Location: Sustainable Biofuels and Co-Products
Title: Enzymatic fractionation of SAA-pretreated barley straw for production of fuel ethanol and astaxanthin as a value-added co-product Authors
Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2013
Publication Date: September 9, 2013
Citation: Nghiem, N.P., Kim, T., Yoo, C., Hicks, K.B. 2013. Enzymatic fractionation of SAA-pretreated barley straw for production of fuel ethanol and astaxanthin as a value-added co-product. Applied Biochemistry and Biotechnology. 171, Issue 2,p.341-351. 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 barley straw as a model biomass feedstock, which includes the following steps: 1. Pretreatment of barley 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 sugar; 3. Use of the solution rich in five-carbon sugar for production of high value-added products such as astaxanthin; 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 cellulose 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 the red yeast Phaffia rhodozyma were used to illustrate the feasibility of using the aforementioned solution rich in five-carbon sugar for production of a value-added product. 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: Barley straw was used to demonstrate a process for production of ethanol and astaxanthin as a value-added co-product. Barley straw was pretreated by soaking in aqueous ammonia (SAA) using the previously determined optimum conditions. The pretreated barley straw was first hydrolyzed with Accellerase® XY (a commercial hemicellulase product) to generate a xylose-rich solution (3.8 g/l glucose, 22.9 g/l xylose, and 2.4 g/l arabinose) with 96 % of the glucan being left intact. The xylose-rich solution was used for production of astaxanthin by the yeast Phaffia rhodozyma. The resulting cellulose-enriched solid residue was used for ethanol production in a fed-batch simultaneous saccharification and fermentation (SSF) using Accellerase® 1500 (a commercial cellulase product) and the industrial yeast Saccharomyces cerevisiae. At the end of the fermentation 70 g/l ethanol was obtained, which was equivalent to 63 % theoretical yield based on the glucan content of the solid substrate.