan in the residue and Saccharomyces cerevisiae, which is the most efficient commercial ethanol-producing organism, was used for ethanol production. Final ethanol concentration of 57 g/l was obtained at 27 wt percent total solid loading.