Improved sugar yields from biomass sorghum feedstocks: comparing low-lignin mutants and pretreatment chemistries

Authors: GODIN, BRUNO - National Renewable Energy Laboatory; NAGLE, NICK - National Renewable Energy Laboatory; Sattler, Scott; AGNEESSENS, RICHARD - Walloon Agricultural Research Center; DELCARTE, JEROME - Walloon Agricultural Research Center; WOLFRUM, EDWARD - National Renewable Energy Laboatory

Submitted to: Biotechnology for Biofuels and Bioproducts
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2016
Publication Date: 11/21/2016
Publication URL: http://handle.nal.usda.gov/10113/5728772
Citation: Godin, B., Nagle, N., Sattler, S.E., Agneessens, R., Delcarte, J., Wolfrum, E. 2016. Improved sugar yields from biomass sorghum feedstocks: comparing low-lignin mutants and pretreatment chemistries. Biotechnology for Biofuels. 9(1):251. doi:10.1186/s13068-016-0667-y.

Interpretive Summary: By 2040, the next generation of biofuels derived from plant biomass, also known as 2nd generation or advance biofuels, are expected to significantly contribute to U.S. bioenergy production. To maximize the production of biofuels from plant biomass and to make the process economically feasible, cellulose and hemicellulose components of biomass need to be efficiently converted into sugars. A third component of biomass, lignin impedes breakdown of cellulose and hemicellulose into the sugars that comprise these biomass components. One strategy to contend with lignin is to use brown midrib (bmr) mutants, which impair the ability of the sorghum plants to make this biomass component. In this study, the bmr sorghum biomass with reduced lignin content showed higher total sugar yields than normal sorghum biomass after dilute acid treatment and enzymatic digestion at two different scales. Removal of a chemical group from lignin through a biomass pre-treatment process also increased the total sugar yields from both bmr and normal sorghum. This study showed that bmr sorghum can reduce a barrier to converting biomass to sugars for bioenergy and bioproduct uses.

Technical Abstract: Background: For biofuel production processes to be economically efficient, it is essential to maximize the production of monomeric carbohydrates from the structural carbohydrates of feedstocks. One strategy for maximizing carbohydrate production is to identify less recalcitrant feedstock cultivars by performing some type of experimental screening on a large and diverse set of candidate materials, or by identifying genetic modifications (random or directed mutations or transgenic plants) that provide decreased recalcitrance. Economic efficiency can also be increased using additional pretreatment processes such as deacetylation, which uses dilute NaOH to remove the acetyl groups of hemicellulose prior to dilute acid pretreatment. In this work, we used a laboratory-scale screening tool that mimics relevant thermochemical pretreatment conditions to compare the total sugar yield of three near-isogenic brown midrib (bmr) mutant lines and the wild-type (WT) sorghum cultivar. We then compared results obtained from the laboratory-scale screening pretreatment assay to a large-scale pretreatment system. Results: After pretreatment and enzymatic hydrolysis, the bmr mutants had higher total sugar yields than the WT sorghum cultivar. Increased pretreatment temperatures increased reactivity for all sorghum samples reducing the differences observed at lower reaction temperatures. Deacetylation prior to dilute acid pretreatment increased the total sugar yield for all four sorghum samples, and reduced the differences in total sugar yields among them, but solubilized a sizable fraction of the non-structural carbohydrates. The general trends of increased total sugar yield in the bmr mutant compared to the WT seen at the laboratory scale were observed at the large-scale system. However, in the larger reactor system, the measured total sugar yields were lower and the difference in total sugar yield between the WT and bmr sorghum was larger. Conclusions: Sorghum bmr mutants, which have a reduced lignin content showed higher total sugar yields than the WT cultivar after dilute acid pretreatment and enzymatic hydrolysis. Deacetylation prior to dilute acid pretreatment increased the total sugar yield for all four sorghum samples. However, since deacetylation also solubilizes a large fraction of the non-structural carbohydrates, the ability to derive value from these solubilized sugars will depend greatly on the proposed conversion process.