A chemical-free pretreatment for biosynthesis of bioethanol and lipids from lignocellulosic biomass: An industrially relevant 2G biorefinery approach

Authors: DESHAVATH, NARENDRA - University Of Illinois; Dien, Bruce; Slininger, Patricia - Pat; JIN, YONG-SU - University Of Illinois; SINGH, VIJAY - University Of Illinois

Submitted to: Fermentation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2022
Publication Date: 12/21/2022
Citation: Deshavath, N., Dien, B.S., Slininger, P.J., Jin, Y., Singh, V. 2022. A chemical-free pretreatment for biosynthesis of bioethanol and lipids from lignocellulosic biomass: An industrially relevant 2G biorefinery approach. Fermentation. 9(1). Article 5. https://doi.org/10.3390/fermentation9010005.
DOI: https://doi.org/10.3390/fermentation9010005

Interpretive Summary: Sorghum grown for biomass production is a promising bioenergy crop because it is drought tolerant, requires low inputs, and can be grown on lands not suitable for row crops. ARS and University of Illinois researchers are developing an industrially friendly method to convert harvested stalks and leaves to sugars, which can be fermented by yeast, to fuel ethanol or oils suitable for use in making biodiesel. The processed stalks yield sugar syrups containing 22-24% concentrated sugars. However, the original technology requires adding 19.2 kg of citric acid per ton of biomass, which is estimated to cost $33 per ton. Not only is this expensive, the citric acid slows down the ethanol fermentation and complicates waste water treatment. Here it was discovered that the amount of citrate added can be reduced 100 fold without reducing the sugar yields. The new sugar syrup was, furthermore, shown to be easily converted to ethanol or lipids by yeast fermentation. This research will be of interest to current corn ethanol producers looking to expand into cellulosic ethanol or adding yeast oil as a new biofuel product.

Technical Abstract: A wide range of inorganic and organic chemicals are used during the pretreatment and enzymatic hydrolysis of lignocellulosic biomass to produce biofuels. Developing an industrially relevant 2G biorefinery process using such chemicals is challenging and requires more unit operations for downstream processing. A sustainable process has been developed to achieve industrially relevant titers of bioethanol with significant ethanol yield. The pretreatment of sorghum biomass was performed by a continuous pilot-scale hydrothermal reactor followed by disk milling. Enzymatic hydrolysis was performed without washing the pretreated biomass. Moreover, citrate buffer strength was reduced to 100-fold (50 mM to 0.5 mM) during the enzymatic hydrolysis. Enzymatic hydrolysis at 0.5 mM citrate buffer strength showed that significant sugar concentrations of 222 ± 2.3 to 241 ± 2.3 g/L (glucose + xylose) were attained at higher solids loadings of 50 to 60% (w/v). Furthermore, hydrolysates were fermented to produce bioethanol using two different xylose-fermenting Saccharomyces cerevisiae strains and a co-culture of xylose-fermenting and non-GMO yeast cultures. Bioethanol titer of 81.7 g/L was achieved with an ethanol yield of 0.48 gp/gs. Additionally, lipids were produced using the oleaginous yeast Rhodosporidium toruloides, yielding 13.2 g/L lipids with cellular lipid accumulation of 38.5% w/w from 100 g/L of sugar concentration. In summary, reducing the strength of the citrate buffer during enzymatic hydrolysis and omitting inorganic chemicals from the pretreatment process enhances the fermentability of hydrolysates and can also reduce operating costs