Cellulosic butanol biorefinery: production of biobutanol from high solid loadings of sweet sorghum bagasse - simultaneous saccharification, fermentation, and product recovery

Authors: Qureshi, Nasib; Saha, Badal; Liu, Siqing; EZEJI, THADDEUS, - The Ohio State University; Nichols, Nancy

Submitted to: Fermentation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2021
Publication Date: 12/14/2021
Citation: Qureshi, N., Saha, B.C., Liu, S., Ezeji, T.C., Nichols, N.N. 2021. Cellulosic butanol biorefinery: production of biobutanol from high solid loadings of sweet sorghum bagasse - simultaneous saccharification, fermentation, and product recovery. Fermentation. 7(4): Article 310. https://doi.org/10.3390/fermentation7040310.
DOI: https://doi.org/10.3390/fermentation7040310

Interpretive Summary: Butanol is a superior biofuel to ethanol and contains 33% more energy on a gallon basis. There is a strong interest in the production of this biofuel. Historically, butanol was produced from corn starch and cane molasses. The use of these feedstocks is no longer economic due to their high prices. Hence, butanol should be produced from renewable cellulosic biomass using energy efficient production technologies. This study used fermentations of sweet sorghum crop residue (called bagasse) to produce butanol. Traditionally, the feedstock concentration must be limited because the product, butanol, is toxic to the fermenting microbe. In the present study, continuous product recovery was incorporated into the process, to prevent the product from reaching toxic levels. The product was captured using vacuum recovery. In this system nearly all the sugars were fermented, with a good rate of productivity (0.18 gL-1h-1). The system thus allowed fermentation of much higher feedstock loadings and recovery of more product compared to systems without continuous product recovery. The study also revealed an area of needed improvement because by the end of fermentation (132 h), the fermenting microbe lost its capability to use one type of sugar. This work is important for sweet sorghum producers as well as the biofuels industry and those interested in converting biomass to value-added products.

Technical Abstract: Fermentations of high solid concentrations of sweet sorghum bagasse (SSB; 160-220 gL-1) were used to produce butanol or acetone butanol and ethanol (ABE). Traditional batch fermentations are limited to approximately 80-90 gL-1 biomass (55-60 gL-1 sugars) because of end-product toxicity from butanol. In the present studies, continuous product recovery was integrated with simultaneous saccharification and fermentation to prevent butanol from achieving toxic levels. The product was recovered using vacuum fermentation. Initially, 220 gL-1 SSB solids were charged into the bioreactor, but fermentation to ABE was not successful under these conditions. This solid concentration was too high and inhibited the butanol fermentation. Following this, SSB solid concentration was reduced to 160 gL-1. In this system all the sugars except 7.78 gL-1 xylose were fermented and ABE productivity of 0.18 gL-1h-1 was achieved. Although in the beginning the culture utilized xylose, by the end of fermentation (132 h), it lost its capability to use this sugar. This suggests that either the fermenting strain should be mutated to alleviate glucose repression or the reactor should be inoculated with new inoculum near the end of fermentation to use residual xylose.