Integration of Pretreatment With Simultaneous Counter-Current Extraction of Energy Sorghum for High-Titer Mixed Sugar Production

Daniel L Williams,Rebecca G Ong,John E Mullet,David B Hodge

doi:10.3389/fenrg.2018.00133

Abstract

Sorghum (Sorghum bicolor L. Moench) offers substantial potential as a feedstock for the production of sugar-derived biofuels and biochemical products from cell wall polysaccharides (i.e., cellulose and hemicelluloses) and water-extractable sugars (i.e., glucose, fructose, sucrose, and starch). A number of preprocessing schemes can be envisioned that involve processes such as sugar extraction, pretreatment, and densification that could be employed in decentralized, regional-scale biomass processing depots. In this work, an energy sorghum exhibiting a combination of high biomass productivity and high sugar accumulation was evaluated for its potential for integration into several potential biomass preprocessing schemes. This included counter-current extraction of water-soluble sugars followed by mild NaOH or liquid hot water pretreatment of the extracted bagasse. A novel processing scheme was investigated that could integrate with current diffuser-type extraction systems for sugar extraction. In this approach, mild NaOH pretreatment (i.e., 8%) at high metabolic yields without detoxification using a Saccharomyces cerevisiae strain metabolically engineered and evolved to ferment xylose.

Highlights

Ethanol derived from sucrose and starch crops has seen substantial growth in recent years (Babcock, 2012), and a non-trivial fraction of the agricultural output of some countries such as the U.S and Brazil are currently devoted to supply these first-generation fuel ethanol processes
An innovative hybrid approach to sugar extraction and pretreatment of energy sorghum was investigated in this work to demonstrate the potential of integrating mild alkaline pretreatment with counter-current sugar extraction
Hybrid energy sorghum TX08001 was found to be comprised of 64% carbohydrates as a percentage of the total dry mass of the sorghum, of which 27% of this sugar is water-extractable sucrose, fructose, glucose, and unidentified glucan

Summary

Introduction

Ethanol derived from sucrose and starch crops has seen substantial growth in recent years (Babcock, 2012), and a non-trivial fraction of the agricultural output of some countries such as the U.S and Brazil are currently devoted to supply these first-generation fuel ethanol processes. A lignocellulosic biorefinery that uses a biological process consisting of pretreatment such as dilute acid or dilute aqueous ammonia, enzymatic hydrolysis, and fermentation to produce cellulosic ethanol will need to process on the order of 2,000 dry tons per day of biomass to generate comparable ethanol yields as a typical dry grind corn ethanol plant (50 MM gal/yr). This presents an enormous set of logistical and storage challenges for these low bulk density feedstocks. As one example, decentralized, depot-scale preprocessing that employs AFEX pretreatment and pelletization could provide a higher bulk-density feedstock that facilitates transportation and storage, enabling year-round supply, decreasing the sensitivity to supply chain disruptions, and potentially yielding a product with additional applications, such as a high-digestibility ruminant feed (Campbell et al, 2013; Lamers et al, 2015)

Methods

Results

Conclusion