Comparison of separate hydrolysis and fermentation and simultaneous saccharification and fermentation processes for ethanol production from wheat straw by recombinant Escherichia coli strain FBR5

Authors: Saha, Badal; Nichols, Nancy; Qureshi, Nasib; Cotta, Michael

Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2011
Publication Date: 10/4/2011
Citation: Saha, B.C., Nichols, N.N., Qureshi, N., Cotta, M.A. 2011. Comparison of separate hydrolysis and fermentation and simultaneous saccharification and fermentation processes for ethanol production from wheat straw by recombinant Escherichia coli strain FBR5. Applied Microbiology and Biotechnology. 92:865-874.

Interpretive Summary: Wheat straw contains about 65% carbohydrates that can be used for production of fuel ethanol. Generally, four steps are involved for its conversion to ethanol: pretreatment, enzymatic hydrolysis, fermentation, and ethanol recovery. Inhibitors are generated during pretreatment that need to be removed prior to fermentation. Integration of two or more process steps is important to lower the cost of production of ethanol from wheat straw. In this research, we have used a recombinant bacterium for fermentation to ethanol and combined the enzymatic hydrolysis and fermentation steps together typically known as simultaneous saccharification and fermentation (SSF). We demonstrated that SSF offered a distinct advantage over separate hydrolysis and fermentation (SHF) with respect to reducing total time required to produce ethanol from pretreated wheat straw. Also, fed-batch SSF (adding the substrate in three equal portions) performed better than the batch SSF with respect to shortening the time requirement and increase in ethanol yield. We showed for the first time that SHF can be easily run at a certain pH without removing the inhibitors. This finding is very important and encouraging.

Technical Abstract: Ethanol production by recombinant Escherichia coli strain FBR5 from dilute acid pretreated wheat straw (WS) by separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) was studied. The WS used in this study contained 32.0±0% cellulose, 32.1±1.3% hemicelluloses, and 11.2±0.4% lignin. The yield of total sugars (glucose, xylose, arabinose, and galactose) from dilute acid (0.5% H2SO4) pretreated (160 deg C, 10 min) and enzymatically saccharified (pH 5.0, 45 deg C, 72 h) WS (86 g/l) using a cocktail of 3 commercial enzyme (cellulase, beta-glucosidase, and hemicellulase) preparations at each enzyme dose level of 150 µl/g straw was 50.0±1.4 g/l which is equivalent to 82% of the theoretical sugar yield. In addition, the hydrolyzate contained 1,184±19 mg furfural and 161±1 mg hydroxymethyl furfural (HMF) per liter. The recombinant E. coli strain FBR5 could not grow at all at pH controlled at 4.5 to 6.5 in the non-abated WSH at 35 deg C. However, it produced 21.9±0.3 g ethanol from non-abated wheat straw hydrolyzate (WSH; total sugars), 44.1±0.4 g/l in 90 h including the lag time of 24 h at controlled pH 7.0 and 35 deg C. The bioabatement of WS was performed by growing the fungus Coniochaeta ligniaria NRRL 30616 in the liquid portion of the pretreated WS aerobically at pH 6.5 and 30 deg C for 15 h. The bacterium produced 21.6±0.5 g ethanol per liter in 40 h from the bioabated enzymatically saccharified WSH (total sugars, 44.1±0.4 g) at pH 6.0. It produced 24.9±0.3 g ethanol in 96 h and 26.7±0.0 g ethanol in 72 h per liter from bioabated WSH by batch SSF and fed-batch SSF, respectively. SSF offered distinct advantage over SHF with respect to reducing total time required to produce ethanol from the bioabated WS. Also, fed-batch SSF performed better than the batch SSF with respect to shortening the time requirement and increase in ethanol yield.