|Kim, Tae Hyun|
|Nghiem, Nhuan - John|
Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2010
Publication Date: 3/24/2010
Citation: Li, X., Kim, T., Nghiem, N.P. 2010. Bioethanol production from corn stover using aqueous ammonia pretreatment and two-phase simultaneouos saccharification and fermentation (TPSSF). Bioresource Technology. 101:5910-5916. Interpretive Summary: Cellulosic ethanol is of great interest since corn ethanol production is limited. Commercialization of cellulosic ethanol is economically feasible only if both of the main fermentable sugars, i.e. glucose and xylose, can be converted to ethanol at high rates and yields. The yeast Saccharomyces cerevisiae ferments glucose to ethanol at high efficiencies but cannot metabolize xylose. On the other hand, recombinant organisms developed for cellulosic ethanol production can ferment both glucose and xylose but normally do not ferment glucose at the same efficiency as the yeast. The aforementioned problems are overcome by our recently developed processing scheme. The biomass, in our particular case, corn stover, was first pretreated by soaking in aqueous ammonia (SAA). The pretreated corn stover was hydrolyzed with commercial xylanase enzyme to produce a xylose-rich solution, which contained only low levels of glucose. The solid residue was recovered and the sugar solution was fermented to ethanol using Escherichia coli KO11, which is a well known ethanol-producing recombinant organism capable of converting xylose to ethanol at high efficiency. The recovered solid was hydrolyzed with commercial cellulase enzymes to produce a glucose-rich solution, which subsequently was fermented to ethanol by the yeast. The result of this work is a two-step fermentation process for converting both key sugars in cellulosic biomass to ethanol at high efficiencies using microorganisms most suitable for utilization of each sugar. The results obtained should be applicable to other types of cellulosic biomass and help push cellulosic ethanol forward and closer to the reality of commercialization.
Technical Abstract: An integrated bioconversion process was developed to convert corn-stover derived pentose and hexose to ethanol effectively. In this study, corn stover was pretreated by soaking in aqueous ammonia (SAA), which resulted in high retention of glucan (~100%) and xylan (>80%) in the solids. The pretreated carbohydrates-rich corn stover was converted to ethanol via sequential pentose-hexose saccharification and fermentation (SePHSaF). This single-reactor process employed sequential simultaneous saccharification and fermentation (SSF), i.e. pentose conversion using recombinant Escherichia coli KO11 in the first phase, followed by hexose conversion with Saccharomyces cerevisiae D5A in the second phase. In the first phase, high xylan digestibility (88%) was achieved through the synergistic action of xylanase and endo-glucanase with minimal glucan hydrolysis (10.5%). The SePHSaF using 12-h SAA resulted in the highest ethanol concentration (22.5 g/L), which was equivalent to 85% of the theoretical ethanol yield based on the total carbohydrates (glucan+xylan) in the untreated corn stover.