Hemicellulosic Hydrolysate Research Articles

Chemical pretreatment of corncob for the selective dissolution of hemicellulose and lignin: influence of pretreatment on the chemical, morphological and thermal features

The aim of this work was to remove the hemicellulose and lignin fraction from corncob using chemical pretreatments, and to stablish the effect of that on the composition, microstructure, and thermal properties of corncob. The biomass was pretreated with H2SO4 and NaOH solution to dissolve hemicellulose and lignin portion. Likewise, the biomass pretreated was characterized by scanning electron microscope, IR spectroscopy and TGA methods. It was found that the acid pretreatment disrupt the hemicellulose structure by hydrolysis of glycoside bonds. Moreover, the corncob morphology becomes flaky after acid pretreatment due to dissolution of hemicellulose. The lignin is removed during alkaline pretreatment by solvation of aryl groups. In addition, the crystalline morphology of cellulose fibers was disrupted, increasing the surface area. The pretreatment of corncob with 1.0% H2SO4 and 0.1 M NaOH solubilized ∼42.6% and ∼20.9% of the biomass, respectively. The soluble products were analyzed by FTIR, Fehling test, and calorimetric analysis, to stablish their composition and heat content. The hydrolysis of hemicellulose leads to the release of the monomeric sugars and soluble oligomers. Moreover, the lignin of corncob releases more energy per unit of mass, making it an energy-denser component of corncob.

Fermentative valorisation of xylose-rich hemicellulosic hydrolysates from agricultural waste residues for lactic acid production under non-sterile conditions

Lactic acid (LA) is a platform chemical with diverse industrial applications. Presently, commercial production of LA is dominated by microbial fermentation using sugary or starch-based feedstocks. Research pursuits emphasizing towards sustainable production of LA using non-edible and renewable feedstocks have accelerated the use of lignocellulosic biomass (LCB). The present study focuses on the valorisation of xylose derived from sugarcane bagasse (SCB) and olive pits (OP) through hydrothermal and dilute acid pretreatment, respectively. The xylose-rich hydrolysate obtained was used for LA production by homo-fermentative and thermophilic Bacillus coagulans DSM2314 strain under non-sterile conditions. The fed-batch mode of fermentation resulted in maximum LA titers of 97.8, 52.4 and 61.3 g/L with a yield of 0.77, 0.66 and 0.71 g/g using pure xylose, xylose-rich SCB and OP hydrolysates, respectively. Further, a two-step aqueous two-phase system (ATPS) extraction technique was employed for the separation and recovery of LA accumulated on pure and crude xylose. The LA recovery was 45 – 65% in the first step and enhanced to 80–90% in the second step.The study demonstrated an efficient integrated biorefinery approach to valorising the xylose-rich stream for cost-effective LA production and recovery.

Effect and mechanism analysis of dual enzyme pretreatment on enhancement of volatile fatty acids production in kitchen waste anaerobic acidification

The high content of cellulose and hemicellulose in kitchen waste (KW) results in a slow hydrolysis rate and subsequently hinders the yields of volatile fatty acids (VFAs) in anaerobic acidification. In this study, cellulase and hemicellulase were used for the pretreatments of KW to tackle the problem of difficult hydrolysis in anaerobic acidification. The results indicated that the joint use of double enzymes significantly improved the hydrolysis of cellulose and hemicellulose during pretreatment, thus enhancing the VFAs production in anaerobic acidification. The highest production of TVFAs was 50.73 ± 2.10 g COD/L and it was improved by 62.77% compared with the unpretreated group. In addition, the 16S rRNA amplicon sequencing analysis showed that the relative abundance of Firmicutes related with the VFAs production increased from 17.77% to 50.55%. The potential metabolism function analysis (PICRUSt2) indicated that the enrichment of glyceraldehyde-3-phosphate dehydrogenase (EC:1.2.1.9) and acetyl-CoA C-acetyltransferase (EC:2.3.1.9) contributed to the production of VFAs in AD. The results suggested that collaborative enzyme pretreatment could be one of the effective methods for the hydrolysis of cellulose and hemicellulose in KW and provided constructive mechanistic insights into recovering valuable products from KW.

Detoxification of areca nut acid hydrolysate and production of xylitol by Candida tropicalis (MTCC 6192)

Areca nut husk is the most promising alternative source of low-cost raw materials because it contains a considerable amount of five-carbon monosaccharide sugar in the form of xylose. This polymeric sugar can be isolated and transformed into a value-added chemical using fermentation. To extract sugars from areca nut husk fibers, preliminary pretreatment, such as dilute acid hydrolysis (H2SO4), was performed. The hemicellulosic hydrolysate of areca nut husk can produce xylitol through fermentation, but toxic components inhibit the growth of microorganisms. To overcome this, a series of detoxification treatments, including pH adjustment, activated charcoal, and ion exchange resin, were carried out to reduce the concentration of inhibitors in the hydrolysate. This study reports a remarkable 99% removal of inhibitors in the hemicellulosic hydrolysate. Subsequently, a fermentation process using Candida tropicalis (MTCC6192) was executed with the detoxified hemicellulosic hydrolysate of areca nut husk, yielding an optimum xylitol yield of 0.66 g/g. This study concludes that detoxification techniques like pH adjustment, activated charcoal, and ion exchange resins are the most economical and effective methods for eliminating toxic compounds in hemicellulosic hydrolysates. Therefore, the medium derived after detoxification from areca nut hydrolysate may be considered to have significant potential for xylitol production.

Simultaneous production of xylitol and arabitol by Candida tropicalis fermentation improving agro-industrial wastes valorization

Xylitol and arabitol are sugar alcohols with similar properties and applications. Although the biotechnological production of xylitol is already being explored, its coproduction with arabitol is still in the early stages. This study aimed to improve the coproduction of xylitol and arabitol by Candida tropicalis from sugarcane bagasse hemicellulosic hydrolysate. The most promising results were achieved with 2.2 g L−1 nitrogen, obtained by mixing 80% yeast hydrolysate and 20% corn steep water. For fermentations conducted in a bioreactor, the concentrations of xylitol (52.1 g L−1) and arabitol (4.2 g L−1) were 65 and 3.7 times higher, respectively, than those reported in the literature for similar processes, with productivities and yields of 0.24 g L−1 h−1 and 0.74 g g−1 for xylitol and 0.02 g L−1 h−1 and 0.43 g g−1 for arabitol. These results show promise for the coproduction of sugar alcohols from sugarcane bagasse hemicellulosic hydrolysate by C tropicalis.

Detoxification strategy of wheat straw hemicellulosic hydrolysate for cultivating Trichoderma reesei: a contribution towards the wheat straw biorefinery

The urgency for reducing the dependence on fossil-based materials is increasing the interest in the utilization of renewable feedstocks. Lignocellulosic residual biomass can be used as feedstock to produce chemicals and energy without generating food security problems. Wheat straw (WS) has a clear potential for developing sustainable processes in a circular bioeconomy context. However, the development of processes requires a strategy for utilizing the hemicellulosic, cellulosic, and lignin fractions. This work covers the utilization of the hemicellulosic fraction as the first stage of a wheat straw biorefinery. The aim was to evaluate the hydrolysis of WS by using liquid hot water (LHW) treatment, the detoxification of the produced wheat straw hydrolysate (WSH), and the cultivation of Trichoderma reesei using it as the only carbon source as proof of detoxification. LHW treatment was performed at 160 °C and 90 min and yielded a WSH rich in monomeric and oligomeric saccharides (~ 14 g/L) and containing degradation products in low concentration (furfural, HMF, and acetic acid). As part of the development of the extraction and detoxification strategy, we determined the specific inhibition thresholds for T. reesei for the mentioned degradation products. Detoxification was carried out by evaporation by modifying the % of volume evaporated and the pH of the solution. Approximately 55.9% of acetic acid and 100% of furfural were removed from the WSH. The fungal biomass obtained in the medium containing WSH was equivalent to 98% of the biomass obtained in the control medium.

Relation of xylitol formation and lignocellulose degradation in yeast

One of the critical steps of the biotechnological production of xylitol from lignocellulosic biomass is the deconstruction of the plant cell wall. This step is crucial to the bioprocess once the solubilization of xylose from hemicellulose is allowed, which can be easily converted to xylitol by pentose-assimilating yeasts in a microaerobic environment. However, lignocellulosic toxic compounds formed/released during plant cell wall pretreatment, such as aliphatic acids, furans, and phenolic compounds, inhibit xylitol production during fermentation, reducing the fermentative performance of yeasts and impairing the bioprocess productivity. Although the toxicity of lignocellulosic inhibitors is one of the biggest bottlenecks of the biotechnological production of xylitol, most of the studies focus on how much xylitol production is inhibited but not how and where cells are affected. Understanding this mechanism is important in order to develop strategies to overcome lignocellulosic inhibitor toxicity. In this mini-review, we addressed how these inhibitors affect both yeast physiology and metabolism and consequently xylose-to-xylitol bioconversion. In addition, this work also addresses about cellular adaptation, one of the most relevant strategies to overcome lignocellulosic inhibitors toxicity, once it allows the development of robust and tolerant strains, contributing to the improvement of the microbial performance against hemicellulosic hydrolysates toxicity. KEY POINTS: • Impact of lignocellulosic inhibitors on the xylitol production by yeasts • Physiological and metabolic alterations provoked by lignocellulosic inhibitors • Cell adaptation as an efficient strategy to improve yeast's robustness.

Xylitol Production by Candida tropicalis from Areca Nut Husk Enzymatic Hydrolysate and Crystallization.

Lignocellulosic biomasses are extensively used by researchers to produce a variety of renewable bioproducts. This research described an environment-friendly technique of xylitol production by an adapted strain of Candida tropicalis from areca nut hemicellulosic hydrolysate, produced through enzymatic hydrolysis. To enhance the activity of xylanase enzymes, lime and acid pretreatment was conducted to make biomass more amenable for saccharification. To improve the efficiency of enzymatic hydrolysis, saccharification parameters like xylanase enzyme loading were varied. Results exposed that the highest yield (g/g) of reducing sugar, about 90%, 83%, and 15%, were achieved for acid-treated husk (ATH), lime-treated husk (LTH), and raw husk (RH) at an enzyme loading of 15.0IU/g. Hydrolysis was conducted at a substrate loading of 2% (w/V) at 30°C, 100rpm agitation, for 12h hydrolysis time at pH 4.5 to 5.0. Subsequently, fermentation of xylose-rich hemicellulose hydrolysate was conducted with pentose utilizing the yeast Candida tropicalis to produce xylitol. The optimum concentration of xylitol was obtained at about 2.47g/L, 3.83g/L, and 5.88g/L, with yields of approximately 71.02%, 76.78%, and 79.68% for raw fermentative hydrolysate (RFH), acid-treated fermentative hydrolysate (ATFH), and lime-treated fermentative gydrolysate (LTFH), respectively. Purification and crystallization were also conducted to separate xylitol crystals, followed by characterization like X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. Results obtained from crystallization were auspicious, and about 85% pure xylitol crystal was obtained.

Pretreatment of beechwood with polyoxometalate-based catalysts towards the production of polyunsaturated fatty acids by Crypthecodinium cohnii

In the present work, a novel OxiOrganosolv pretreatment was developed for the fractionation of beechwood. Τhe process employs a mixture of water and common organic solvents, polyoxometalate complexes (POMs) as homogeneous catalysts and O2 that acts as an oxidative agent enabling their regeneration. Due to their strong oxidizing ability and redox reversibility, POMs can achieve efficient biomass fractionation by catalyzing simultaneously hemicellulose hydrolysis and oxidation of lignin ether bonds that leads to its depolymerization, dissolution in the organic solvent and removal from the cellulose-rich pulp. According to the results, both commercially available phosphomolybdic acid (HPMo) and also the in-house produced Fe- and Cu- exchanged salts successfully promoted delignification and hemicellulose removal compared to the OxiOrganosolv pretreatment in the absence of catalyst. Most importantly, catalytic lignin depolymerization was achieved without the use of mineral acids that are typically used in a traditional organosolv. Evaluation of the enzymatic digestibility of the pulps revealed HPMo as the most favorable catalyst for increased saccharification yields and production of sugar-rich hydrolysates that were further used as carbon sources for fermentation and production of ω-3 polyunsaturated fatty acids (ω-3 PUFAs) from the heterotrophic microalgae Crypthecodinium cohnii. Hydrolysates produced from HPMo and Fe-PMo-assisted OxiOrganosolv pretreatment supported cell growth and led to high accumulation of ω-3 PUFAs, while severe inhibition was observed with the hydrolysates from Cu-PMo pretreated biomass. This study demonstrates the possibility of using POMs as catalysts for efficient biomass delignification with the aim to obtain sugar-rich streams that can be utilized for the production of nutraceuticals.

In-situ detoxification and enhanced oxygen mass transfer for C5 sugar acid production from corncob hemicellulose hydrolysates using activated carbon particles

In this work, an easily implemented and cost-effective activated carbon (AC) enhanced technique was proposed for C5 sugar acid production from corncob hemicellulose hydrolysates. The optimum AC detoxification conditions were obtained by response surface methodology, and the fermentation kinetics behaviors were comparatively evaluated using two fermentation modes: separate detoxification and fermentation (SDF) and in-situ detoxification and fermentation (In-situDF). An investigation of the AC function demonstrates it offers synchronous detoxification and oxygen transfer enhancement, with a final increase of XA fermentation yield and productivity. This study shows a potential of detoxification and oxygen transfer enhancement in one-pot in C5 sugar acid production, which may smooth the way for commercial utilization of C5 sugars from lignocellulosic biomass in the future.

Enhanced glycerol assimilation and lipid production in Rhodotorula toruloides CBS14 upon addition of hemicellulose primarily correlates with early transcription of energy-metabolism-related genes

BackgroundLipid formation from glycerol was previously found to be activated in Rhodotorula toruloides when the yeast was cultivated in a mixture of crude glycerol (CG) and hemicellulose hydrolysate (CGHH) compared to CG as the only carbon source. RNA samples from R. toruloides CBS14 cell cultures grown on either CG or CGHH were collected at different timepoints of cultivation, and a differential gene expression analysis was performed between cells grown at a similar physiological situation.ResultsWe observed enhanced transcription of genes involved in oxidative phosphorylation and enzymes localized in mitochondria in CGHH compared to CG. Genes involved in protein turnover, including those encoding ribosomal proteins, translation elongation factors, and genes involved in building the proteasome also showed an enhanced transcription in CGHH compared to CG. At 10 h cultivation, another group of activated genes in CGHH was involved in β-oxidation, handling oxidative stress and degradation of xylose and aromatic compounds. Potential bypasses of the standard GUT1 and GUT2-glycerol assimilation pathway were also expressed and upregulated in CGHH 10 h. When the additional carbon sources from HH were completely consumed, at CGHH 36 h, their transcription decreased and NAD+-dependent glycerol-3-phosphate dehydrogenase was upregulated compared to CG 60 h, generating NADH instead of NADPH with glycerol catabolism. TPI1 was upregulated in CGHH compared to cells grown on CG in all physiological situations, potentially channeling the DHAP formed through glycerol catabolism into glycolysis. The highest number of upregulated genes encoding glycolytic enzymes was found after 36 h in CGHH, when all additional carbon sources were already consumed.ConclusionsWe suspect that the physiological reason for the accelerated glycerol assimilation and faster lipid production, was primarily the activation of enzymes that provide energy.

Functional identification of two novel carbohydrate-binding modules of glucuronoxylanase CrXyl30 and their contribution to the lignocellulose saccharification

BackgroundGlycoside hydrolase (GH) family 30 xylanases are a distinct group of xylanases, most of which have a highly specific catalytic activity for glucuronoxylan. Since GH30 xylanases do not normally carry carbohydrate-binding modules (CBMs), our knowledge of the function of their CBMs is lacking.ResultsIn this work, the CBM functions of CrXyl30 were investigated. CrXyl30 was a GH30 glucuronoxylanase containing tandem CBM13 (CrCBM13) and CBM2 (CrCBM2) at its C terminus, which was identified in a lignocellulolytic bacterial consortium previously. Both CBMs could bind insoluble and soluble xylan, with CrCBM13 having binding specificity for the xylan with l-arabinosyl substitutions, whereas CrCBM2 targeted l-arabinosyl side chains themselves. Such binding abilities of these two CBMs were completely different from other CBMs in their respective families. Phylogenetic analysis also suggested that both CrCBM13 and CrCBM2 belong to novel branches. Inspection of the simulated structure of CrCBM13 identified a pocket that just accommodates the side chain of 3(2)-alpha-l-arabinofuranosyl-xylotriose, which forms hydrogen bonds with three of the five amino acid residues involved in ligand interaction. The truncation of either CrCBM13 or CrCBM2 did not alter the substrate specificity and optimal reaction conditions of CrXyl30, whereas truncation of CrCBM2 decreased the kcat/Km value by 83% (± 0%). Moreover, the absence of CrCBM2 and CrCBM13 resulted in a 5% (± 1%) and a 7% (± 0%) decrease, respectively, in the amount of reducing sugar released by the synergistic hydrolysis of delignified corncob whose hemicellulose is arabinoglucuronoxylan, respectively. In addition, fusion of CrCBM2 with a GH10 xylanase enhanced its catalytic activity against the branched xylan and improved the synergistic hydrolysis efficiency by more than fivefold when delignified corncob was used as substrate. Such a strong stimulation of hydrolysis resulted from the enhancement of hemicellulose hydrolysis on the one hand, and the cellulose hydrolysis is also improved according to the lignocellulose conversion rate measured by HPLC.ConclusionsThis study identifies the functions of two novel CBMs in CrXyl30 and shows the good potential of such CBMs specific for branched ligands in the development of efficient enzyme preparations.

Role of levulinic acid in catalytic wet torrefaction of oil palm trunks: Insights into the hydrochar physicochemical properties, liquid phase composition, and reaction mechanisms

This study evaluates the effects of levulinic acid addition and different operating conditions on the physicochemical and structural properties of the resultant OPT hydrochar as well as on the liquid phase composition to identify its potential reaction pathway during wet torrefaction. The addition of levulinic acid enhances the solid yield and HHV of the hydrochar and promote repolymerisation and condensation reactions for the formation of secondary char with high HHV. The highest energy yield of 66.47% was attained from the OPT hydrochar wet torrefied at 220 °C for 24 h with 1.0 M of levulinic acid. Increasing the levulinic acid concentration promoted secondary chars to agglomerate. Liquid phase analyses showed that wet torrefaction of OPT in the presence of levulinic acid promotes the hydrolysis of hemicellulose and cellulose to glucose at mild operating conditions and enhances the rehydration of 5-HMF to levulinic acid.

Fungal pretreatment and acid post-treatment for fractionation and biovalorization of palm biomass wastes into fungal oil, bioethanol, and lactic acid

This research aimed to develop prototype for fully utilizing palm empty fruit brunches (EFB) for production of sugars and biofuels. EFB was biologically pretreated using oleaginous fungus and post-treated using mild acid. This process can remove lignin and fractionate hemicellulose from cellulose pulp, which was confirmed by chemical composition, functional groups, and crystallinity changes. The cellulose pulp was used for ethanol production via simultaneous saccharification and fermentation (SSF). To increase initial sugar concentration prior to ethanol production, repeated-prehydrolysis (RPH) was attempted. The RPH for four cycles gave the maximum sugars of 60.9 ± 1.9 g/L, and the subsequent SSF gave the highest ethanol of 35.2 ± 2.7 g/L with an ethanol yield of 0.40 g/g-sugar. Moreover, during fungal pretreatment the fungus also accumulated lipids with high potential as biodiesel feedstocks, and the hemicellulose hydrolysate obtained after acid post-treatment could be used for lactic acid fermentation. This study has shown a practical biorefinery roadmap for biovalorization of palm biomass wastes into multiple bioproducts, which may greatly contribute to Bio-Circular-Green (BCG) economy.

Lactic Acid: A Comprehensive Review of Production to Purification

Lactic acid (LA) has broad applications in the food, chemical, pharmaceutical, and cosmetics industries. LA production demand rises due to the increasing demand for polylactic acid since LA is a precursor for polylactic acid production. Fermentative LA production using renewable resources, such as lignocellulosic materials, reduces greenhouse gas emissions and offers a cheaper alternative feedstock than refined sugars. Suitable pretreatment methods must be selected to minimize LA cost production, as the successful hydrolysis of lignocellulose results in sugar-rich feedstocks for fermentation. This review broadly focused on fermentative LA production from lignocellulose. Aspects discussed include (i). low-cost materials for fermentative LA production, (ii). pretreatment methods, (iii). enzymatic hydrolysis of cellulose and hemicellulose, (iv). lactic acid-producing microorganisms, including fungi, bacteria, genetically modified microorganisms, and their fermentative pathways, and (v). fermentation modes and methods. Industrial fermentative lactic acid production and purification, difficulties in using lignocellulose in fermentative LA production, and possible strategies to circumvent the challenges were discussed. A promising option for the industrial production and purification of LA that contains enzyme and cell recycling continuous simultaneous saccharification and fermentation coupled with membrane-based separation was proposed. This proposed system can eliminate substrate-, feedback-, and end-product inhibition, thereby increasing LA concentration, productivity, and yield.

Evaluation of Fermentative Xylitol Production Potential of Adapted Strains of Meyerozyma caribbica and Candida tropicalis from Rice Straw Hemicellulosic Hydrolysate

Dilute acid hydrolysis of lignocellulosic biomass generates inhibitors in the hydrolysate which hamper yeast metabolism and the fermentation process. Therefore, understanding the effect of these compounds on the performance of microorganisms becomes essential to achieve improved product yields. In this study, the effect of acetic acid, furfural, and hydroxymethylfurfural was evaluated on yeast growth and fermentation efficiency. Various parameters for the pretreatment of rice straw, such as an acid catalyst, and its concentration and residence time, were optimized for the maximum liberation of sugars in the hydrolysate. Further, the yeast strains Candida tropicalis and Meyerozyma caribbica were adapted for the tolerance of inhibitors at higher concentrations. A comparative analysis was carried out using un-adapted and adapted strains of Candida tropicalis and Meyerozyma caribbica for xylitol production. The findings of this study revealed that sulfuric acid (1.25% v/v) at 121 ரC for 30 min can efficiently convert rice straw xylan to xylose, with the release of 16.07 g/L xylose in the hydrolysate. Further, the adaptation results showed an increase of 76.42% and 69.33% in xylose assimilation by C. tropicalis and M. caribbica, respectively. The xylitol production with the adapted C. tropicalis was increased by 7.54% to 28.03 g/L xylitol. However, the xylitol production with the adapted M. caribbica was increased by 8.33%, yielding 26.02 g/L xylitol in the non-detoxified hydrolysate when compared to the un-adapted strains. Repeated batch fermentation was carried out for seven batches, and xylitol was found to be efficiently produced by the yeasts during five successive batches without any significant loss in the xylitol yield. Moreover, the results suggest that M. caribbica is a promising microorganism for the transformation of rice straw-derived xylose to xylitol.

Detoxification of furanic aldehydes of the hemicellulosic hydrothermal hydrolysate of brewers’ spent grains by continuous adsorption on a fixed‐bed column

AbstractDuring the process of subcritical water hydrolysis of lignocellulosic compounds, such as brewers' spent grains, some toxic compounds can be formed due to the breakdown of the cellulose structure. These compounds are known as furanic aldehydes, and they are inhibitors of microorganisms that are used in different processes, such as fermentation and anaerobic digestion, among others. The detoxification and conditioning of these hydrolysates are required before their use in other processes that require low concentrations of toxins. One of the mechanisms that can be used to diminish those compounds is the adsorption process. For this reason, this study evaluated at‐line and in‐line purification strategies using different adsorbents in fixed‐bed columns to remove furanic aldehydes from a model feed simulating hydrolysate from subcritical water hydrolysis. The model feed consisted of simple sugars and furanic aldehydes at appropriate concentrations. The selected adsorbents were silica‐C18, hydrocarbons, and activated carbon. The results indicate that, among the adsorbents that were studied, activated carbon was the most efficient for the removal of both 5‐hydroxymethylfurfural and furfural. The activated carbon removed >99% of furanic aldehydes in the experiments conducted in the at‐line system. Sequential subcritical water hydrolysis followed by either at‐line or in‐line purification is promising for integrated inexpensive and efficient hydrolysate purification.

Гидролиз гемицеллюлоз древесины при ультранизких концентрациях серной кислоты

Studies in the field of hydrolysis of plant polysaccharides are ordinary classified according to protic reactions with diluted or concentrated acids. Such classification is based on the significant difference in the mechanisms of the reactions. The hydrolysis of polysaccharides of plant materials with the diluted acids is indicated by the concentrations of the mineral acids 0.5–10.0 % or happens by acid-free autohydrolysis, without any use of acids. Each of these reactions has considerably different kinetic and temperature-time parameters. They have both advantages and disadvantages. In particular, the hydrolysis using dilute acids is specified by a significant consumption of reagents and the presence of a large amount of carbohydrate degradation products in the hydrolysate. Autohydrolysis is characterized by a relatively low monosaccharide yield, high energy consumption for the process and the formation of many by-products. To date, studies regarding hydrolysis of polysaccharides of plant materials with acids in a concentration range of less than 0.5 % are absent. The reason for the lack of interest in research in this area, in our opinion, was the statement that acid in the process of hydrolysis is spent on the neutralization of ash components of plant materials at a flow rate of 5 to 20 g/kg of dry raw materials. Accordingly, when hydrolysis is carried out with ultra-low concentrations of acid, it is possible to completely neutralize it and switch the hydrolysis process from acid to acid-free autohydrolysis. The purpose of the work was to establish the efficiency of the hydrolysis process at ultra-low acid consumption. A study of the process of hydrolysis of hemicelluloses of birch wood at ultra-low concentrations of sulfuric acid was carried out. The possibility of almost complete hydrolysis of hemicelluloses with sulfuric acid with concentration of 0.10–0.25 % is shown. The process of hydrolysis of hemicelluloses with ultra-low acid concentrations is well described by the first order reaction. The general kinetic constants are calculated according to the experimental data. They show that the process occupies an intermediate position between acid-free autohydrolysis and traditional hydrolysis of hemicelluloses with sulfuric acid with a concentration of more than 0.5 %. The developed technique is advantageously different from the known methods of hydrolysis of hemicelluloses by low consumption of sulfuric acid (more than 5 times) and energy resources. Hemicellulose hydrolysates obtained by ultra-low acid concentration regimes have high benign properties and can be used in xylitol production.

Genetically Engineering Escherichia coli to Produce Xylitol from Corncob Hydrolysate without Lime Detoxification.

Before fermentation with hemicellulosic hydrolysate as a substrate, it is generally necessary to detoxify the toxic substances that are harmful to microorganism growth. Cyclic AMP receptor protein (CRP) is a global regulator, and mutation of its key sites may have an important impact on E. coli virulence tolerance. Using corncob hydrolysate without ion-exchange or lime detoxification as the substrate, shake flask fermentation experiments showed that CRP mutant IS5-dG (I112L, T127G, A144T) produced 18.4 g/L of xylitol within 34 h, and the OD600 was 9.7 at 24 h; these values were 41.5% and 21.3% higher than those of the starting strain, IS5-d, respectively. This mutant produced 82 g/L of xylitol from corncob hydrolysate without ion-exchange or lime detoxification during fed-batch fermentation in a 15-L bioreactor, with a productivity of 1.04 g/L/h; these values were 173% and 174% higher than the starting strain, respectively. To our knowledge, this is the highest xylitol concentration and productivity produced by microbial fermentation using completely non-detoxified hemicellulosic hydrolysate as the substrate to date. This study also showed that alkali neutralization, high temperature sterilization, and fermentation of the hydrolysate had important effects on the xylose loss rate and xylitol production.

Pretreatment of Grape Pomaces and Stalks Using Deep Eutectic Solvents for Succinic Acid Production Integrated in a Biorefinery Concept

Deep eutectic solvents (DES) have been employed for the pretreatment of grape pomace and stalks within a biorefinery concept. Four DES, produced with choline chloride (ChCl) as hydrogen bond acceptor and four carboxylic acids as hydrogen bond donors, namely formic acid (FA), acetic acid (AA), lactic acid (LA) and oxalic acid (OA), were evaluated considering lignin removal efficiency, polysaccharide hydrolysis efficiency into C5 and C6 sugars, recyclability and reusability. The mixture of ChCl:LA at 1:10 molar ratio, 120°C and 1 h pretreatment, led to 40% lignin removal over four pretreatment cycles. Enzymatic hydrolysis of the remaining solids after the first pretreatment cycle resulted in 92.7% glucan and 36.6% hemicellulose hydrolysis yield. The hydrolysate was used as fermentation feedstock in batch Actinobacillus succinogenes bioreactor cultures leading to 36 g/L succinic acid with a yield of 0.62 gSA per g total sugars and 0.65 g/(L⋅h) productivity. Using 2 kg of grape pomace and stalks in each pretreatment cycle, the succinic acid that could be produced in five consecutive cycles is 200.8 g, 208 g, 204.9 g, 184.5 g and 94.3 g.Graphical