Yadhu N. Guragain

Importance of Biomass-Specific Pretreatment Methods for Effective and Sustainable Utilization of Renewable Resources

Pretreatment is the central unit operation that plays a key role for efficient bioconversion of lignocellulosic biomass. However, a single pretreatment method cannot be an effective choice for all types of biomass. A distinct variation in biomass composition and structure is clearly evident, which complicates further the pretreatment optimization process. Fundamental understanding of pretreatment mechanisms and composition of biomass feedstocks is vital to develop an appropriate pretreatment method for each type of feedstock. Our study showed that at same processing conditions, acid pretreatment is significantly less effective than alkali pretreatment for grasses (switchgrass, sorghum stalk, and corn stover) and hardwood (poplar). Sugar released during enzymatic hydrolysis was 38, 65, 72, and 63 % less from acid pretreated biomass compared to alkali pretreated biomass for switchgrass, sorghum stalk, corn stover, and poplar, respectively. Among the various alkaline organic solvent pretreatments, an equal mixture (by volume) of isopropanol and ethanol led to the highest sugar yield (0.50 g/g) from corn stover, while glycerol yielded the highest sugar (0.40 g/g) from poplar. None of these pretreatment methods were effective for softwood (Douglas fir); sugar release was less than 0.1 g/g biomass. Brown midrib (bmr)-12 mutation of sorghum resulted in an increased total fermentable sugar recovery by 36 and 30 % in Atlas, and Kansas Collier cultivars, respectively, while a decreased sugar recovery by 12 % was observed in Early Hegari cultivar. These results showed that decreased lignin in biomass is not necessarily beneficial to increase the pretreatment efficiency, indicating that lignin composition and inter-lignin unit linkages also have significant impact in addition to total lignin content on biomass pretreatment efficiency. Furthermore, biomass feedstocks have various amounts of non-structural sugars, and a number of high-value phytochemicals, which must be extracted prior to biomass pretreatment. Consequently, a targeted biorefining strategy is required for each type of feedstock.

Evaluation of Brown Midrib Sorghum Mutants as a Potential Biomass Feedstock for 2,3-Butanediol Biosynthesis

Three sorghum backgrounds [Atlas, Early Hegari (EH), and Kansas Collier (KC)] and two bmr mutants (bmr6 and bmr12) of each line were evaluated and compared for grain and biomass yield, biomass composition, and 2,3-butanediol production from biomass. The data showed that the bmr6 mutation in EH background led to a significant decrease in stover yield and increase in grain yield, whereas the stover yield was increased by 64% without affecting grain yield in KC background. The bmr mutants had 10 to 25% and 2 to 9% less lignin and structural carbohydrate contents, respectively, and 24 to 93% more non-structural sugars than their parents in all sorghum lines, except EH bmr12. The total fermentable sugars released were 22 to 36% more in bmr mutants than in parents for Atlas and KC, but not for EH. The bmr6 mutation in KC background produced the most promising feedstock, among the evaluated bmr mutants, for 2,3-butanediol production without affecting grain yield, followed by KC bmr12 and Atlas bmr6, but the bmr mutation had an adverse effect in EH background. This indicated that the genetic background of the parent line and type of bmr mutation significantly affect the biomass quality as a feedstock for biochemical production.

Production of free fatty acids from switchgrass using recombinant Escherichia coli

Switchgrass is a promising feedstock to generate fermentable sugars required for the sustainable operation of biorefineries because of their abundant availability, easy cropping system, and high cellulosic content. The objective of this study was to investigate the potentiality of switchgrass as an alternative sugar supplier for free fatty acid (FFA) production using engineered Escherichia coli strains. Recombinant E. coli strains successfully produced FFAs using switchgrass hydrolysates. A total of about 3 g/L FFAs were attained from switchgrass hydrolysates by engineered E. coli strains. Furthermore, overall yield assessments of our bioconversion process showed that 88 and 46% of the theoretical maximal yields of glucose and xylose were attained from raw switchgrass during sugar generation. Additionally, 72% of the theoretical maximum yield of FFAs were achieved from switchgrass hydrolysates by recombinant E. coli during fermentation. These shake‐flask results were successfully scaled up to a laboratory scale bioreactor with a 4 L working volume. This study demonstrated an efficient bioconversion process of switchgrass‐based FFAs using an engineered microbial system for targeting fatty acid production that are secreted into the fermentation broth with associated lower downstream processing costs, which is pertinent to develop an integrated bioconversion process using lignocellulosic biomass.