Surface and ultrastructural characterization of raw and pretreated switchgrass

LPMO-supported saccharification of biomass: effects of continuous aeration of reaction mixtures with variable fractions of water-insoluble solids and cellulolytic enzymes

BACKGROUND

High substrate concentrations and high sugar yields are important aspects of enzymatic saccharification of lignocellulosic substrates. The benefit of supporting the catalytic action of lytic polysaccharide monooxygenase (LPMO) through continuous aeration of slurries of pretreated softwood was weighed against problems associated with increasing substrate content (quantitated as WIS, water-insoluble solids, in the range 12.5-17.5%), and was compared to the beneficial effect on the saccharification reaction achieved by increasing the enzyme preparation (Cellic CTec3) loadings. Aerated reactions were compared to reactions supplied with N2 to assess the contribution of LPMO to the saccharification reactions. Analysis using 13C NMR spectroscopy, XRD, Simons' staining, BET analysis, and SEM analysis was used to gain further insights into the effects of the cellulolytic enzymes on the substrate under different reaction conditions.

RESULTS

Although glucose production after 72 h was higher at 17.5% WIS than at 12.5% WIS, glucan conversion decreased with 24% (air) and 17% (N2). Compared to reactions with N2, the average increases in glucose production for aerated reactions were 91% (12.5% WIS), 70% (15.0% WIS), and 67% (17.5% WIS). Improvements in glucan conversion through aeration were larger (55-86%) than the negative effects of increasing WIS content. For reactions with 12.5% WIS, increased enzyme dosage with 50% improved glucan conversion with 25-30% for air and N2, whereas improvements with double enzyme dosage were 30% (N2) and 39% (air). Structural analyses of the solid fractions revealed that the enzymatic reaction, particularly with aeration, created increased surface area (BET analysis), increased disorder (SEM analysis), decreased crystallinity (XRD), and increased dye adsorption based on the cellulose content (Simons' staining).

CONCLUSIONS

The gains in glucan conversion with aeration were larger than the decreases observed due to increased substrate content, resulting in higher glucan conversion when using aeration at the highest WIS value than when using N2 at the lowest WIS value. The increase in glucan conversion with double enzyme preparation dosage was smaller than the increase achieved with aeration. The results demonstrate the potential in using proper aeration to exploit the inherent capacity of LPMO in enzymatic saccharification of lignocellulosic substrates and provide detailed information about the characteristics of the substrate after interaction with cellulolytic enzymes.

Real-time imaging of enzymatic degradation of pretreated maize internodes reveals different cell types have different profiles

This work presents a dynamic view of the enzymatic degradation of maize cell walls, and sheds new light on the recalcitrance of hot water pretreated maize stem internodes. Infra-red microspectrometry, mass spectrometry, fluorescence recovery after photobleaching and fluorescence imaging were combined to investigate enzymatic hydrolysis at the cell scale. Depending on their polymer composition and organisation, cell types exhibits different extent and rate of enzymatic degradation. Enzymes act sequentially from the cell walls rich in accessible cellulose to the most recalcitrant cells. This phenomenon can be linked to the heterogeneous distribution of enzymes in the liquid medium and the adsorption/desorption mechanisms that differ with the type of cell.

Cellulosic and hemicellulosic fractions of sugarcane bagasse: Potential, challenges and future perspective

Sugarcane bagasse is a rich source of cellulose (32-45%), hemicellulose (20-32%) and lignin (17-32%), 1.0-9.0% ash and some extractives. Huge amount of the generation of sugarcane bagasse has been a great challenge to industries and environment at global level for many years. Though cellulosic and hemicellulosic fractions in bagasse makes it a potential raw substrate for the production of value-added products at large scale, the presence of lignin hampers its saccharification which further leads to low yields of the value-added products. Therefore, an appropriate pretreatment strategy is of utmost importance that effectively solubilizes the lignin that exposes cellulose and hemicellulose for enzymatic action. Pretreatment also reduces the biomass recalcitrance i.e., cellulose crystallinity, structural complexity of cell wall and lignification for its effective utilization in biorefinery. Sugarcane bagasse served as nutrient medium for the cultivation of diverse microorganisms for the production of industrially important metabolites including enzymes, reducing sugars, prebiotic, organic acids and biofuels. Sugarcane bagasse has been utilized in the generation of electricity, syngas and as biosorbant in the bioremediation of heavy metals. Furthermore, the ash generated from bagasse is an excellent source for the synthesis of high strength and light weight bricks and tiles. Present review describes the utility of sugarcane bagasse as sustainable and renewable lignocellulosic substrate for the production of industrially important multifarious value-added products.

The influence of lignin on the effectiveness of using a chemithermomechanical pulping based process to pretreat softwood chips and pellets prior to enzymatic hydrolysis

Over the last century the pulp and paper sector has assessed various technologies to fractionate woody biomass to produce strong, bright fibers. Several of these processes have also been assessed for their potential to pretreat and fractionate biomass to enhance the subsequent enzymatic hydrolysis of the cellulosic component. Although many of these pretreatments are effective on agricultural residues, softwoods have proven more recalcitrant, primarily due to their high lignin content and structure. As delignification is too expensive to be used routinely a more economically attractive approach might be to alter the lignin. Recent work has shown that, using a modified chemithermomechanical pulping (CTMP) "front end", lignin can be modified and relocated. This significantly enhanced hemicellulose recovery and enzyme-mediated cellulose hydrolysis of woody biomass. As well as being effective on wood chips, the modified CTMP pretreatment process also enhanced the bioconversion of densified feedstocks such as pellets.

Effect of Lignin Content on Cellulolytic Saccharification of Liquid Hot Water Pretreated Sugarcane Bagasse

Lignin contributes to the rigid structure of the plant cell wall and is partially responsible for the recalcitrance of lignocellulosic materials to enzymatic digestion. Overcoming this recalcitrance is one the most critical issues in a sugar-flat form process. This study addresses the effect of low lignin sugarcane bagasse on enzymatic hydrolysis after liquid hot water pretreatment at 190 °C and 20 min (severity factor: 3.95). The hydrolysis of bagasse from a sugarcane line selected for a relatively low lignin content, gave an 89.7% yield of cellulose conversion to glucose at 40 FPU/g glucan versus a 68.3% yield from a comparably treated bagasse from the high lignin bred line. A lower enzyme loading of 5 FPU/g glucan (equivalent to 3.2 FPU/g total solids) resulted in 31.4% and 21.9% conversion yields, respectively, for low and high lignin samples, suggesting the significance of lignin content in the saccharification process. Further increases in the enzymatic conversion of cellulose to glucose were achieved when the bagasse sample was pre-incubated with a lignin blocking agent, e.g., bovine serum albumin (50 mg BSA/g glucan) at 50 °C for 1 h prior to an actual saccharification. In this work, we have demonstrated that even relatively small differences in lignin content can result in considerably increased sugar production, which supports the dissimilarity of bagasse lignin content and its effects on cellulose digestibility. The increased glucose yields with the addition of BSA helped to decrease the inhibition of non-productive absorption of cellulose enzymes onto lignin and solid residual lignin fractions.

Deep Eutectic Solvent Pretreatment of Transgenic Biomass With Increased C6C1 Lignin Monomers

The complex and heterogeneous polyphenolic structure of lignin confers recalcitrance to plant cell walls and challenges biomass processing for agroindustrial applications. Recently, significant efforts have been made to alter lignin composition to overcome its inherent intractability. In this work, to overcome technical difficulties related to biomass recalcitrance, we report an integrated strategy combining biomass genetic engineering with a pretreatment using a bio-derived deep eutectic solvent (DES). In particular, we employed biomass from an Arabidopsis line that expressed a bacterial hydroxycinnamoyl-CoA hydratase-lyase (HCHL) in lignifying tissues, which results in the accumulation of unusual C₆C₁ lignin monomers and a slight decrease in lignin molecular weight. The transgenic biomass was pretreated with renewable DES that can be synthesized from lignin-derived phenols. Biomass from the HCHL plant line containing C₆C₁ monomers showed increased pretreatment efficiency and released more fermentable sugars up to 34% compared to WT biomass. The enhanced biomass saccharification of the HCHL line is likely due to a reduction of lignin recalcitrance caused by the overproduction of C₆C₁ aromatics that act as degree of polymerization (DP) reducers and higher chemical reactivity of lignin structures with such C₆C₁ aromatics. Overall, our findings demonstrate that strategic plant genetic engineering, along with renewable DES pretreatment, could enable the development of sustainable biorefinery.

Surface treated Pteris vittata L. pinnae powder used as an efficient biosorbent of Pb(II), Cd(II), and Cr(VI) from aqueous solution

Biosorption is a surface-dependent phenomenon. Surface modifications by chemical treatment methods could either improve or reduce the biosorption capacity of potential biosorbents. In the present work, pristine Pteris vittata L. pinnae (PPV) powder was treated separately with sodium hydroxide (NaOH), calcium chloride (CaCl₂), and nitric acid (HNO₃). The pristine and treated biosorbents were used to assess the biosorption of Pb(II), Cd(II), and Cr(VI) as a function of pH. Kinetics and adsorption isotherms were studied. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscope combined with energy dispersive x-ray (SEM-EDX) spectroscopic techniques were used to characterize the biosorbents before and after chemical treatments. The possible functional groups contributing to the metal sorption were identified. Results revealed favorable biosorption of Pb(II), Cd(II), and Cr(VI) described by pseudo-second order kinetics. NaOH-treated P. vittata (NPV) showed higher biosorption capacity for Pb(II) and Cd(II) compared to that of PPV. ATR-FTIR studies indicated that -OH, -COOH, and -NH₂ groups were mainly involved in Cr(VI) and -OH in Pb(II) and Cd(II) biosorption. The enhanced efficiency of NPV and CaCl₂ treated P. vittata (CPV) in the uptake of Pb(II) and Cd(II) compared to PPV can be associated with their altered physicochemical characters.

Lignocellulose degradation patterns, structural changes, and enzyme secretion by Inonotus obliquus on straw biomass under submerged fermentation

This study examined the white rot fungus I. obliquus on the degradation of three types of straw biomass and the production of extracellular lignocellulolytic enzymes under submerged fermentation. The fungus process resulted in a highest lignin loss of 72%, 39%, and 47% in wheat straw, rice straw, and corn stover within 12days, respectively. In merely two days, the fungus selectively degraded wheat straw lignin by 37%, with only limited cellulose degradation (13%). Fourier transform infrared spectroscopy revealed that the fungus most effectively degraded the wheat straw lignin and rice straw crystalline cellulose. Scanning electronic microscopy showed the most pronounced structural changes in wheat straw. High activities of manganese peroxidase (159.0U/mL) and lignin peroxidase (123.4U/mL) were observed in wheat straw culture on Day 2 and 4, respectively. Rice straw was the best substrate to induce the production of cellulase and xylanase.

Understanding the Changes to Biomass Surface Characteristics after Ammonia and Organosolv Pretreatments by Using Time-of-Flight Secondary-Ion Mass Spectrometry (TOF-SIMS)

Surface characteristic changes to poplar after ammonia and organosolv pretreatments were investigated by means of time-of-flight secondary-ion mass spectrometry (TOF-SIMS) analysis. Whereas normalized total polysaccharides and lignin contents on the surface differed from bulk chemical compositions, the surface cellulose ions detected by TOF-SIMS showed the same value trend as the cellulose content in the biomass. In addition, the lignin syringyl/guaiacyl ratio according to TOF-SIMS results showed the same trend as the ratio measured by means of NMR spectroscopic analysis, even though the ratio scales for each method were different. A similar correlation was determined between the surface cellulose and glucose release after enzymatic hydrolysis. These results demonstrate that surface characterization using TOF-SIMS can provide important information about the effects of pretreatment on biomass properties and its hydrolysis.

Effect of pretreatment and enzymatic hydrolysis on the physical-chemical composition and morphologic structure of sugarcane bagasse and sugarcane straw

The present work aimed to study the effect of the pretreatment of sugarcane bagasse and straw with microwave irradiation in aqueous and acid glycerol solutions on their chemical composition, fiber structure and the efficiency of subsequent enzymatic hydrolysis. Thermogravimetric analysis showed that the pretreatment acted mainly on the lignin and hemicellulose fractions of the bagasse, whereas, in the straw, lesser structural and chemical changes were observed. The images from transmission electron microscopy (TEM) revealed that treating bagasse and straw with acid glycerol solution loosened the cell walls and there was a breakdown in the pit membrane. The treated material was submitted to hydrolysis for 72h and higher yields of reducing sugars were observed compared to the untreated material (250.9mg/g from straw and 197.4mg/g from bagasse). TEM images after hydrolysis confirmed the possible points of access of the enzymes to the secondary cell wall region of the pretreated biomass.

A critical review of analytical methods in pretreatment of lignocelluloses: Composition, imaging, and crystallinity

Lignocelluloses are widely investigated as renewable substrates to produce biofuels, e.g., ethanol, methane, hydrogen, and butanol, as well as chemicals such as citric acid, lactic acid, and xanthan gum. However, lignocelluloses have a recalcitrance structure to resist microbial and enzymatic attacks; therefore, many physical, thermal, chemical, and biological pretreatment methods have been developed to open up their structure. The efficiency of these pretreatments was studied using a variety of analytical methods that address their image, composition, crystallinity, degree of polymerization, enzyme adsorption/desorption, and accessibility. This paper presents a critical review of the first three categories of these methods as well as their constraints in various applications. The advantages, drawbacks, approaches, practical details, and some points that should be considered in the experimental methods to reach reliable and promising conclusions are also discussed.

Ethylenediamine pretreatment changes cellulose allomorph and lignin structure of lignocellulose at ambient pressure

BACKGROUND

Pretreatment of lignocellulosic biomass is essential to increase the cellulase accessibility for bioconversion of lignocelluloses by breaking down the biomass recalcitrance. In this work, a novel pretreatment method using ethylenediamine (EDA) was presented as a simple process to achieve high enzymatic digestibility of corn stover (CS) by heating the biomass-EDA mixture with high solid-to-liquid ratio at ambient pressure. The effect of EDA pretreatment on lignocellulose was further studied.

RESULTS

High enzymatic digestibility of CS was achieved at broad pretreatment temperature range (40-180 °C) during EDA pretreatment. Herein, X-ray diffractogram analysis indicated that cellulose I changed to cellulose III and amorphous cellulose after EDA pretreatment, and cellulose III content increased along with the decrease of drying temperature and the increase of EDA loading. Lignin degradation was also affected by drying temperature and EDA loading. Images from scanning electron microscope and transmission electron microscope indicated that lignin coalesced and deposited on the biomass surface during EDA pretreatment, which led to the delamination of cell wall. HSQC NMR analysis showed that ester bonds of p-coumarate and ferulate units in lignin were partially ammonolyzed and ether bonds linking the phenolic monomers were broken during pretreatment. In addition, EDA-pretreated CS exhibited good fermentability for simultaneous saccharification and co-fermentation process.

CONCLUSIONS

EDA pretreatment improves the enzymatic digestibility of lignocellulosic biomass significantly, and the improvement was caused by the transformation of cellulose allomorph, lignin degradation and relocalization in EDA pretreatment.

Chemical and structural changes associated with Cu-catalyzed alkaline-oxidative delignification of hybrid poplar

BACKGROUND

Alkaline hydrogen peroxide pretreatment catalyzed by Cu(II) 2,2'-bipyridine complexes has previously been determined to substantially improve the enzymatic hydrolysis of woody plants including hybrid poplar as a consequence of moderate delignification. In the present work, cell wall morphological and lignin structural changes were characterized for this pretreatment approach to gain insights into pretreatment outcomes and, specifically, to identify the extent and nature of lignin modification.

RESULTS

Through TEM imaging, this catalytic oxidation process was shown to disrupt cell wall layers in hybrid poplar. Cu-containing nanoparticles, primarily in the Cu(I) oxidation state, co-localized with the disrupted regions, providing indirect evidence of catalytic activity whereby soluble Cu(II) complexes are reduced and precipitated during pretreatment. The concentration of alkali-soluble polymeric and oligomeric lignin was substantially higher for the Cu-catalyzed oxidative pretreatment. This alkali-soluble lignin content increased with time during the catalytic oxidation process, although the molecular weight distributions were unaltered. Yields of aromatic monomers (including phenolic acids and aldehydes) were found to be less than 0.2 % (wt/wt) on lignin. Oxidation of the benzylic alcohol in the lignin side-chain was evident in NMR spectra of the solubilized lignin, whereas minimal changes were observed for the pretreatment-insoluble lignin.

CONCLUSIONS

These results provide indirect evidence for catalytic activity within the cell wall. The low yields of lignin-derived aromatic monomers, together with the detailed characterization of the pretreatment-soluble and pretreatment-insoluble lignins, indicate that the majority of both lignin pools remained relatively unmodified. As such, the lignins resulting from this process retain features closely resembling native lignins and may, therefore, be amenable to subsequent valorization.

Experimental approaches to study plant cell walls during plant-microbe interactions

Plant cell walls provide physical strength, regulate the passage of bio-molecules, and act as the first barrier of defense against biotic and abiotic stress. In addition to providing structural integrity, plant cell walls serve an important function in connecting cells to their extracellular environment by sensing and transducing signals to activate cellular responses, such as those that occur during pathogen infection. This mini review will summarize current experimental approaches used to study cell wall functions during plant-pathogen interactions. Focus will be paid to cell imaging, spectroscopic analyses, and metabolic profiling techniques.

The impact of alkali pretreatment and post-pretreatment conditioning on the surface properties of rice straw affecting cellulose accessibility to cellulases

Rice straw was pretreated with sodium hydroxide and subsequently conditioned to reduce the pH to 5-6 by either: (1) extensive water washing or (2) acidification with hydrochloric acid then water washing. Alkali pretreatment improved the enzymatic digestibility of rice straw by increasing the cellulose accessibility to cellulases. However, acidification after pretreatment reversed the gains in cellulose accessibility to cellulases and enzymatic digestibility due to precipitation of solubilized compounds. Surface composition analyses by ToF-SIMS confirmed a reduction in surface lignin by pretreatment and water washing, and suggested that acidification precipitated a chemically modified form of lignin on the surfaces of rice straw. The spin-spin relaxation times (T2) of the samples indicated increased porosity in alkali pretreated rice straw. The acidified pretreated rice straw had reduced amounts of water in the longer T2 proton pools associated with water in the pores of the biomass likely due to back-filling by the precipitated components.

Topochemistry of environmentally friendly pretreatments to enhance enzymatic hydrolysis of sugar cane bagasse to fermentable sugar

In this work, dilute alkaline and alkaline peroxide pretreatments were conducted in comparison with hydrotropic pretreatment to improve the delignification of bagasse prior to enzymatic hydrolysis. The surface chemical composition of bagasse after pretreatments was investigated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The surface distribution of lignin and extractives on the bagasse fiber was significantly changed by dilute alkaline, alkaline peroxide, and hydrotropic pretreatments. Hydrotropic pretreatment typically showed, other than the decrease of surface coverage by lignin and extractives, dramatic removal of xylan, thereby leading to more cellulose exposed on the fiber surface after pretreatment. Fiber morphology after pretreatments was more favorable for enzyme hydrolysis as well. However, the hydrotropic treatment had clear advantages because the enzymatic hydrolysis yields of glucan and xylan of pretreated bagasse were 83.9 and 14.3%, respectively.

Effect of mechanical disruption on the effectiveness of three reactors used for dilute acid pretreatment of corn stover Part 2: morphological and structural substrate analysis

BACKGROUND

Lignocellulosic biomass is a renewable, naturally mass-produced form of stored solar energy. Thermochemical pretreatment processes have been developed to address the challenge of biomass recalcitrance, however the optimization, cost reduction, and scalability of these processes remain as obstacles to the adoption of biofuel production processes at the industrial scale. In this study, we demonstrate that the type of reactor in which pretreatment is carried out can profoundly alter the micro- and nanostructure of the pretreated materials and dramatically affect the subsequent efficiency, and thus cost, of enzymatic conversion of cellulose.

RESULTS

Multi-scale microscopy and quantitative image analysis was used to investigate the impact of different biomass pretreatment reactor configurations on plant cell wall structure. We identify correlations between enzymatic digestibility and geometric descriptors derived from the image data. Corn stover feedstock was pretreated under the same nominal conditions for dilute acid pretreatment (2.0 wt% H2SO4, 160°C, 5 min) using three representative types of reactors: ZipperClave® (ZC), steam gun (SG), and horizontal screw (HS) reactors. After 96 h of enzymatic digestion, biomass treated in the SG and HS reactors achieved much higher cellulose conversions, 88% and 95%, respectively, compared to the conversion obtained using the ZC reactor (68%). Imaging at the micro- and nanoscales revealed that the superior performance of the SG and HS reactors could be explained by reduced particle size, cellular dislocation, increased surface roughness, delamination, and nanofibrillation generated within the biomass particles during pretreatment.

CONCLUSIONS

Increased cellular dislocation, surface roughness, delamination, and nanofibrillation revealed by direct observation of the micro- and nanoscale change in accessibility explains the superior performance of reactors that augment pretreatment with physical energy.

Mapping out the structural changes of natural and pretreated plant cell wall surfaces by atomic force microscopy single molecular recognition imaging

BACKGROUND

Enzymatic hydrolysis of lignocellulosic biomass (mainly plant cell walls) is a critical process for biofuel production. This process is greatly hindered by the natural complexity of plant cell walls and limited accessibility of surface cellulose by enzymes. Little is known about the plant cell wall structural and molecular level component changes after pretreatments, especially on the outer surface. Therefore, a more profound understanding of surface cellulose distributions before and after pretreatments at single-molecule level is in great need. In this study, we determined the structural changes, specifically on crystalline cellulose, of natural, dilute sulfuric acid pretreated and delignified cell wall surfaces of poplar, switchgrass, and corn stover using single molecular atomic force microscopy (AFM) recognition imaging.

RESULTS

The AFM tip was first functionalized by a family 3 carbohydrate-binding module (CBM3a) (Clostridium thermocellum Scaffoldin) which specifically recognizes crystalline cellulose by selectively binding to it. The surface structural changes were studied at single molecule level based on the recognition area percentage (RAP) of exposed crystalline cellulose over the imaged cell wall surface. Our results show that the cell wall surface crystalline cellulose coverage increased from 17-20% to 18-40% after dilute acid pretreatment at 135°C under different acid concentrations and reached to 40-70% after delignification. Pretreated with 0.5% sulfuric acid, the crystalline cellulose surface distributions of 23% on poplar, 28% on switchgrass and, 38% on corn stover were determined as an optimized result. Corn stover cell walls also show less recalcitrance due to more effective pretreatments and delignification compared to poplar and switchgrass.

CONCLUSIONS

The dilute acid pretreatment can effectively increase the cellulose accessibility on plant cell wall surfaces. The optimal acid concentration was determined to be 0.5% acid at 135°C, especially for corn stover. This study provides a better understanding of surface structural changes after pretreatment such as lignin relocation, re-precipitation, and crystalline cellulose distribution, and can lead to potential improvements of biomass pretreatment.

Optimization of AFEX™ pretreatment conditions and enzyme mixtures to maximize sugar release from upland and lowland switchgrass

Switchgrass is a North American grass that is considered to be a highly promising herbaceous bioenergy feedstock. Differences in processing conditions and yields specifically related to switchgrass cultivar or cytotype (upland or lowland) can be confounded by differences in harvest date or region of growth. For this research, AFEX™ pretreatment conditions and hydrolysis enzyme mixtures were statistically optimized for Alamo (lowland) and Shawnee (upland) switchgrass that had been harvested in December in Oklahoma. Optimal pretreatment conditions and enzyme mixtures were almost identical for both varieties and gave similar mass sugar yields. Inclusion of hemicellulases in the enzyme mixture maintained total sugar yields with 50% reduction in enzyme loading. Regardless of variety, the biorefinery should be able to obtain high sugar yields using the same pretreatment and hydrolysis conditions to process switchgrass grown under the same environmental conditions, in the same location, and harvested at the same time of the year.

Ethanol yields and cell wall properties in divergently bred switchgrass genotypes

Genetic modification of herbaceous plant cell walls to increase biofuels yields is a primary bioenergy research goal. Using two switchgrass populations developed by divergent breeding for ruminant digestibility, the contributions of several wall-related factors to ethanol yields was evaluated. Field grown low lignin plants significantly out yielded high lignin plants for conversion to ethanol by 39.1% and extraction of xylans by 12%. However, across all plants analyzed, greater than 50% of the variation in ethanol yields was attributable to changes in tissue and cell wall architecture, and responses of stem biomass to dilute-acid pretreatment. Although lignin levels were lower in the most efficiently converted genotypes, no apparent correlation were seen in the lignin monomer G/S ratios. Plants with higher ethanol yields were associated with an apparent decrease in the lignification of the cortical sclerenchyma, and a marked decrease in the granularity of the cell walls following dilute-acid pretreatment.