Combined pretreatment using ozonolysis and ball milling to improve enzymatic saccharification of corn straw

Extraction of homogeneous lignin oligomers by ozonation of Miscanthus giganteus and vine shoots in a pilot scale reactor

Lignin, a complex phenolic polymer crucial for plant structure, is mostly used as fuel but it can be harnessed for environmentally friendly applications. This article explores ozonation as a green method for lignin extraction from lignocellulosic biomass, aiming to uncover the benefits of the extracted lignin. A pilot-scale ozonation reactor was employed to extract lignin from Miscanthus giganteus (a grass variety) and vine shoots (a woody biomass). The study examined the lignin extraction and modification of the fractions and identified the generation of phenolic and organic acids. About 48 % of lignin was successfully extracted from both biomass types. Phenolic monomers were produced, vine shoots yielding fewer monomers than Miscanthus giganteus. Ozonation generated homogeneous lignin oligomers, although their molecular weight decreased during ozonation, with vine shoot oligomers exhibiting greater resistance to ozone. Extracted fractions were stable at 200 °C, despite the low molecular weight, outlining the potential of these phenolic fractions.

Mutation breeding of high-stress resistant strains for succinic acid production from corn straw

The production of succinic acid from corn stover is a promising and sustainable route; however, during the pretreatment stage, byproducts such as organic acids, furan-based compounds, and phenolic compounds generated from corn stover inhibit the microbial fermentation process. Selecting strains that are resistant to stress and utilizing nondetoxified corn stover hydrolysate as a feedstock for succinic acid production could be effective. In this study, A. succinogenes CICC11014 was selected as the original strain, and the stress-resistant strain A. succinogenes M4 was obtained by atmospheric and room temperature plasma (ARTP) mutagenesis and further screening. Compared to the original strain, A. succinogenes M4 exhibited a twofold increase in stress resistance and a 113% increase in succinic acid production when hydrolysate was used as the substrate. By conducting whole-genome resequencing of A. succinogenes M4 and comparing it with the original strain, four nonsynonymous gene mutations and two upstream regions with base losses were identified. KEY POINTS: • A high-stress-resistant strain A. succinogenes M4 was obtained by ARTP mutation •  The production of succinic acid increased by 113% • The mutated genes of A. succinogenes M4 were detected and analyzed.

Quantitative comparison of the delignification performance of lignocellulosic biomass pretreatment technologies for enzymatic saccharification

Pretreatments for delignification are required for the enzymatic saccharification of lignocellulosic biomasses. However, in the current literature, various pretreatment approaches have been applied for the same kinds of biomass. To find the optimum pretreatments for biomaterials containing various lignin contents, in this study, a quantitative comparison was carried out on the delignification performance of 15 categories of pretreatments. In total, 1729 sets of biomass, cellulose, hemicellulose, and lignin recovery data were collected from 214 relevant studies. Box plots and Cate-Nelson-like graphs were applied for analyses. The results showed that alkali, oxidation, organic solvent, and multistep pretreatments generally were better at removing lignin and recovering cellulose. Moreover, among these four categories, alkali pretreatments had the best performance, increasing the saccharification efficiency by approximately five-fold. Considering both delignification performance and saccharification improvement, alkali pretreatments are currently considered to be the optimum pretreatment methods for enzymatic saccharification.

Structural Changes of Alkali Lignin under Ozone Treatment and Effect of Ozone-Oxidized Alkali Lignin on Cellulose Digestibility

In this study, the structural changes of alkali lignin induced by ozonation were investigated, and the effect of ozone-treated alkali lignin and its mechanism on Avicel enzymatic hydrolysis was examined. The physicochemical properties of alkali lignin were analyzed by FTIR, 1H-13C HSQC NMR, and GPC. It was revealed that ozone pretreatment increased the content of carboxyl and/or aldehyde groups and the negative zeta potential of alkali lignin, which enhanced the electrostatic repulsion between alkali lignin and cellulase; The S/G ratio was reduced, indicating the hydrophobic interaction was diminished. The Langmuir adsorption isotherm showed that the cellulase binding strength of ozone pretreated alkali lignin (OL-pH3, OL-pH7, and OL-pH12 were 16.67, 13.87, and 44.05 mL/g, respectively) was significantly lower than that of alkali lignin (161.29 mL/g). The 72 h hydrolysis yields of Avicel added with OL-pH3, OL-pH7, and OL-pH12 were 55.4%, 58.6%, and 54.9% respectively, which were 2.6–6.3% higher than that of Avicel added with AL (52.3%). This research aimed to reduce the non-productive adsorption between cellulase and lignin by investigating the structural changes of lignin caused by ozone treatment. For the first time, we discovered that ozone-treated alkali lignin has a further promotion effect on the enzymatic digestion of cellulose, providing a green and feasible pretreatment process for the enzymatic hydrolysis of lignocellulose and aiding in the more efficient utilization of biomass.

Advanced Bioethanol Production: From Novel Raw Materials to Integrated Biorefineries

The production of so-called advanced bioethanol offers several advantages compared to traditional bioethanol production processes in terms of sustainability criteria. This includes, for instance, the use of nonfood crops or residual biomass as raw material and a higher potential for reducing greenhouse gas emissions. The present review focuses on the recent progress related to the production of advanced bioethanol, (i) highlighting current results from using novel biomass sources such as the organic fraction of municipal solid waste and certain industrial residues (e.g., residues from the paper, food, and beverage industries); (ii) describing new developments in pretreatment technologies for the fractionation and conversion of lignocellulosic biomass, such as the bioextrusion process or the use of novel ionic liquids; (iii) listing the use of new enzyme catalysts and microbial strains during saccharification and fermentation processes. Furthermore, the most promising biorefinery approaches that will contribute to the cost-competitiveness of advanced bioethanol production processes are also discussed, focusing on innovative technologies and applications that can contribute to achieve a more sustainable and effective utilization of all biomass fractions. Special attention is given to integrated strategies such as lignocellulose-based biorefineries for the simultaneous production of bioethanol and other high added value bioproducts.

Integrated Ozonation-Enzymatic Hydrolysis Pretreatment of Sugarcane Bagasse: Enhancement of Sugars Released to Expended Ozone Ratio

The combined effects of three key ozonation process parameters on the integrated ozonation-enzymatic hydrolysis pretreatment of sugarcane bagasse (SCB) were investigated, with emphasis on the relationship between sugar release and ozone consumption. A lab-scale fixed bed reactor was employed for ozonation at varying ozone doses (50, 75 and 100 mg O3/g SCB), particle sizes (420, 710 and 1000 µm) and moisture contents (30, 45 and 60% w/w) in multifactorial experiments, keeping a residence time of 30 min. The ozonated SCB showed a reduction in the content of acid-insoluble lignin from 26.6 down to 19.1% w/w, while those of cellulose and hemicellulose were retained above 45.5 and 13.6% w/w, with recoveries of 100–89.9 and 83.5–72.7%, respectively. Ozone-assisted enzymatic hydrolysis allowed attaining glucose and xylose yields as high as 45.0 and 37.8%, respectively. The sugars released/ozone expended ratio ranged between 2.3 and 5.7 g sugars/g O3, being the higher value achieved with an applied ozone input of 50 mg O3/g SCB and SCB with 420 µm particle size and 60% moisture. Such operating conditions led to efficient ozone utilization (<2% unreacted ozone) with a yield of 0.29 g sugars/g SCB. Overall, the amount of sugars released relative to the ozone consumed was improved, entailing an estimated cost of ozonation of USD 34.7/ton of SCB, which could enhance the profitability of the process.

Atmospheric Low-Temperature Plasma-Induced Changes in the Structure of the Lignin Macromolecule: An Experimental and Theoretical Investigation

Atmospheric low-temperature plasma has emerged as a promising pretreatment for lignocellulose to improve bio-refining. Herein, we investigated plasma-induced changes in the chemical structure of lignin to obtain a fundamental understanding of the plasma-lignocellulose interaction. Based on the results, plasma possesses a strong capacity to cleave C-C covalent bonds in the aliphatic region of lignin, accompanied by oxidation. Plasma treatment leads to the degradation and fragmentation of lignin. Pronounced deconstruction of β-O-4 aryl ether is observed in plasma. The relative content of β-O-4 aryl ether was reduced from the initial value of 65.1/100Ar to 58.7/100Ar for lignin from corncob and from the initial value of 72.5/100Ar to 63.8/100Ar for lignin from poplar after plasma treatment, respectively. According to the density functional theory analysis, the oxygen atom of β-O-4 aryl ether is the most likely potential reaction site and the C_β-O covalent bond exhibits the lowest decomposition free energy (50.5 kcal mol^-1), which will easily be cleaved in plasma. The dominant reaction pathway of lignin degradation is the cleavage of the C_β-O covalent bond followed by the cleavage of the C_β-C_α bond. We propose that this investigation is beneficial to optimize and expand the applications of plasma treatment in pretreatment of lignocellulose.

Pretreatment and fermentation of lignocellulosic biomass: reaction mechanisms and process engineering

At a time of rapid depletion of oil resources, global food shortages and solid waste problems, it is imperative to encourage research into the use of appropriate pre-treatment techniques using regenerative raw materials such as lignocellulosic biomass.

A comparative investigation of fast pyrolysis with enzymatic hydrolysis for fermentable sugars production from cellulose

In this study, ball milling and ionic liquid pretreatments were utilized to alter cellulose structure prior to fast pyrolysis and enzymatic hydrolysis. The variations in the products distribution of cellulose fast pyrolysis, and their dependence on the structure of cellulose, and the temperature of fast pyrolysis were illustrated. Fast pyrolysis of pretreated cellulose yielded more levoglucosan than crystalline cellulose (14.7%) at 300 °C. Nevertheless, the levoglucosan achieved higher yield (64.3%) from crystalline cellulose at 400 °C. At last, a comparison between fast pyrolysis and enzymatic hydrolysis for cellulose saccharifaction was made. Fast pyrolysis was a promising alternative to liberate levoglucosan from cellulose. Further investigation and development were required to maximize the levoglucosan production.

Understanding the synergistic effect and the main factors influencing the enzymatic hydrolyzability of corn stover at low enzyme loading by hydrothermal and/or ultrafine grinding pretreatment

A thorough assessment of the microstructural changes and synergistic effects of hydrothermal and/or ultrafine grinding pretreatment on the subsequent enzymatic hydrolysis of corn stover was performed in this study. The mechanism of pretreatment was elucidated by characterizing the particle size, specific surface area (SSA), pore volume (PV), average pore size, cellulose crystallinity (CrI) and surface morphology of the pretreated samples. In addition, the underlying relationships between the structural parameters and final glucose yields were elucidated, and the relative significance of the factors influencing enzymatic hydrolyzability were assessed by principal component analysis (PCA). Hydrothermal pretreatment at a lower temperature (170 °C) combined with ultrafine grinding achieved a high glucose yield (80.36%) at a low enzyme loading (5 filter paper unit (FPU)/g substrate) which is favorable. The relative significance of structural parameters in enzymatic hydrolyzability was SSA > PV > average pore size > CrI/cellulose > particle size. PV and SSA exhibited logarithmic correlations with the final enzymatic hydrolysis yield.

Increased sugar yield from pre-milled Douglas-fir forest residuals with lower energy consumption by using planetary ball milling

Impact of planetary ball milling on pre-milled wood fiber was studied to improve efficiency of energy consumption for bioconversion using post-harvest forest residuals. Crystalline cellulose decreased from 40.73% to 11.70% by ball milling. Crystallinity index of ball milled wood samples had a negative correlation with glucose yield (r = -0.97, p < .01), xylose/mannose (r = -0.96, p < .01), and a positive correlation with median particle size (r = 0.77, p < .01). Range of glucose yield and xylose/mannose yield for ball milled samples was found to be 24.45-59.67% and from 11.92% to 23.82%, respectively. Morphological changes of the lignocellulosic biomass were observed; the compact fiber bundles of the forest residuals were cleaved to smaller particles with lower aspect ratio with increasing intensity of ball milling. The required energy ranged from 0.50 to 2.15 kWh/kg for 7-30 min of milling respectively.

Oxygen-radical pretreatment promotes cellulose degradation by cellulolytic enzymes

BACKGROUND

The efficiency of cellulolytic enzymes is important in industrial biorefinery processes, including biofuel production. Chemical methods, such as alkali pretreatment, have been extensively studied and demonstrated as effective for breaking recalcitrant lignocellulose structures. However, these methods have a detrimental effect on the environment. In addition, utilization of these chemicals requires alkali- or acid-resistant equipment and a neutralization step.

RESULTS

Here, a radical generator based on non-thermal atmospheric pressure plasma technology was developed and tested to determine whether oxygen-radical pretreatment enhances cellulolytic activity. Our results showed that the viscosity of carboxymethyl cellulose (CMC) solutions was reduced in a time-dependent manner by oxygen-radical pretreatment using the radical generator. Compared with non-pretreated CMC, oxygen-radical pretreatment of CMC significantly increased the production of reducing sugars in culture supernatant containing various cellulases from Phanerochaete chrysosporium. The production of reducing sugar from oxygen-radical-pretreated CMC by commercially available cellobiohydrolases I and II was 1.7- and 1.6-fold higher, respectively, than those from non-pretreated and oxygen-gas-pretreated CMC. Moreover, the amount of reducing sugar from oxygen-radical-pretreated wheat straw was 1.8-fold larger than those from non-pretreated and oxygen-gas-pretreated wheat straw.

CONCLUSIONS

Oxygen-radical pretreatment of CMC and wheat straw enhanced the degradation of cellulose by reducing- and non-reducing-end cellulases in the supernatant of a culture of the white-rot fungus P. chrysosporium. These findings indicated that oxygen-radical pretreatment of plant biomass offers great promise for improvements in lignocellulose-deconstruction processes.

Ozonolysis of straw from Secale cereale L. for anaerobic digestion

The effect of different ozonation conditions on straw from Secale cereale (rye straw) pretreatment has been investigated. Using the Taguchi method, this study analyzed the optimum conditions for pretreatment of rye straw by ozonation. After 60min of rye straw ozonation the concentration of reducing sugars (RS) and volatile fatty acid (VFA), chemical oxygen demand (COD) were 7.4, 32.3 and 11.7 times higher, respectively compared to samples raw rye straw. The most effective rye straw ozonation occurred while using the highest amount of the rye straw (15g) treated with lower ozone dose (100gO₃/m³) in the longest period of time (60min). For this variant of experiment the increment of methane production was 291.71dm³CH₄/kgVS. Moreover, co-digestion of sewage sludge with addition of 20% ozonated rye straw allowed to obtain 269.1dm³CH₄/kgVS. The positive effect of ozone on changes in the rye straw structure has been confirmed by SEM and FTIR analysis.

Quantitative characterization of enzyme adsorption and hydrolytic performance for ultrafine grinding pretreated corn stover

Quantitative analysis of enzyme adsorption and hydrolysis were performed for sieve-based grinding corn stover (SGCS) and ultrafine grinding corn stover (UGCS)¹ with different enzyme consumptions. The UGCS presented significantly higher enzyme adsorption quantity (5.15mg/g for UGCS, 1.33mg/g for SGCS), higher glucose yield (49.75% for UGCS, 28.75% for SGCS) under 20FPU/g and higher binding enzyme proportion (41.32% for UGCS, 10.64% for SGCS under 5FPU/g) which can be attributed to the more accessible microstructure properties. The relationship between enzyme adsorption and hydrolytic production was directly proportional for SGCS (GY₁=21.04×AQ₁+1.86 (R²=0.95)) while was exponential for UGCS (GY₂=49.42×(1-e^-0.57×AQ₂) (R²=0.99)),² indicating that overmuch enzyme consumption was not advisable for UGCS at economical aspect.

Unconventional Pretreatment of Lignocellulose with Low-Temperature Plasma

Lignocellulose represents a potential supply of sustainable feedstock for the production of biofuels and chemicals. There is, however, an important cost and efficiency challenge associated with the conversion of such lignocellulosics. Because its structure is complex and not prone to undergo chemical reactions very easily, chemical and mechanical pretreatments are usually necessary to be able to refine them into the compositional building blocks (carbohydrates and lignin) from which value-added platform molecules, such as glucose, ethylene glycol, 5-hydroxymethylfurfural, and levulinic acid, and biofuels, such as bioderived naphtha, kerosene, and diesel fractions, will be produced. Conventional (wet) methods are usually polluting, aggressive, and highly energy consuming, so any alternative activation procedure of the lignocellulose is highly recommended and anticipated in recent and future biomass research. Lignocellulosic plasma activation has emerged as an interesting (dry) treatment technique. In the long run, in particular, in times of fairly accessible renewable electricity, plasma may be considered as an alternative to conventional pretreatment methods, but current knowledge is too little and examples too few to guarantee that statement. This review therefore highlights recent knowledge, advancements, and shortcomings in the field of plasma treatment of cellulose and lignocellulose with regard to the (structural and chemical) effects and impact on the future of pretreatment methods.

Effect of ozonolysis parameters on the inhibitory compound generation and on the production of ethanol by Pichia stipitis and acetone-butanol-ethanol by Clostridium from ozonated and water washed sugarcane bagasse

Sugarcane bagasse (SCB) was ozone pretreated and detoxified by water washing, applying a L9(3)(4) orthogonal array (OA) design of experiments to study the effect of pretreatment parameters (moisture content, ozone concentration, ozone/oxygen flow and particle size) on the generation of inhibitory compounds and on the composition of hydrolysates of ozonated-washed samples. Ozone concentration resulted the highest influence process parameter on delignification and sugar release after washing; while, for inhibitory compound formation, moisture content also had an important role. Ozone expended in pretreatment related directly with sugar release and inhibitory compound formation. Washing detoxification was effective, providing non-inhibitory hydrolysates. Maximum glucose and xylose release yields obtained were 84% and 67%, respectively, for ozonated-washed SCB. Sugar concentration resulted in the decisive factor for biofuels yields. Ethanol production achieved an 88% yield by Pichia stipitis, whereas Clostridium acetobutylicum produced 0.072gBUTANOL/gSUGAR and 0.188gABE/gSUGAR, and, Clostridium beijerinckii 0.165gBUTANOL/gSUGAR and 0.257gABE/gSUGAR.

Mechanical fragmentation of corncob at different plant scales: Impact and mechanism on microstructure features and enzymatic hydrolysis

In this work, corncob samples at different scales, i.e., plant scale (>1mm), tissue scale (500-100μm) and cellular scale (50-30μm), were produced to investigate the impact and mechanisms of different mechanical fragmentations on microstructure features and enzymatic hydrolysis. The results showed that the microstructure features and enzymatic hydrolysis of corncob samples, either at a plant scale or tissue scale, did not change significantly. Conversely, corncob samples at a cellular scale exhibited some special properties, i.e., an increase in the special surface area with the inner mesopores and macropores exposed to the surface; breakage of crystalline cellulose and linkages in polysaccharides; and a higher proportion of polysaccharides on the surface, which significantly enhanced enzymatic digestibility resulting in a 98.3% conversion yield of cellulose to glucose which is the highest conversion ever reported. In conclusion, mechanical fragmentation at the cellular scale is an effective pretreatment for corncob.

Ozonolysis: An advantageous pretreatment for lignocellulosic biomass revisited

Ozonolysis, as a lignocellulosic biomass pretreatment, goes back to 80s; however, in the last years it is becoming widespread again owing to its efficiency and mild operation conditions. Ozone reacts preferably with lignin than carbohydrates, promoting biomass destructuration and delignification, and so the sugar release by enzymatic hydrolysis. The hydrolysate from pretreated biomass has being used as sugars source for second-generation fuels production, mainly ethanol, methane and hydrogen. Short-chain carboxylic acids are the main inhibitory compounds generated, being properly removed by water washing. The most common inhibitory compounds reported for other pretreatments, furfural and HMF (5-hydroxymethylfurfural), are not found in ozone-pretreated hydrolysates. Composition of pretreated biomass and ozone consumption depends on several process parameters: reactor design, moisture content, particle size, pH, reaction time, ozone/air flow and ozone concentration. Additional studies are necessary to clarify process parameters effect and to optimize the process to achieve high yields with economic feasibility.

Promise of combined hydrothermal/chemical and mechanical refining for pretreatment of woody and herbaceous biomass

Production of advanced biofuels from woody and herbaceous feedstocks is moving into commercialization. Biomass needs to be pretreated to overcome the physicochemical properties of biomass that hinder enzyme accessibility, impeding the conversion of the plant cell walls to fermentable sugars. Pretreatment also remains one of the most costly unit operations in the process and among the most critical because it is the source of chemicals that inhibit enzymes and microorganisms and largely determines enzyme loading and sugar yields. Pretreatments are categorized into hydrothermal (aqueous)/chemical, physical, and biological pretreatments, and the mechanistic details of which are briefly outlined in this review. To leverage the synergistic effects of different pretreatment methods, conducting two or more pretreatments consecutively has gained attention. Especially, combining hydrothermal/chemical pretreatment and mechanical refining, a type of physical pretreatment, has the potential to be applied to an industrial plant. Here, the effects of the combined pretreatment (combined hydrothermal/chemical pretreatment and mechanical refining) on energy consumption, physical structure, sugar yields, and enzyme dosage are summarized.

The comparison of obtaining fermentable sugars from cellulose by enzymatic hydrolysis and fast pyrolysis

Sugars are one of intermediates in the biological and chemical conversion of biomass. The objective of this study was to make comparison of obtaining fermentable sugars by enzymatic hydrolysis and fast pyrolysis of ball milling pretreated cellulose. After ball milling pretreatment for 0-18h, with the accumulation of alkali and alkali earth metals (from 50.8 to 276.4ppm) and decrease of the crystalline structure (from 89.8% to 10.1%), the hydrolysis yields increased from 23.6% to 56.0% in enzymatic saccharification, while the yields of levoglucosan diminished from 61.5% to 45.6% gradually in fast pyrolysis. Both enzymatic saccharification and fast pyrolysis had unique attractive features and unfavorable limitations. The present research provided a concept for considering choices among the technologies and feedstocks currently available.