Production of xylooligosaccharides and monosaccharides from poplar by a two-step acetic acid and peroxide/acetic acid pretreatment

Unlocking the potential of vinegar residue: A novel biorefining strategy for amino acid-enriched xylooligosaccharides and humic-like acid

In order to address the issue of low amino acid retention in the production of xylooligosaccharides (XOS) through hydrothermal pretreatment at high temperatures, a novel approach combining low temperature acid hydrolysis and enzymatic hydrolysis was employed. This innovative method not only allows for the production of amino acid-rich XOS, but also yields a valuable byproduct known as humic-like acid (HLA) from vinegar residue (VR). Under the optimal pretreatment conditions (89 °C, 6 h, 1.2 % sulfuric acid), the yield of XOS was 19.88 %. Furthermore, the hydrolyzate extracted from the acid pretreated VR had a content of 2.65 g/L amino acids (corresponding to the retention rate of 82.0 %), and the HLA yield of the sample was 10.51 %. Comprehensive analyses, such as Fourier transform infrared spectroscopy, elemental analysis, total acidic functional group, and nuclear magnetic resonance were employed to examine the structure and composition of the produced HLA, indicating that it was similar to that of natural commercial humic acid (CHA) extracted from minerals. Through this innovative approach, the production of amino acid-rich XOS and HLA from VR offers a sustainable solution that not only addresses the issue of low amino acid retention but also maximizes the potential of VR as a valuable resource.

Efficient production of xylobiose and xylotriose from xylan in moso bamboo by the combination of pH-controlled lactic acid and xylanase hydrolysis

Short-chain xylo-oligosaccharides (XOS) such as xylobiose (X2) and xylotriose (X3) have higher biological activities. Therefore, it is interesting to produce highly active XOS enriched with X2 and X3. In this work, pH-controlled lactic acid (LA) hydrolysis was used to produce XOS from xylan in moso bamboo and xylanase was used to convert high DP XOS into low DP XOS to increase the percentage of X2 + X3 in XOS. A 33.1 % XOS yield was obtained from 2 % LA hydrolysis (pH = 3.2). After xylanase hydrolysis of the LA hydrolysate, the total XOS yield reached 64.1 %, with X2 + X3 yield reaching 58.4 %. The percentage of X2 + X3 increased from 52.3 % to a high level of 91.0 %. The deep eutectic solvent pretreatment removed 87.2 % lignin from the residue and the glucose yield of the delignified residue hydrolyzed by cellulase was 96.9 %. The results suggested that the integrated process of LA and xylanase hydrolysis could effectively produce X2 + X3 from xylan in moso bamboo.

Comparison of lactic and propionic acid hydrolysis for production of xylo-oligosaccharides and ethanol from polysaccharides in Toona sinensis branch

Xylan-type hemicellulose hydrolysis by an organic acid solution for the production of xylo-oligosaccharides (XOS) is efficient and eco-friendly, but the effects of different organic acids on XOS production from Toona sinensis branch (TB) biomass is limited. In this work, under the conditions of 170 °C for 60 min, 33.1 % and 38.7 % XOS yields were obtained from polysaccharides present in TB by 2 % lactic acid (LA) and 6 % propionic acid (PA), respectively. Then 77 % of the lignin was removed by hydrogen peroxide-acetic acid pretreatment system, and 39.5 % and 44.7 % XOS yield were obtained from polysaccharides in delignification TB by 2 % LA and 6 % PA, respectively. It was found that PA hydrolysis, especially from delignified TB, resulted in higher XOS yield and purity compared to LA hydrolysis. Moreover, the content of byproducts (xylose, hydroxymethyl-furfural and furfural) in PA hydrolysate was lower. Following the hydrolysis process, the simultaneous saccharification and fermentation of the TB solid residue achieved an ethanol yield of 71.5 %. This work proposed an integrated process to preferentially convert the TB hemicellulose into valuable XOS and then convert the cellulose into ethanol. This process had the advantages of eliminating the need for isolation and purification of xylan, and the potential to obtain multiple products from the same raw material.

Bioconversion of cellulose and hemicellulose in corn cob into L-lactic acid and xylo-oligosaccharides

With the banning of antibiotic chemical feed additives, multi-functional bioactive feed additives have been extensively sought after by the feed industry. In this study, low-cost and renewable corn cobs were treated with liquid hot water and converted into bioactive xylo-oligosaccharides and L-lactic acid after enzymatic hydrolysis, strain activation, and fermentation under mild conditions, which achieved a full utilization of cellulose and hemicellulose in corn cobs. Simultaneous saccharification fermentation after strain activation with enzymatic hydrolysate delivered the highest conversion rate of glucose to L-lactic acid (93.00 %) and yielded 17.38 g/L L-lactic acid and 2.68 g/L xylo-oligosaccharides. On this basis, batch-feeding fermentation resulted in a 78.03 % conversion rate of glucose to L-lactic acid, 18.99 g/L L-lactic acid, and 2.84 g/L xylo-oligosaccharides. This work not only provided a green and clean bioconversion strategy to produce multi-functional feed additives but can also boost the full utilization of renewable and cheap biomass resources.

Discovery of novel alkaline-tolerant xylanases from fecal microbiota of dairy cows

Xylo-oligosaccharides (XOS) are considered as a promising type of prebiotics that can be used in foods, feeds, and healthcare products. Xylanases play a key role in the production of XOS from xylan. In this study, we conducted a metagenomic analysis of the fecal microbiota from dairy cows fed with different types of fodders. Despite the diversity in their diets, the main phyla observed in all fecal microbiota were Firmicutes and Bacteroidetes. At the genus level, one group of dairy cows that were fed probiotic fermented herbal mixture-containing fodders displayed decreased abundance of Methanobrevibacter and increased growth of beneficial Akkermansia bacteria. Additionally, this group exhibited a high microbial richness and diversity. Through our analysis, we obtained a comprehensive dataset comprising over 280,000 carbohydrate-active enzyme genes. Among these, we identified a total of 163 potential xylanase genes and subsequently expressed 34 of them in Escherichia coli. Out of the 34 expressed genes, two alkaline xylanases with excellent temperature stability and pH tolerance were obtained. Notably, CDW-xyl-8 exhibited xylanase activity of 96.1 ± 7.5 U/mg protein, with an optimal working temperature of 55 ℃ and optimal pH of 8.0. CDW-xyl-16 displayed an activity of 427.3 ± 9.1 U/mg protein with an optimal pH of 8.5 and an optimal temperature at 40 ℃. Bioinformatic analyses and structural modeling suggest that CDW-xyl-8 belongs to GH10 family xylanase, and CDW-xyl-16 is a GH11 family xylanase. Both enzymes have the ability to hydrolyze beechwood xylan and produce XOS. In conclusion, this metagenomic study provides valuable insights into the fecal microbiota composition of dairy cows fed different fodder types, revealing main microbial groups and demonstrating the abundance of xylanases. Furthermore, the characterization of two novel xylanases highlights their potential application in XOS production.

Combined iron (III) chloride/sodium citrate with enzymatic hydrolysis for xylo-oligosaccharides and monosaccharides production from poplar

Currently, the production of xylo-oligosaccharides (XOS) from lignocelluloses by chelating system hydrolysis has not been investigated. Herein, iron (III) chloride/sodium citrate (IC/SC) chelating system hydrolysis and xylanase hydrolysis were used to produce XOS from poplar. Then, the delignification of IC/SC-hydrolyzed poplar was performed by p-toluenesulfonic acid (p-TsOH) pretreatment to increase the accessibility of cellulase. The results demonstrated that 42.3% of XOS with an extremely low by-product (xylose/XOS = 0.11) was produced from poplar by 50 mM IC/SC hydrolysis (molar ratio of 1:1, 170 °C, 60 min) and xylanase hydrolysis. The second step IC/SC hydrolysis and xylanase hydrolysis of poplar increased the yield of XOS to 51.3%. Finally, the glucose yield of p-TsOH-pretreated poplar (60% p-TsOH, 70 °C, 30 min) was greatly increased from 37.5% to 83.8% by cellulase hydrolysis with Tween 80 addition. The novel strategy proposed in this work was feasible for XOS and monosaccharides production from poplar.

Glycolic acid-based deep eutectic solvents boosting co-production of xylo-oligomers and fermentable sugars from corncob and the related kinetic mechanism

BACKGROUND

Xylo-oligomers are a kind of high value-added products in biomass fractionation. Although there are several chemical methods to obtain xylo-oligomers from biomass, the reports about the deep eutectic solvents (DESs)-mediated co-production of xylo-oligomers and fermentable sugars and the related kinetic mechanism are limited.

RESULTS

In this work, glycolic acid-based DESs were used to obtain xylo-oligomers from corncob. The highest xylo-oligomers yield of 65.9% was achieved at 120 °C for 20 min, of which the functional xylo-oligosaccharides (XOSs, DP 2-5) accounted for up to 31.8%. Meanwhile, the enzymatic digestion of cellulose and xylan in residues reached 81.0% and 95.5%, respectively. Moreover, the addition of metal inorganic salts significantly accelerated the hydrolysis of xylan and even the degradation of xylo-oligomers in DES, thus resulting in higher selectivity of xylan removal. AlCl3 showed the strongest synergistic effect with DES on accelerating the processes, while FeCl2 is best one for xylo-oligomers accumulation, affording the highest xylo-oligomers yield of 66.1% for only 10 min. Furthermore, the kinetic study indicates that the 'potential hydrolysis degree' model could well describe the xylan hydrolysis processes and glycolic acid/lactic acid (3:1) is a promising solvent for xylo-oligomers production, in particular, it worked well with FeCl2 for the excellent accumulation of xylo-oligomers.

CONCLUSIONS

Glycolic acid-based deep eutectic solvents can be successfully applied in corncob fractionation with excellent xylo-oligomers and fermentable sugars yields on mild conditions, and the large amount of xylo-oligosaccharides accumulation could be achieved by specific process controlling. The strategies established here can be useful for developing high-valued products from biomass.

Novel dual-action vanillic acid pretreatment for efficient hemicellulose separation with simultaneous inhibition of lignin condensation

Aromatic acids play a selective role in the separation of hemicellulose. Phenolic acids have demonstrated an inhibitory effect on lignin condensation. In the current study, vanillic acid (VA), which combines the characteristics of aromatic and phenolic acids, is used to separate eucalyptus. The efficient and selective separation of hemicellulose is achieved simultaneously at 170 °C, 8.0% VA concentration, and 80 min. The separation yield of xylose increased from 78.80% to 88.59% compared to acetic acid (AA) pretreatment. The separation yield of lignin decreased from 19.32% to 11.19%. In particular, the β-O-4 content of lignin increased by 5.78% after pretreatment. The results indicate that VA, as a "carbon positive ion scavenger", it preferentially reacts with the carbon-positive ion intermediate of lignin. Surprisingly, the inhibition of lignin condensation is achieved. This study provides a new starting point for the development of an efficient and sustainable commercial technology by organic acid pretreatment.

Rapid and mild fractionation of hemicellulose through recyclable mandelic acid pretreatment

The development of organic acid pretreatments from biological sources is essential to facilitate the progress of green and sustainable chemistry. In this study, the effectiveness of mandelic acid pretreatment (MAP) was analyzed for eucalyptus hemicellulose separation. 83.66% of xylose was separated under optimal conditions (temperature: 150 °C; concentration: 6.0 wt%; time: 80 min). The hemicellulose separation selectivity is higher than acetic acid pretreatment (AAP). The stable and effective separation efficiency (56.55%) is observed even after six reuses of the hydrolysate. Higher thermal stability, larger crystallinity index and optimized surface element distribution in the samples were demonstrated by MAP. Lignin condensation is effectively inhibited through MAP, as determined from the structural of different lignin. In particular, the demethoxylation of lignin by MA was found. These results open up a new way to construct a novel organic acid pretreatment for separating hemicellulose with high efficiency.

Pretreatment of poplar with eco-friendly levulinic acid to achieve efficient utilization of biomass

Organic acid pretreatment is an effective method for green separation of lignocellulosic biomass. However, repolymerization of lignin seriously affects the dissolution of hemicellulose and the conversion of cellulose during organic acid pretreatment. Therefore, a new organic acid pretreatment, levulinic acid (Lev) pretreatment, was studied for the deconstruction of lignocellulosic biomass without adding additional additives. The preferred separation of hemicellulose was realized at Lev concentration 7.0%, temperature 170 °C, and time 100 min. The separation of hemicellulose increased from 58.38% to 82.05% compared with acetic acid pretreatment. It was found that the repolymerization of lignin was effectively inhibited in the efficient separation of hemicellulose. This was attributed to the fact that γ-valerolactone (GVL) is a good green scavenger of lignin fragments. The lignin fragments in the hydrolysate were effectively dissolved. The results provided theoretical support for creating green and efficient organic acid pretreatment and effectively inhibiting lignin repolymerization.

Xylopentose production from crop residue employing xylanase enzyme

To produce xylo-oligosaccharides (XOS) from the agriculture waste, which included, green coconut and vegetable cocktail. The two pretreatment - hydrogen peroxide-acetic acid (HP-AC) and sodium hypochlorite-sodium hydroxide (SH-SH) - were used for this study. The optimal conditions for the pretreatment were 80 °C, 4.0 % NaClO, and 2 h, followed by 0.08 % NaOH, 55 °C, and 1 h. Further enzymatic hydrolysis of green coconut (GC) and vegetable cocktail (VC) were performed and found in case of GC, the best outcomes were observed. Different types of XOS were obtained from the treated biomass whereas a single type of XOS xylo-pentose was obtained in high quantity (96.44 % and 93.09 % from CG and VC respectively) with the production of other XOS < 2 %. This study presents a reasonably secure and economical method for turning secondary crop residue into XOS and fermentable sugars.

An integrated process using acetic acid hydrolysis and deep eutectic solvent pretreatment for xylooligosaccharides and monosaccharides production from wheat bran

Organic acid hydrolysis for xylooligosaccharides (XOS) production from lignocelluloses provides the benefits of simple operation, rapid reaction and high XOS yield. However, no literature reported the XOS production from wheat bran (WB) by organic acid hydrolysis. In this paper, acetic acid (AA) hydrolysis was employed to produce XOS from WB. After AA hydrolysis (5 %, v/v, 170 °C, 20 min) of 100 g/L WB, the concentrations of X2, X3, X4, X5 and X6 were 2.4, 5.0, 1.9, 1.9 and 1.4 g/L respectively and the total XOS yield was 62.9 %, which was the highest among the previous researches. The arabinose yield reached 76.1 %. Then, AA-hydrolyzed WB was delignified by deep eutectic solvent (DES) pretreatment and the resulting residue had the glucose and xylose yields of 83.8 % and 54.8 %, respectively. This work offers a productive method for the conversion of WB into XOS, arabinose and glucose by AA hydrolysis and DES pretreatment.

Production of xylo-oligosaccharides and ethanol from corncob by combined tartaric acid hydrolysis with simultaneous saccharification and fermentation

In organic acid hydrolysis for xylo-oligosaccharides (XOS) production, organic acids with low flash points and explosion limits can lead to explosion and fire risk. Herein, tartaric acid (TA) as an organic acid with high flash point and no explosion limit was used in the hydrolysis of corncob to produce XOS. Then, the TA-hydrolyzed corncob was used for ethanol production. In TA hydrolysis of corncob, a 56.4 % XOS yield was obtained from the hydrolysate with the conditions of 170 °C, 60 mM TA and 10 min. Meanwhile, 92.1 % TA was recovered from the hydrolysate by the addition of calcium hydroxide. After simultaneous saccharification and fermentation of TA-hydrolyzed corncob, an 82.4 % ethanol yield was obtained with a solid loading of 25 % (w/v, 250 g/L) by Saccharomyces cerevisiae H06. This research provided a relatively safe, simple, and efficient technology for producing XOS and ethanol from corncob.

Predicting xylose yield from prehydrolysis of hardwoods: A machine learning approach

Hemicelluloses are amorphous polymers of sugar molecules that make up a major fraction of lignocellulosic biomasses. They have applications in the bioenergy, textile, mining, cosmetic, and pharmaceutical industries. Industrial use of hemicellulose often requires that the polymer be hydrolyzed into constituent oligomers and monomers. Traditional models of hemicellulose degradation are kinetic, and usually only appropriate for limited operating regimes and specific species. The study of hemicellulose hydrolysis has yielded substantial data in the literature, enabling a diverse data set to be collected for general and widely applicable machine learning models. In this paper, a dataset containing 1955 experimental data points on batch hemicellulose hydrolysis of hardwood was collected from 71 published papers dated from 1985 to 2019. Three machine learning models (ridge regression, support vector regression and artificial neural networks) are assessed on their ability to predict xylose yield and compared to a kinetic model. Although the performance of ridge regression was unsatisfactory, both support vector regression and artificial neural networks outperformed the simple kinetic model. The artificial neural network outperformed support vector regression, reducing the mean absolute error in predicting soluble xylose yield of test data to 6.18%. The results suggest that machine learning models trained on historical data may be used to supplement experimental data, reducing the number of experiments needed.

Coproduction of xylooligosaccharides and monosaccharides from hardwood by a combination of acetic acid pretreatment, mechanical refining and enzymatic hydrolysis

Sequential biorefinery treatments of acetic acid (HAC) pretreatment, Papir Forsknings Institutet (PFI) milling and enzymatic hydrolysis were demonstrated for coproduction of xylooligosaccharides (XOS) and fermentable monosaccharides. Results indicated that 36.2% XOS (50.8% X2-X3) and 17.0% low DP xylans were achieved using a HAC pretreatment with a combined severity factor of 0.78. The HAC pretreatment resulted in a XOS-rich prehydrolyzate with a low molecular weight of 1.28 kDa. The endo-xylanase hydrolysis was conducted on the pretreatment liquor to elevate XOS yield and the content of higher-value X2-X3. Moreover, fermentable glucose production from the pretreated residue increased by 2.3 folds when introducing an additional step of PFI refining prior to enzymatic digestion. Properties of substrate including cellulose accessibility, crystallite size, crystalline index and water retention value were in close relationships with enzymatic digestibility. The implementation of proposed biorefinery process will give more insights into the efficient construction of a wood-derived sugar platform.

p-Toluenesulfonic acid combined with hydrogen peroxide-assisted pretreatment improves the production of fermentable sugars from walnut (Juglans regia L.) shells

This study presents p-toluenesulfonic acid (p-TsOH) pretreatment combined with subsequent hydrogen-peroxide pretreatment for the fractionation of all lignocellulosic components from walnut shells (WNS). The main focus of this study is the production of fermentable sugars. During p-TsOH pretreatment (55% p-TsOH, 1:10 solid-to-liquid ratio, 90 °C, 120 min), 50.2% of lignin and 88.3% of xylan were dissolved. Subsequently, the p-TsOH pretreated WNS without washing (to reduce water consumption) was further pretreated with 40% (v/v) H₂O₂ and 74.6% of lignin was removed at 60 °C for 120 min. Compared with the glucose yield of 10.2% from p-TsOH pretreated WNS, the glucose yield from the p-TsOH/H₂O₂ pretreated WNS could be significantly improved to 94.4%. Structural characterization analysis showed increases in porosity, biomass disruption, and cellulose crystallinity during p-TsOH/H₂O₂ pretreatment. Mass balance demonstrated that 1000 g of WNS produced 295.1 g of fermentable sugars (263.6 g glucose and 31.5 g xylose).

One-step sodium bisulfate hydrolysis for efficient production of xylooligosaccharides from poplar

Acid salts have been shown to catalyze xylan hydrolysis selectively and efficiently for xylooligosaccharides (XOS) production while using acid salts that are non-toxic and available as feed additives can avoid separation from resulting XOS-rich hydrolysates. There is no report on XOS production with sodium bisulfate (NaHSO₄) hydrolysis, of significance is that NaHSO₄ as feed additive does not need to be separated. In this work, NaHSO₄ hydrolysis was firstly employed to produce XOS from poplar. XOS yield of 42.7% was reached under optimal conditions of 0.04 mol/L NaHSO₄, 170 °C and 60 min. After hydrogen peroxide/acetic acid and sodium hydroxide treatments of NaHSO₄-pretreated poplar, high yields of glucose (92.0%) and xylose (91.3%) were obtained at a low cellulase dose of 5 FPU/g dry mass. NaHSO₄ hydrolysis was a novel strategy to prepare XOS efficiently with simple operation steps, and XOS-rich hydrolysates could be potentially used as feed additives without NaHSO₄ separation.

Lignin removal improves xylooligosaccharides production from poplar by acetic acid hydrolysis

Organic acid hydrolysis is a potential method for xylooligosaccharides (XOS) production from lignocelluloses. However, the effect of lignin content on XOS production using organic acid hydrolysis remains unclear. In this work, the effect of delignification on XOS production from poplar by acetic acid (AC) hydrolysis was investigated. Hydrogen peroxide-acetic acid (HPAC) pretreatment catalyzed by 0-200 mM H₂SO₄ (HPAC0-HPAC200) removed 21.6-86.5% of lignin in poplar. HPAC pretreatment increased the xylan accessibility to AC solution, thus increasing the xylan removal during AC hydrolysis. An appropriate delignification (61.7%) resulted in the highest XOS yield of 37.4% by AC hydrolysis, increased by 29.9% compared to the optimal XOS yield (28.8%) from raw poplar. After alkaline post-incubation, the glucose yield of poplar residue rose from 57.1% to 78.6%. This work developed a delignification process to efficiently improve XOS and monosaccharides production from poplar.

Optimized production of xylooligosaccharides from poplar: A biorefinery strategy with sequential acetic acid/sodium acetate hydrolysis followed by xylanase hydrolysis

The preparation of xylooligosaccharides (XOS) from lignocelluloses by organic acid hydrolysis has the advantages of high efficiency and simplicity, but reducing the production of by-products, especially xylose, is a prerequisite for commercial preparation of XOS using organic acid. In this work, to reduce the production of by-products, the acetic acid/sodium acetate conjugate system (AC/SA) was used to prepare XOS from poplar. Under the optimal conditions (0.15 M AC/SA, molar ratio of 3.0, 175 °C, 60 min), the maximum XOS yield was 33.6% with a low xylose/XOS ratio of 0.19. Xylanase hydrolysis effectively converted XOS with DP above 6 in the AC/SA hydrolysate to X2-X6 with little xylose produced. The XOS yield increased to 42.1%, with a xylose/XOS ratio was only 0.17. This work shows that AC/SA in combination with xylanase hydrolysis of poplar successfully achieved high XOS yield with low by-products yields without the extraction of xylan from the substrate.

Structural properties and hydrolysability of recycled poplar residues (Populus L.): Effects of two-step acetic acid and sodium sulphite pre-treatment

Poplar trees rapidly yield wood and are therefore suitable as a biofuel feedstock; however, the quality of poplar is modest, and the profitability of poplar cultivation depends on the efficiency of the harvesting process. This study offers a simple and sustainable technique to harvest lignocellulosic resources from poplar for bioethanol production. The proposed two-step pretreatment method increased the surface lignin content and decreased the surface polysaccharide content. The cellulose content increased to 54.9% and the xylan content decreased to 6.7% at 5% AC. The cellulose yield of poplar residues (Populus L.) reached 65.5% by this two-step acetic acid (AC) and sodium sulphite (SS) treatment method. Two-step pretreatment using 5% AC and 4% SS obtained a recovery of nearly 80% of the total available fermentable sugar. The surface characterization showed a higher porosity in treated samples, which improved their hydrolysability. This method decreased the amount of lignin in plant biomass, making it applicable for further wood resource recovery or waste recycling for biorefinery purposes at very low costs.

Molecular Characteristics and Antioxidant Activity of Spruce (Picea abies) Hemicelluloses Isolated by Catalytic Oxidative Delignification

Spruce (Piceaabies) wood hemicelluloses have been obtained by the noncatalytic and catalytic oxidative delignification in the acetic acid-water-hydrogen peroxide medium in a processing time of 3–4 h and temperatures of 90–100 °C. In the catalytic process, the H₂SO₄, MnSO₄, TiO₂, and (NH₄)₆Mo₇O₂₄ catalysts have been used. A polysaccharide yield of up to 11.7 wt% has been found. The hemicellulose composition and structure have been studied by a complex of physicochemical methods, including gas and gel permeation chromatography, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. The galactose:mannose:glucose:arabinose:xylose monomeric units in a ratio of 5:3:2:1:1 have been identified in the hemicelluloses by gas chromatography. Using gel permeation chromatography, the weight average molar mass M_w of hemicelluloses has been found to attain 47,654 g/mol in noncatalytic delignification and up to 42,793 g/mol in catalytic delignification. Based on the same technique, a method for determining the α and k parameters of the Mark–Kuhn–Houwink equation for hemicelluloses has been developed; it has been established that these parameters change between 0.33–1.01 and 1.57–472.17, respectively, depending on the catalyst concentration and process temperature and time. Moreover, the FTIR spectra of the hemicellulose samples contain all the bands characteristic of heteropolysaccharides, specifically, 1069 cm⁻¹ (C–O–C and C–O–H), 1738 cm⁻¹ (ester C=O), 1375 cm⁻¹ (–C–CH₃), 1243 cm⁻¹ (–C–O–), etc. It has been determined by the thermogravimetric analysis that the hemicelluloses isolated from spruce wood are resistant to heating to temperatures of up to ~100 °C and, upon further heating, start destructing at an increasing rate. The antioxidant activity of the hemicelluloses has been examined using the compounds simulating the 2,2-diphenyl-2-picrylhydrazyl free radicals.

Investigation of hydrogen peroxide-acetic acid pretreatment to enhance the enzymatic digestibility of bamboo residues

Bamboo biomass was widely considered as a promising substitute for lignocellulose to produce fermentable sugars and biofuels in the south of China. When P. amarus were treated using hydrogen peroxide and acetic Acid pretreatment in the presence of sulphuric acid at 60 ℃ for 2 h, 82.63% lignin was removed from the bamboo residue, and enzymatic saccharification yield of 79.3% and ethanol content of 13.31 g/L were obtained. Analysis indicated that HPAC pretreatment increased the hydrophilic and porous nature of substrate, which can improve the enzyme accessibility to cellulose. When HPAC-pretreated D. sinicus, B. lapidea, N. affinis, andD. giganteus were used as the substrates of enzymatic saccharification, glucose yields of 71-84% at 72 h were achieved. HPAC pretreatment was a highly efficient and environmentally friendly method for bamboo biorefinery in the south of China.

Delignification of poplar for xylo-oligosaccharides production using lactic acid catalysis

Recently, xylo-oligosaccharides (XOS) production from lignocelluloses by organic acid catalysis has been widely reported. However, the effect of delignification of lignocelluloses on XOS production by organic acid catalysis was unclear, and lactic acid (LA) catalysis in XOS production from lignocelluloses has not been reported. In this work, the effect of delignification on XOS production from poplar by LA catalysis was investigated. Results demonstrated that hydrogen peroxide-acetic acid (HPAA) pretreatment removed 83.2% of lignin and retained 95.4% of xylan. After 2% LA catalysis (170 °C, 30 min), a high XOS yield of 42.7% was obtained from HPAA₁-LA-pretreated poplar. Lignin removal from poplar was positively correlated with XOS yield. Glucose yield of HPAA₁-LA-pretreated poplar by cellulase was 88.9%. Compared with LA-catalyzed poplar, the XOS and glucose production from HPAA₁-LA-pretreated poplar by cellulase increased by 1.4-fold and 6.8-fold, respectively. This work presents a novel strategy for efficient producing XOS and monosaccharides from poplar.

Efficient production of xylooligosaccharides and fermentable sugars from corncob by propionic acid and enzymatic hydrolysis

Xylooligosaccharides (XOS) are usually produced by xylan isolation from lignocellulose by alkaline followed by enzymatic hydrolysis, but the process is complicated. Recently acid hydrolysis for XOS preparation has become popular as it is faster and easier. This study investigated a novel strategy for producing XOS from corncob using propionic acid (PA) hydrolysis, then producing monosaccharides from solid residues by cellulase hydrolysis. The effect of alkaline post-treatment on enzymatic hydrolysis was studied. The maximum XOS yield of 68.5% was achieved using 5% PA at 170 °C for 50 min. About 84% of lignin in PA-hydrolyzed corncob was removed using alkaline post-treatment. The yields of glucose and xylose reached 89.8% and 80.1%, respectively, using 5 FPU cellulase/g dry matter. The results indicated that alkaline post-treatment reduced 50% cellulase loading and improved the saccharification of PA-hydrolyzed corncob. This study presents an innovative option for efficient production of XOS and monosaccharides from corncob.

High solid loading enzymatic hydrolysis of acetic acid-peroxide/acetic acid pretreated poplar and cellulase recycling

High solid loading saccharification is the premise of preparing high-concentration sugar which is beneficial to bioethanol production, but the limited sugar concentration and high enzyme dosage are two challenges. In this work, the glucan-rich acetic acid-hydrogen peroxide/acetic acid (AC-HPAC)-pretreated poplar (85.8%) were prepared for enzymatic hydrolysis at 10%-40% solid loading and the strategies for reducing cellulase dosage were explored. Results showed that the maximum glucose concentration reached to 250.8 g/L at 40% solid loading, which was the highest concentration in previous literatures. As the solid loading was 20%, the addition of Tween 80 saved 50% of cellulase and the recycling of unhydrolyzed residue (0.2 g/g DM) saved another 25% of cellulase, resulting in 152.2 g/L of glucose concentration with yield of 79.9%. This work showed potential of poplar to produce the high concentration glucose solution with low enzyme loading through the recycling of enzyme bound onto unhydrolyzed residue.

Effects of post-washing on pretreated biomass and hydrolysis of the mixture of acetic acid and sodium hydroxide pretreated biomass and their mixed filtrate

Effects of post-washing [one-volume water (I-VW) or double-volume water (Ⅱ-VW)] on pretreated hemp and poplar biomass and enzymatic hydrolysis of the mixture of HOAc and NaOH pretreated biomass and their mixed filtrate were investigated. Compared to I-VW, Ⅱ-VW increased 3.76-6.80% of glucan content in NaOH pretreated biomass, diminished lignin recondensation, and heightened cellulose-related FTIR peak intensities, crystallinity index, and lignin removal. The pH of mixed filtrate was around 4.80, precipitating the NaOH soluble lignin partially. Although Ⅱ-VW showed lower lignin recoveries than I-VW, their FTIR characteristics were equivalent to the commercial alkali lignin. Enzymatic hydrolysis at solid loadings of 2.5-10% (w/v) demonstrated that I-VW and Ⅱ-VW had marginal variations in sugar concentration and conversion efficiency, indicating that I-VW is sufficient for post-washing pretreated biomass. Glucose concentration exhibited a quadratic correlation with solid loading and hemp biomass reached the maximum glucose (43.88 g/L) and total sugar (57.08 g/L) concentrations with I-VW.

Ozone assisted autohydrolysis of wheat bran enhances xylooligosaccharide production with low generation of inhibitor compounds: A comparative study

In the present study, ozone assisted autohydrolysis (OAAH) was evaluated for enhanced generation of xylooligosaccharide (XOS) from wheat bran. The total XOS yield with optimum ozone dose of 3% (OAAH-3) was found to be 8.9% (w/w biomass) at 110 °C in comparison to 7.96% at 170 °C by autohydrolysis (AH) alone. Although, there was no significant difference in oligomeric composition (DP 2-6), significant decrease in degradation products namely furfural (2.78-fold), HMF (3.15-fold), acrylamide (nil) and acetic acid (1.06-fold), was observed with OAAH-3 as a pretreatment option. There was 1-fold higher xylan to XOS conversion and OAAH-hydrolysate had higher DPPH radical scavenging activity than AH. PCA plots indicated clear enhancement in XOS production and lower generation of inhibitors with decrease in treatment temperature. Results of the study therefore suggest OAAH can be an effective pretreatment option that can further be integrated with downstream processing for concentration and purification of XOS.

Minimizing water consumption for sugar and lignin recovery via the integration of acid and alkali pretreated biomass and their mixed filtrate without post-washing

Excessive post-washing of pretreated biomass leads to huge water consumption and chemical loss. To address this issue, parallel HOAc and NaOH pretreatments of biomass followed by integration of their biomass and filtrate were investigated. Pretreatment effectiveness including morphology, crystallinity, and component recovery, were elucidated. Results showed that HOAc and NaOH in the mixed filtrate were neutralized to achieve a pH of around 4.80 prompting the alkali lignin precipitation. Lignin (46.01 and 48.38 g/kg-biomass for hemp and poplar, respectively) exhibiting comparable FTIR characteristics with the commercial alkali lignin was recovered. Compared to sodium acetate buffer as a control, integrating HOAc and NaOH pretreated biomass and their mixed filtrate for enzymatic hydrolysis boosted total sugar concentration (hemp: 42.90 vs. 38.27 g/L; poplar: 43.18 vs. 38.76 g/L) without compromising glucose yield (hemp: 70.86 vs. 70.69%; poplar: 66.48 vs. 69.48%) but improving xylose yield (hemp: 60.10 vs. 35.92%; poplar: 56.90 vs. 29.39%).

Alkaline post-incubation improves the saccharification of poplar after hydrogen peroxide-acetic acid pretreatment

BACKGROUND

Hydrogen peroxide-acetic acid (HPAA) is widely used in pretreatment of lignocellulose because it has a good capability in selective delignification. However, high concentration (more than 60%) of HPAA increases the cost of pretreatment and the risk of explosion. In this work, alkaline post-incubation was employed to decrease the HPAA loading and improve the saccharification of poplar.

RESULTS

Pretreatment with 100% HPAA removed 91.0% lignin and retained 89.9% glucan in poplar. After poplar was pretreated by 100% HPAA at 60 °C for 2 h, the glucan conversion in enzymatic hydrolysis by cellulase increased to 90.1%. Alkaline incubation reduced the total lignin, surface lignin, and acetyl group of HPAA-pretreated poplar. More than 92% acetyl groups of HPAA-pretreated poplar were removed by alkaline incubation with 1.0% NaOH at 50 °C for 1 h. After incubation of 60% HPAA-pretreated poplar with 1.0% NaOH, the glucan conversion enhanced to 95.0%. About 40% HPAA loading in pretreatment was reduced by alkaline incubation without the decrease of glucose yield.

CONCLUSIONS

Alkaline post-incubation had strong ability on the deacetylation and delignification of HPAA-pretreated poplar, exhibiting a strong promotion on the enzymatic hydrolysis yield. This report represented alkaline incubation reduced the HPAA loading, improved pretreatment safety, exhibiting excellent potential application in saccharification of poplar.

Production of xylo-oligosaccharides from poplar by acetic acid pretreatment and its impact on inhibitory effect of poplar lignin

Recently, efficient production of xylo-oligosaccharides (XOS) from poplar by acetic acid (AA) pretreatment was developed; but the effect of residual lignin on subsequent cellulase hydrolysis was unclear. Herein, XOS was produced from poplar by AA pretreatment and the effect of AA pretreatment on lignin inhibition to cellulase hydrolysis was investigated. The results indicated that a high XOS yield of 55.8% was obtained, and the inhibition degree of lignin in poplar increased from 1.0% to 6.8% after AA pretreatment. Lignin was acetylated and its molecular weight decreased from 12,211 to 2871 g/mol after AA pretreatment. The increase of S/G ratio, phenolic hydroxyl, and condensed units of lignin after AA pretreatment might be reasons for this intensified inhibition. The results advanced our understanding of the structural and inhibitory properties of lignin after production of XOS from poplar with AA pretreatment, and provided references for efficient cellulase hydrolysis of poplar after AA pretreatment.

Co-production of Xylooligosaccharides and Xylose From Poplar Sawdust by Recombinant Endo-1,4-β-Xylanase and β-Xylosidase Mixture Hydrolysis

As is well-known, endo-1,4-β-xylanase and β-xylosidase are the rate-limiting enzymes in the degradation of xylan (the major hemicellulosic component), main functions of which are cleavaging xylan to release xylooligosaccharides (XOS) and xylose that these two compounds have important application value in fuel, food, and other industries. This study focuses on enzymatic hydrolysis of poplar sawdust xylan for production of XOS and xylose by a GH11 endo-1,4-β-xylanase MxynB-8 and a GH39 β-xylosidase Xln-DT. MxynB-8 showed excellent ability to hydrolyze hemicellulose of broadleaf plants, such as poplar. Under optimized conditions (50°C, pH 6.0, dosage of 500 U/g, substrate concentration of 2 mg/mL), the final XOS yield was 85.5%, and the content of XOS2-3 reached 93.9% after 18 h. The enzymatic efficiency by MxynB-8 based on the poplar sawdust xylan in the raw material was 30.5%. Xln-DT showed excellent xylose/glucose/arabinose tolerance, which is applied as a candidate to apply in degradation of hemicellulose. In addition, the process and enzymatic mode of poplar sawdust xylan with MxynB-8 and Xln-DT were investigated. The results showed that the enzymatic hydrolysis yield of poplar sawdust xylan was improved by adding Xln-DT, and a xylose-rich hydrolysate could be obtained at high purity, with the xylose yield of 89.9%. The enzymatic hydrolysis yield was higher (32.2%) by using MxynB-8 and Xln-DT together. This study provides a deep understanding of double-enzyme synergetic enzymolysis of wood polysaccharides to valuable products.

Assessment of combination of pretreatment of Sorghum durra stalk and production of chimeric enzyme (β-glucosidase and endo β-1,4 glucanase, CtGH1-L1-CtGH5-F194A) and cellobiohydrolase (CtCBH5A) for saccharification to produce bioethanol

Optimization of pretreatment and saccharification of Sorghum durra stalk (Sds) was carried out. The chimeric enzyme (CtGH1-L1-CtGH5-F194A) having β-glucosidase (CtGH1) and endo β-1,4 glucanase activity (CtGH5-F194A) and cellobiohydrolase (CtCBH5A) from Clostridium thermocellum were used for saccharification. Chimeric enzyme will save production cost of two enzymes, individually. Stage 2 pretreatment by 1% (w/v) NaOH assisted autoclaving + 1.5% (v/v) dilute H₂SO₄ assisted oven heating gave lower total sugar yield (366.6 mg/g of pretreated Sds) and total glucose yield (195 mg/g of pretreated Sds) in pretreated hydrolysate with highest crystallinity index 55.6% than the other stage 2 pretreatments. Optimized parameters for saccharification of above stage 2 pretreated biomass were 3% (w/v) biomass concentration, enzyme (chimera: cellobiohydrolase) ratio, 2:3 (U/g) of biomass, total enzyme loading (350 U/g of pretreated biomass), 24 h and 30 °C. Best stage 2 pretreated Sds under optimized enzyme saccharification conditions gave maximum total reducing sugar yield 417 mg/g and glucose yield 285 mg/g pretreated biomass in hydrolysate. Best stage 2 pretreated Sds showed significantly higher cellulose, 71.3% and lower lignin, 2.0% and hemicellulose, 12.2% (w/w) content suggesting the effectiveness of method. This hydrolysate upon SHF using Saccharomyces cerevisiae under unoptimized conditions produced ethanol yield, 0.12 g/g of glucose. Abbreviation: Ct-Clostridium thermocellum, Sds-Sorghum durra stalk, TRS-Total reducing sugar, HPLC-High performance liquid chromatography, RI-Refractive index, ADL-acid insoluble lignin, GYE-Glucose yeast extract, MGYP-Malt glucose yeast extract peptone, SHF-separate hydrolysis and fermentation, OD-Optical density, PVDF-Poly vinylidene fluoride, TS-total sugar, FESEM-Field emission scanning electron microscopy, XRD-X-ray diffraction, FTIR-Fourier transform infra-red spectroscopy and CrI-Crystallinity index.

Production of xylooligosaccharides and monosaccharides from hydrogen peroxide-acetic acid-pretreated poplar by two-step enzymatic hydrolysis

The severe pretreatment of poplar makes xylan difficult to utilize efficiently. In this work, poplar was pretreated by hydrogen peroxide-acetic acid (HPAC) with H₂SO₄ as catalyst to remove lignin, and the solid residues were used to produce xylooligosaccharides (XOS) and monosaccharides by two-step xylanase and cellulase hydrolysis. The results indicated that higher H₂SO₄ concentrations in the HPAC pretreatment of poplar afforded stronger lignin removal ability. An increased XOS yield of 19.8% was obtained from 200 mM H₂SO₄-catalyzed poplar by xylanase and the XOS purity was high, with a very low xylose/XOS ratio of 0.14. Higher glucose (75.2%) and xylose (61.4%) yields were obtained from the HPAC-pretreated poplar using 50 mM H₂SO₄ as catalyst. Finally, 16.9 g XOS and 296.4 g glucose were produced from 1 kg poplar by xylanase and cellulase. This study provides a method for producing functional XOS and monosaccharides from poplar using a simple reduced-pollution strategy.

Improving enzymatic saccharification of hardwood through lignin modification by carbocation scavengers and the underlying mechanisms

In this work, the beneficial effect of carbocation scavenger additives on hardwood pretreatment was revealed by significantly improved biomass saccharification: cellulose hydrolysis yield was increased by over 15% after steam pretreatment of poplar, while that was enhanced by more than 48% after dilute acid pretreatment. Besides, the relative contributions of lignin towards enzyme binding and physical barrier effect for proposed mechanisms were quantified. Results indicated that the addition of carbocation scavenger, 2-naphthol-7-sulfonate, resulted in acid groups incorporation of 62.36 mmol/kg to lignin, which mitigated enzyme non-productive binding. Moreover, enlarged biomass porosity and reduced surface lignin coverage were detected through BET and XPS analysis, respectively, which mostly related to the diminished physical barrier effect of lignin. As a result, the lignin inhibitions were significantly suppressed through the addition of carbocation scavenger, giving rise to significantly improved enzymatic hydrolysis of hardwood.