Integrated in situ gas stripping-salting-out process for high-titer acetone-butanol-ethanol production from sweet sorghum bagasse

Evaluation of free and immobilized cellulase on chitosan-modified magnetic nanoparticles for saccharification of sorghum residue

Enzymatic hydrolysis plays a pivotal role in transforming lignocellulosic biomass. Addressing alternate techniques to optimize the utilization of cellulolytic enzymes is one strategy to improve its efficiency and lower process costs. Cellulases are highly specific and environmentally benign biocatalysts that break down intricate polysaccharides into simple forms of sugars. In contrast to the most difficult and time-consuming enzyme immobilization processes, in this research, we studied simple, mild, and successful techniques for immobilization of pure cellulase on magnetic nanocomposites using glutaraldehyde as a linker and used in the application of sorghum residue biomass. Fe3O4 nanoparticles were coated with chitosan from the co-precipitation method, which served as an enzyme carrier. The nanoparticles were observed under XRD, Zeta Potential, FESEM, VSM, and FTIR. The size morphology results presented that the Cs@Fe3O4 have 42.2 nm, while bare nanoparticles (Fe3O4) have 31.2 nm in size. The pure cellulase reaches to 98.07% of loading efficiency and 71.67% of recovery activity at optimal conditions. Moreover, immobilized enzyme's pH stability, thermostability, and temperature tolerance were investigated at suitable conditions. The kinetic parameters of free and immobilized enzyme were estimated as Vmax; 29 ± 1.51 and 27.03 ± 2.02 µmol min-1 mg-1, Km; 4.7 ± 0.49 mM and 2.569 ± 0.522 mM and Kcat; 0.13 s-1, and 0.89 s-1. Sorghum residue was subjected to 2% NaOH pre-treatment at 50 ℃. Pre-treated biomass contains cellulose of 64.8%, used as a raw material to evaluate the efficiency of reducing sugar during hydrolysis and saccharification of free and immobilized cellulase, which found maximum concentration of glucose 5.42 g/L and 5.12 g/L on 72 h. Thus, our study verifies the use of immobilized pure cellulase to successfully hydrolyze raw material, which is a significant advancement in lignocellulosic biorefineries and the reusability of enzymes.

Probing efficient microbial CO2 utilisation through metabolic and process modelling

Acetogenic gas fermentation is increasingly studied as a promising technology to upcycle carbon-rich waste gasses. Currently the product range is limited, and production yields, rates and titres for a number of interesting products do not allow for economically viable processes. By pairing process modelling and host-agnostic metabolic modelling, we compare fermentation conditions and various products to optimise the processes. The models were then used in a simulation of an industrial-scale bubble column reactor. We find that increased temperatures favour gas transfer rates, particularly for the valuable and limiting H2 , while furthermore predicting an optimal feed composition of 9:1 mol H2 to mol CO2 . Metabolically, the increased non-growth associated maintenance requirements of thermophiles favours the formation of catabolic products. To assess the expansion of the product portfolio beyond acetate, both a product volatility analysis and a metabolic pathway model were implemented. In-situ recovery of volatile products is shown to be within range for acetone but challenging due to the extensive evaporation of water, while the direct production of more valuable compounds by acetogens is metabolically unfavourable compared to acetate and ethanol. We discuss alternative approaches to overcome these challenges to utilise acetogenic CO2 fixation to produce a wider range of carbon negative chemicals.

Fed-batch simultaneous saccharification and fermentation including in-situ recovery for enhanced butanol production from rice straw

This paper describes a study of fed-batch SSFR (simultaneous saccharification, fermentation and recovery) for butanol production from alkaline-pretreated rice straw (RS) in a 2-L stirred tank reactor. The initial solid (9.2% w/v) and enzyme (19.9 FPU g-dw^-1) loadings were previously optimized by 50-mL batch SSF assays. Maximum butanol concentration of 24.80 g L^-1 was obtained after three biomass feedings that doubled the RS load (18.4% w/v). Butanol productivity (0.344 g L^-1h^-1) also increased two-fold in comparison with batch SSF without recovery (0.170 g L^-1h^-1). Although fed-batch SSFR was able to operate with a single initial enzyme dosage, an extra dosage of nutrients was required with the biomass additions to achieve this high productivity. The study showed that SSFR can efficiently improve butanol production from a lignocellulosic biomass accompanied by the efficient use of the enzyme.

Optimization of a pretreatment and hydrolysis process for the efficient recovery of recycled sugars and unknown compounds from agricultural sweet sorghum bagasse stem pith solid waste

Background

Sweet sorghum bagasse (SSB), comprising both a dermal layer and pith, is a solid waste generated by agricultural activities. Open burning was previously used to treat agricultural solid waste but is harmful to the environment and human health. Recent reports showed that certain techniques can convert this agricultural waste into valuable products. While SSB has been considered an attractive raw material for sugar extraction and the production of value-added products, the pith root in the SSB can be difficult to process. Therefore, it is necessary to pretreat bagasse before conventional hydrolysis.

Methods

A thorough analysis and comparison of various pretreatment methods were conducted based on physicochemical and microscopic approaches. The responses of agricultural SSB stem pith with different particle sizes to pretreatment temperature, acid and alkali concentration and enzyme dosage were investigated to determine the optimal pretreatment. The integrated methods are beneficial to the utilization of carbohydrate-based and unknown compounds in agricultural solid waste.

Results

Acid (1.5−4.5%, v/v) and alkali (5−8%, w/v) reagents were used to collect cellulose from different meshes of pith at 25–100 °C. The results showed that the use of 100 mesh pith soaked in 8% (w/v) NaOH solution at 100 °C resulted in 32.47% ± 0.01% solid recovery. Follow-up fermentation with 3% (v/v) acid and 6.5% (w/v) alkali at 50 °C for enzymolysis was performed with the optimal enzyme ratio. An analysis of the surface topography and porosity before and after pretreatment showed that both the pore size of the pith and the amount of exposed cellulose increased as the mesh size increased. Interestingly, various compounds, including 42 compounds previously known to be present and 13 compounds not previously known to be present, were detected in the pretreatment liquid, while 10 types of monosaccharides, including D-glucose, D-xylose and D-arabinose, were found in the enzymatic solution. The total monosaccharide content of the pith was 149.48 ± 0.3 mg/g dry matter.

Discussion

An integrated technique for obtaining value-added products from sweet sorghum pith is presented in this work. Based on this technique, lignin and hemicellulose were effectively broken down, amorphous cellulose was obtained and all sugars in the sweet sorghum pith were hydrolysed into monosaccharides. A total of 42 compounds previously found in these materials, including alcohol, ester, acid, alkene, aldehyde ketone, alkene, phenolic and benzene ring compounds, were detected in the pretreatment pith. In addition, several compounds that had not been previously observed in these materials were found in the pretreatment solution. These findings will improve the transformation of lignocellulosic biomass into sugar to create a high-value-added coproduct during the integrated process and to maximize the potential utilization of agricultural waste in current biorefinery processing.

Effect of salts formed by neutralization for the enzymatic hydrolysis of cellulose and acetone–butanol–ethanol fermentation

Neutralization is essential to maintain the pH for enzymatic hydrolysis of cellulose followed by fermentation of biofuels. This study investigated the effect of salts formed during the neutralization on the enzymatic hydrolysis of cellulosic materials and acetone–butanol–ethanol (ABE) fermentation. The results showed that the formed Ca-citrate salt considerably decreased the glucose release by 26.9% and 26.1% from Avicel and sulfuric acid-pretreated hybrid Pennisetum, respectively, which was probably due to the unproductive adsorption of cellulases by Ca-citrate solids. On the other hand, the formed soluble Na and Ca salts severely inhibited ABE fermentation, thereby decreasing the ABE concentration from 12.8 g L⁻¹ to 0–10.7 g L⁻¹ in different degrees, but no or slight inhibition was observed when the Ca salts formed as precipitates. In particular, Ca-sulfate did not show apparent inhibition of both hydrolysis and fermentation. Therefore, the selection of suitable pretreatment and neutralizing reagents is an alternative way to avoid process inhibition in biofuel production from lignocellulosic materials.

The salts formed by neutralization after sulfuric, acetic, and citric acid pretreatments affected enzymatic hydrolysis of lignocellulosic materials and acetone–butanol–ethanol (ABE) fermentation to various degrees.