Detoxification of Organosolv-Pretreated Pine Prehydrolysates with Anion Resin and Cysteine for Butanol Fermentation

Carbon catabolite repression during the simultaneous utilization of lignocellulose-derived sugars in lactic acid production: Influencing factors and mitigation strategies

Lignocellulose is the most abundant, sustainable, and comparatively economical renewable biomass containing ample fermentable sugars for bio-based chemical production, such as lactic acid (LA). LA is a versatile chemical with substantial global demand. However, the concurrent utilization of mixed sugars derived from lignocellulose, including glucose, xylose, and arabinose, remains a formidable challenge because of the metabolic regulation of carbon catabolite repression (CCR), in which glucose is preferentially utilized over non-glucose sugars, resulting in the loss of carbon resources and a decrease in biorefinery efficacy. Most current studies on CCR have concentrated on elucidating the principles and their impact on specific bacterial species using mixed carbon sources. However, there remains a notable dearth of comprehensive reviews summarizing the underlying principles and corresponding mitigation strategies across other bacterial strains encountering similar challenges. In light of this, this article delineates the possible factors that lead to CCR, including signal transduction and metabolic pathways. Additionally, the fermentation conditions and nutrients are described. Finally, this study proposes appropriate mitigation strategies to overcome the aforementioned obstacles and presents new insights into the rapid and simultaneous consumption of mixed sugars to bolster the production yields of biofuels and chemicals in the future.

Distinct effects of dilute acid prehydrolysate inhibitors on enzymatic hydrolysis and yeast fermentation

The effects of dilute acid prehydrolysate from poplar were investigated and compared in the enzymatic hydrolysis, fermentation, and simultaneous saccharification fermentation (SSF) in this study. The improvement of enzymatic hydrolysis and fermentation with resin adsorption and surfactant addition has also been represented. A total of 16 phenolic alcohols, aldehydes, acids and 3 furan derivatives in the prehydrolysates were identified and quantified by gas chromatography/mass spectrometry (GC/MS). The degree of inhibition from the phenolic compounds (26.55%) in prehydrolysate on the enzymatic hydrolysis was much higher than carbohydrates-derived inhibitors (0.52-4.64%). Around 40% degree of inhibition was eliminated in Avicel enzymatic hydrolysis when 75% of prehydrolysates phenolic compounds were removed by resin adsorption. This showed distinguishing inhibition degrees of various prehydrolysate phenolic compounds. Inhibition of prehydrolysate on enzymatic hydrolysis was more dosage-dependent, while their suppression on the fermentation showed a more complicated mode: fermentation could be terminated by the untreated prehydrolysate, while a small number of prehydrolysate inhibitors even improved the glucose consumption and ethanol production in the fermentation. Correlated with this distinct inhibition modes of prehydrolysate, the improvement of Tween 80 addition in SSF was around 7.10% for the final ethanol yield when the glucose accumulation was promoted by 76.6%.

Co-fermentation Approach of Fructose and Glucose to Ethanol from Chinese Elaeagnus angustifolia Fruit (EAF)

The soluble and fermentable carbohydrate contents was detected over 47% of glucose and fructose in Chinese Elaeagnus angustifolia fruit powder (EAF), being over 47 wt% sugar content more than that of grape. Ethanol was therefore fermented directly from EAF, and different submerged fermentation modes were comparatively employed to optimize ethanol harvest. The results indicated that glucose has certain competitive inhibition on fructose bio-utilization, as well as the EAF solid residue involved fermentation mode also hindered the fermented-ethanol titer. Pectinase addition and in situ hydrolysis seemed to assist somewhat the fermentation. The water-solute fermentation mode is preferable, and glucose and fructose components were completely consumed and converted to 80.96 g/L ethanol at 87.6% ethanol yield even under tannin and pectin inhibition. The fermentation result could provide some experimental data and an approach to not only new biomass resource explores of bioethanol and alcohol beverage production, but also the technological development on valorization commercials of EAF in global draught areas.

Cysteine supplementation enhanced inhibitor tolerance of Zymomonas mobilis for economic lignocellulosic bioethanol production

Inhibitors in lignocellulosic hydrolysates are toxic to Zymomonas mobilis and reduce its bioethanol production. This study revealed cysteine supplementation enhanced furfural tolerance in Z. mobilis with a 2-fold biomass increase. Transcriptomic study illustrated that cysteine biosynthesis pathway was down-regulated while cysteine catabolism was up-regulated with cysteine supplementation. Mutants for genes involved in cysteine metabolism were constructed, and metabolites in cysteine metabolic pathway including methionine, glutathione, NaHS, glutamate, and pyruvate were supplemented into media. Cysteine supplementation boosted glutathione synthesis or H₂S release effectively in Z. mobilis leading to the reduced accumulation of reactive oxygen species (ROS) induced by furfural, while pyruvate and glutamate produced in the H₂S generation pathway promoted cell growth by serving as the carbon or nitrogen source. Finally, cysteine supplementation was confirmed to enhance Z. mobilis tolerance against ethanol, acetate, and corncob hydrolysate with an enhanced ethanol productivity from 0.38 to 0.55 g^-1∙L^-1∙h^-1.

Laccase-mediated delignification and detoxification of lignocellulosic biomass: removing obstacles in energy generation

The rising global population and worldwide industrialization have led to unprecedented energy demand that is causing fast depletion of fossil reserves. This has led to search for alternative energy sources that are renewable and environment friendly. Use of lignocellulosic biomass for energy generation is considered a promising approach as it does not compete with food supply. However, the lignin component of the biomass acts as a natural barrier that prevents its efficient utilization. In order to remove the lignin and increase the amount of fermentable sugars, the lignocellulosic biomass is pretreated using physical and chemical methods which are costly and hazardous for environment. Moreover, during the traditional pretreatment process, numerous inhibitory compounds are generated that adversely affect the growth of fermentative microbes. Alternatively, biological methods that use microbes and their enzymes disrupt lignin polymers and increase the accessibility of the carbohydrates for the sugar generation. Microbial laccases have been considered as an efficient biocatalyst for delignification and detoxification offering a green initiative for energy generation process. The present review aims to bring together recent studies in bioenergy generation using laccase biocatalyst in the pretreatment processes. The work provides an overview of the sustainable and eco-friendly approach of biological delignification and detoxification through whole-cell and enzymatic methods, use of laccase-mediator system, and immobilized laccases for this purpose. It also summarizes the advantages, associated challenges, and potential prospects to overcome the limitations.

Effective isolation of antioxidant Phelligridin LA from the fermentation broth of Inonotus baumii by macroporous resin

Phelligridin LA (PLA) is a natural product with vigorous free radical scavenging activities accumulated in the liquid fermentation of herbal medicinal fungus Inonotus baumii. Aiming to establish an efficient isolation method of PLA from the fermentation broth, we evaluated the adsorption of PLA by macroporous resins. The best resin ADS-17 was screened for six candidates with various physical properties and adsorption behaviors. Studies on the thermodynamics and kinetics of the process revealed that the adsorption reaction could take place spontaneously, which implied that the heat generated in adsorption might compensate for the decrease in entropy. The Freundlich theory could be utilized to fit the experimental data. The pseudo-second-order equation could describe the process, and the adsorption rate was primarily controlled by liquid film diffusion and pore diffusion. The influencing operation factors (temperature, pH, and the ratio of fermentation broth to resin) of the adsorption process were optimized with response surface methodology. The optimized condition (temperature 22.81 °C, pH 5.19, and the ratio of fermentation broth to resin or RLS 5.11) supported an adsorption rate of 97.03%. These findings would be indispensable for further optimization of the efficient separation of PLA from the fermentation broth, and the fermentation production of PLA in which separation would be included.

Effect of residual extractable lignin on acetone-butanol-ethanol production in SHF and SSF processes

BACKGROUND

Lignin plays an important role in biochemical conversion of biomass to biofuels. A significant amount of lignin is precipitated on the surface of pretreated substrates after organosolv pretreatment. The effect of this residual lignin on enzymatic hydrolysis has been well understood, however, their effect on subsequent ABE fermentation is still unknown.

RESULTS

To determine the effect of residual extractable lignin on acetone-butanol-ethanol (ABE) fermentation in separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) processes, we compared ABE production from ethanol-washed and unwashed substrates. The ethanol organosolv pretreated loblolly pine (OPLP) was used as the substrate. It was observed that butanol production from OPLP-UW (unwashed) and OPLP-W (washed) reached 8.16 and 1.69 g/L, respectively, in SHF. The results showed that ABE production in SHF from OPLP-UW prevents an "acid crash" as compared the OPLP-W. In SSF process, the "acid crash" occurred for both OPLP-W and OPLP-UW. The inhibitory extractable lignin intensified the "acid crash" for OPLP-UW and resulted in less ABE production than OPLP-W. The addition of detoxified prehydrolysates in SSF processes shortened the fermentation time and could potentially prevent the "acid crash".

CONCLUSIONS

The results suggested that the residual extractable lignin in high sugar concentration could help ABE production by lowering the metabolic rate and preventing "acid crash" in SHF processes. However, it became unfavorable in SSF due to its inhibition of both enzymatic hydrolysis and ABE fermentation with low initial sugar concentration. It is essential to remove extractable lignin of substrates for ABE production in SSF processes. Also, a higher initial sugar concentration is needed to prevent the "acid crash" in SSF processes.

Improving the economy of lignocellulose-based biorefineries with organosolv pretreatment

Lignocellulose-based processes for production of value-added products still face bottlenecks to attain feasibility. The key might lie on the biorefining of all lignocellulose main polymers, that is, cellulose, hemicellulose and lignin. Lignin, considered an impediment in the access of cellulose and normally considered for energy recovery purposes, can give a higher contribution towards profitability of lignocellulosic biorefineries. Organosolv pretreatment allows selective fractionation of lignocellulose into separate cellulose-, hemicellulose- and lignin-rich streams. Ethanol organosolv and wood substrates dominated the research studies, while a wide range of substrates need definition on the most suitable organosolv pretreatment systems. Techno-economic and environmental analyses of organosolv-based processes as well as proper valorization strategies of the hemicellulose-rich fraction are still scarce. In view of dominance of ethanol organosolv with high delignification yields and high-purity of the recovered cellulose-rich fractions, close R & D collaboration with 1st generation ethanol plants might boost commercialization.

Effect of detoxification methods on ABE production from corn stover hydrolysate by Clostridium acetobutylicum CICC 8016

In this study, effects of different single biomass derived inhibitors on acetone-butanol-ethanol (ABE) production by Clostridium acetobutylicum CICC 8016 were first investigated. The results showed that formic acid, coumaric acid, and furfural at 0.5 g/L (sodium formate equivalent) inhibited ABE production. Furthermore, corn stover hydrolysate media were prepared following dilute acid pretreatment, enzymatic hydrolysis, and detoxification with different methods. Among overliming, steam stripping, acetone-ethyl ether extraction, and ion exchange with five anion resins, adsorption with resin D301 showed the highest efficiency for inhibitor removal (99-100% of phenolics and 87-99% of sugar degradation products). Without detoxification, ABE production was lower than 1.0 g/L from 28.1 g/L sugars whereas ABE production with medium detoxified by D301 resin achieved higher ABE concentrations and yields than control with synthetic medium. Correlation analysis further revealed that formic acid, coumaric acid, and total phenolics were the major compounds inhibiting ABE production. The results also showed that the single detoxification method was sufficient to detoxify the hydrolysate for ABE production at the pretreatment conditions used in this study.

Semi-hydrolysate of paper pulp without pretreatment enables a consolidated fermentation system with in situ product recovery for the production of butanol

Utilization of lignocellulosic biomasses for biobutanol fermentation usually requires costly processes of pretreatment and enzymatic hydrolysis. In this study, paper pulp (93.2% glucan) was used as a starting biomass material to produce biobutanol. We conducted enzymatic semi-hydrolysis of paper pulp without pretreatment and with low enzyme loading, which produced high concentrations of cellobiose (13.9 g L^-1) and glucose (21.3 g L^-1). In addition, efficient fermentation of the semi-hydrolysate was achieved similar to that with the use of commercial sugars without inhibitors. Finally, we designed a novel non-isothermal simultaneous saccharification and fermentation with in situ butanol recovery, which was composed of a repeated semi-hydrolysis process and successive butanol-extractive fermentation process under the respective optimal conditions. The consolidated system improved butanol production, butanol yields, and butanol productivities and enabled repeated use of medium when compared with other integrated hydrolysis and fermentation processes.