Bioconversion of lignocellulosic biomass to xylitol: An overview

Characterization of two GH10 enzymes with ability to hydrolyze pretreated Sorghum bicolor bagasse

In this study, we characterized two novel enzymes of the glycoside hydrolase family 10 (GH10), Xyl10 C and Xyl10E, identified in the termite gut microbiome. The activities of both enzymes were assayed using beechwood xylan, barley β-glucan, and pretreated Sorghum bicolor bagasse (SBB) as substrates. Both enzymes, assessed individually and in combination, showed activity on beechwood xylan and pretreated SBB, whereas Xyl10E also showed activity on barley β-glucan. The composition of pretreated SBB mainly consisted of xylose and arabinose content. Purified Xyl10 C showed optimum xylanase activity in the pH range 7.0-8.0 and at a temperature of 50-60 °C, while Xyl10E was active at a wider pH range (5.0-10.0) and at 50 °C. The residual activities of Xyl10 C and Xyl10E after 8 h of incubation at 40 °C were 85% and 70%, respectively. The enzymatic activity of Xyl10 C increased to 115% in the presence of 5 M NaCl, was only inhibited in the presence of 0.5% sodium dodecyl sulfate (SDS), and decreased with β-mercaptoethanol. The xylanase and glucanase activities of Xyl10E were inhibited only in the presence of MnSO4, NaCl, and SDS. The main hydrolysis enzymatic product of Xyl10 C and Xyl10E on pretreated SBB was xylobiose. In addition, the xylo-oligosaccharides produced by xylanase Xyl10E on pretreated SBB demonstrated promising antioxidant activity. Thus, the hydrolysis products using Xyl10E on pretreated SBB indicate potential for antioxidant activity and other valuable industrial applications. KEY POINTS: • Two novel GH10 xylanases from the termite gut microbiome were characterized. • Xylo-oligosaccharides obtained from sorghum bagasse exhibited antioxidant potential. • Both enzymes and their hydrolysis product have potential to add value to agro-waste.

Unveiling the fitness of Saccharomyces cerevisiae strains for lignocellulosic bioethanol: a genomic exploration through fermentation stress tests

Lignocellulosic biomass holds significant promise as a substrate for bioethanol production, yet the financial viability of lignocellulosic fermentation poses challenges. The pre-treatment step needed for lignocellulosic substrates generates inhibitors that impede Saccharomyces cerevisiae growth, affecting the fermentation process and overall yield. In modern sugarcane-to-ethanol plants, a rapid succession of yeast strains occurs, with dominant strains prevailing. Therefore, yeast strains with both dominance potential and inhibitor tolerance are crucial towards the development of superior strains with industrial fitness. This study adopted a hybrid approach combining biotechnology and bioinformatics to explore a cluster of 20 S. cerevisiae strains, including industrial and oenological strains exhibiting diverse phenotypic features. In-depth genomic analyses focusing on gene copy number variations (CNVs) and single nucleotide polymorphisms (SNPs) were conducted and compared with results from fermentation tests once inoculated in multiple strains kinetics under stressing conditions such as low nitrogen availability and high formic or acetic acid levels. Some strains showed high resistance to biotic stress and acetic acid. Moreover, four out of 20 strains - namely S. cerevisiae YI30, Fp89, Fp90 and CESPLG05 - displayed promising resistance also to formic acid, the most impactful weak acids in pre-treated lignocellulosic biomass. These strains have the potential to be used for the development of superior S. cerevisiae strains tailored for lignocellulosic bioethanol production.

Xylitol production from passion fruit peel hydrolysate: Optimization of hydrolysis and fermentation processes

The passion fruit peel (PFP) has a high cellulose and hemicellulose content, which can be used to produce fermentable sugars. In this context, this study aims to optimize the release of xylose and the production of xylitol from PFP. The optimized conditions were 0.71 M dilute sulfuric acid and a 21.84-minute treatment, yielding 19.03 g/L of xylose (PFP-1). Different PFP hydrolysates were evaluated to improve xylitol production by the yeast Kluyveromyces marxianus ATCC 36907: PFP-2 (PFP1 treated with Ca(OH)2), PFP-3 (PFP-1 treated with Ca(OH)2 and activated carbon), PFP-4 (PFP-3 with biological elimination of glucose with S. cerevisiae, and concentrated at different xylose concentrations). The applied methods resulted in higher xylitol production (14.97 g/L), when PFP hydrolysate was detoxified with Ca(OH)2, treated with activated charcoal for 1 h, biotreated for glucose removal, and concentrated to 40 g/L of xylose.

Exploring industrial lignocellulosic waste: Sources, types, and potential as high-value molecules

Lignocellulosic biomass has a promising role in a circular bioeconomy and may be used to produce valuable molecules for green chemistry. Lignocellulosic biomass, such as food waste, agricultural waste, wood, paper or cardboard, corresponded to 15.7% of all waste produced in Europe in 2020, and has a high potential as a secondary raw material for industrial processes. This review first presents industrial lignocellulosic waste sources, in terms of their composition, quantities and types of lignocellulosic residues. Secondly, the possible high added-value chemicals obtained from transformation of lignocellulosic waste are detailed, as well as their potential for applications in the food industry, biomedical, energy or chemistry sectors, including as sources of polyphenols, enzymes, bioplastic precursors or biofuels. In a third part, various available transformation treatments, such as physical treatments with ultrasound or heat, chemical treatments with acids or bases, and biological treatments with enzymes or microorganisms, are presented. The last part discusses the perspectives of the use of lignocellulosic waste and the fact that decreasing the cost of transformation is one of the major issues for improving the use of lignocellulosic biomass in a circular economy and green chemistry approach, since it is currently often more expensive than petroleum-based counterparts.

Computation-Aided Phylogeny-Oriented Engineering of β-Xylosidase: Modification of "Blades" to Enhance Stability and Activity for the Bioconversion of Hemicellulose to Produce Xylose

Hemicellulose is a highly abundant, ubiquitous, and renewable natural polysaccharide, widely present in agricultural and forestry residues. The enzymatic hydrolysis of hemicellulose has generally been accomplished using β-xylosidases, but concomitantly increasing the stability and activity of these enzymes remains challenging. Here, we rationally engineered a β-xylosidase from Bacillus clausii to enhance its stability by computation-aided design combining ancestral sequence reconstruction and structural analysis. The resulting combinatorial mutant rXYLOM25I/S51L/S79E exhibited highly improved robustness, with a 6.9-fold increase of the half-life at 60 °C, while also exhibiting improved pH stability, catalytic efficiency, and hydrolytic activity. Structural analysis demonstrated that additional interactions among the propeller blades in the catalytic module resulted in a much more compact protein structure and induced the rearrangement of the opposing catalytic pocket to mediate the observed improvement of activity. Our work provides a robust biocatalyst for the hydrolysis of agricultural waste to produce various high-value-added chemicals and biofuels.

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.

Improved xylitol production by the novel inhibitor-tolerant yeast Candida tropicalis K2

Production of potential value-added products from different lignocellulosic biomass is becoming more common due to the availability of the feedstocks in abundance and the environment- friendly nature of the microbial production process. Due to the large array of its applications in the pharmaceutical and food sectors, xylitol is considered as potential value-added compound for production. In this study, organic waste samples were collected from various habitats and screened for potential yeast isolates for xylitol production. Among 124 tested isolates, Candida tropicalis K2 showed the highest potential for xylitol production as well as inhibitors tolerance (Furfural, 5-hydroxymethyl furfural and acetic acid) phenotypes. C. tropicalis K2 produced 90 g/L of xylitol in batch fermentation (100 g/L xylose supplemented with 20 g/L of glycerol as co-substrate) with the yield and productivity of 0.90 g/g and 1.5 g/L.h, respectively, at pH 5.5 and 30°C temperature. Together, >10% higher xylitol yield was achieved when glycerol was used as a co-substrate with pure xylose. Moreover, with non-detoxified corncob and Albizia pod hydrolysates, C. tropicalis K2 isolate produced 0.62 and 0.69 g/g of xylitol yields and 1.04 and 0.75 g/L.h xylitol productivities, respectively. Thus, C. tropicalis K2 isolate could be considered as promising candidate for xylitol production from different lignocellulosic biomass.HIGHLIGHTS Candia tropicalis K2 isolate was screened from natural sites of biomass degradation and characterized for xylitol production.Non-detoxified Albizia pod and corncob hydrolysates were explored for xylitol production using selected C. tropicalis K2 isolate.A maximum of 0.90 g/g yield and 1.07 g/L.h xylitol productivity was achieved with pure xylose.A >10% increase in xylitol yield was achieved using glycerol as a co-substrate.

Exploring pineapple peel hydrolysate as a sustainable carbon source for xylitol production

This study explores utilizing pineapple peel (PP) hydrolysate as a promising carbon source for xylitol production, covering scopes from the pre-treatment to the fermentation process. The highest xylose concentration achieved was around 20 g/L via mild acid hydrolysis (5% nitric acid, 105 °C, 20-min residence time) with a solid loading of 10%. Two sets fermentability experiments were carried out of varying pH levels in synthetic media that includes acetic acid as the main inhibitors and hydrolysate supplemented with diverse nitrogen source. The results revealed that pH 7 exhibited the highest xylitol production, yielding 0.35 g/g. Furthermore, urea was found to be a highly promising and cost-effective substitute for yeast extract, as it yielded a comparable xylitol production of 0.31 g/g with marginal difference of only 0.01 g/g compared to yeast extract further highlights the viability of urea as the preferred option for reducing xylitol production cost. The absence of a significant difference between the synthetic media and hydrolysate, with only a marginal variance of 0.35 to 0.32 g/g, implies that acetic acid is indeed the primary constraint in xylitol production using PP hydrolysate. The study sheds light on PP biomass's potential for xylitol production, aligning economic benefits with environmental sustainability and waste management.

Development of engineered Candida tropicalis strain for efficient corncob-based xylitol-ethanol biorefinery

BACKGROUND

Xylitol has a wide range of applications in the pharmaceuticals, cosmetic, food and beverage industry. Microbial xylitol production reduces the risk of contamination and is considered as environment friendly and sustainable compared to the chemical method. In this study, random mutagenesis and genetic engineering approaches were employed to develop Candida tropicalis strains with reduced xylitol dehydrogenase (XDH) activity to eliminate co-substrate requirement for corn cob-based xylitol-ethanol biorefinery.

RESULTS

The results suggest that when pure xylose (10% w/v) was fermented in bioreactor, the Ethyl methane sulfonate (EMS) mutated strain (C. tropicalis K2M) showed 9.2% and XYL2 heterozygous (XYL2/xyl2Δ::FRT) strain (C. tropicalis K21D) showed 16% improvement in xylitol production compared to parental strain (C. tropicalis K2). Furthermore, 1.5-fold improvement (88.62 g/L to 132 g/L) in xylitol production was achieved by C. tropicalis K21D after Response Surface Methodology (RSM) and one factor at a time (OFAT) applied for media component optimization. Finally, corncob hydrolysate was tested for xylitol production in biorefinery mode, which leads to the production of 32.6 g/L xylitol from hemicellulosic fraction, 32.0 g/L ethanol from cellulosic fraction and 13.0 g/L animal feed.

CONCLUSIONS

This work, for the first time, illustrates the potential of C. tropicalis K21D as a microbial cell factory for efficient production of xylitol and ethanol via an integrated biorefinery framework by utilising lignocellulosic biomass with minimum waste generation.

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.

Strategy to reduce acetic acid in sugarcane bagasse hemicellulose hydrolysate concomitantly with xylitol production by the promising yeast Cyberlindnera xylosilytica in a bioreactor

The yeast Cyberlindnera xylosilytica UFMG-CM-Y309 has been identified as a promising new xylitol producer from sugarcane bagasse hemicellulosic hydrolysate (SCHH). However, SCHH pretreatment process generates byproducts, which are toxic to cell metabolism, including furans, phenolic compounds, and carboxylic acids, such as acetic acid, typically released at high concentrations. This research aims to reduce acetic acid in sugarcane hemicellulose hydrolysate concomitantly with xylitol production by yeast strain Cy. xylosilytica UFMG-CM-Y309 in a bioreactor by strategically evaluating the influence of volumetric oxygen transfer coefficient (k_La) (21 and 35 h^-1). Experiments were conducted on a bench bioreactor (2 L volumetric capacity) at different initial k_La values (21 and 35 h^-1). SCHH medium was supplemented with rice bran extract (10 g L^-1) and yeast extract (1 g L^-1). Cy. xylosilytica showed high xylitol production performance (19.56 g L^-1), xylitol yield (0.56 g g^-1) and, maximum xylitol-specific production rate (μp_máx 0.20 g_xylitol·g^-1 h^-1) at k_La value of 21 h^-1, concomitantly slowing the rate of acetic acid consumption. A faster acetic acid consumption (100%) by Cy. xylosilytica was observed at k_La of 35 h^-1, concomitantly with an increase in maximum cellular growth (14.60 g L^-1) and reduction in maximum xylitol production (14.56 g L^-1 and Y_p/s 0.34 g g^-1). This study contributes to pioneering research regarding this yeast performance in bioreactors, emphasizing culture medium detoxification and xylitol production.

Lignocellulosic xylitol production from corncob using engineered Kluyveromycesmarxianus

Xylitol production from lignocellulose hydrolysate is a sustainable and environment-friendly process. In this study, a systematic process of converting corncob waste into xylitol is described. First, the corncobs are hydrolyzed with acid to a hydrolysate. Second, Kluyveromyces marxianus YZJQ016 derived from K. marxianus YZJ074, constructed by overexpressing ScGAL2-N376F from Saccharomyces cerevisiae, CtXYL1 from Candida tropicalis, and KmZWF1 from K. marxianus, produces xylitol from the hydrolysate. A total of ten xylose reductase genes were evaluated, and CtXYL1 proved best by showing the highest catalytic activity under the control of the KmGAPDH promoter. A 5 L fermenter at 42°C produced 105.22 g/L xylitol using K. marxianus YZJQ016—the highest production reported to date from corncob hydrolysate. Finally, for crystallization of the xylitol, the best conditions were 50% (v/v) methanol as an antisolvent, at 25°C, with purity and yield of 99%–100% and 74%, respectively—the highest yield reported to date.

Use of carbohydrate-directed enzymes for the potential exploitation of sugarcane bagasse to obtain value-added biotechnological products

Microorganisms, such as fungi and bacteria, are crucial players in the production of enzymatic cocktails for biomass hydrolysis or the bioconversion of plant biomass into products with industrial relevance. The biotechnology industry can exploit lignocellulosic biomass for the production of high-value chemicals. The generation of biotechnological products from lignocellulosic feedstock presents several bottlenecks, including low efficiency of enzymatic hydrolysis, high cost of enzymes, and limitations on microbe metabolic performance. Genetic engineering offers a route for developing improved microbial strains for biotechnological applications in high-value product biosynthesis. Sugarcane bagasse, for example, is an agro-industrial waste that is abundantly produced in sugar and first-generation processing plants. Here, we review the potential conversion of its feedstock into relevant industrial products via microbial production and discuss the advances that have been made in improving strains for biotechnological applications.

Extraction, Isolation, and Purification of Value-Added Chemicals from Lignocellulosic Biomass

This review covers the operating conditions for extracting top value-added chemicals, such as levulinic acid, lactic acid, succinic acid, vanillic acid, 3-hydroxypropionic acid, xylitol, 2,5-furandicarboxylic acid, 5-hydroxymethyl furfural, chitosan, 2,3-butanediol, and xylo-oligosaccharides, from common lignocellulosic biomass. Operating principles of novel extraction methods, beyond pretreatments, such as Soxhlet extraction, ultrasound-assisted extraction, and enzymatic extraction, are also presented and reviewed. Post extraction, high-value biochemicals need to be isolated, which is achieved through a combination of one or more isolation and purification steps. The operating principles, as well as a review of isolation methods, such as membrane filtration and liquid–liquid extraction and purification using preparative chromatography, are also discussed.

Integrated production of ethanol and xylitol from Brassica juncea using Candida sojae JCM 1644

The present study demonstrates a novel strategy involving two-step fermentation of lignocellulosic hydrolysate for the integrated production of ethanol and xylitol using a newly isolated yeast strain, Candida sojae JCM 1644. The isolated strain was characterised by its carbohydrate assimilation efficiency and tolerance towards inhibitors generated during pretreatment and fermentation of lignocellulosic biomass. In brief, the study comprised alkali treatment of Brassica juncea followed by its saccharification with cellulase consortia. An isolated strain was used for the co-production of xylitol and ethanol from sugar hydrolysate, and several parameters were systematically optimised for maximum co-production of ethanol and xylitol. Out of total glucose (149.72 g/L) and xylose (84.21 g/L) present in biomass hydrolysate, a product yield of 0.45 g/g (ethanol) and 0.62 g/g (xylitol) was achieved for a two-step fermentation process, which was 15.57% and 11.78% higher than the yield achieved for ethanol and xylitol, respectively, in a one-step fermentation process.

Advancement of biorefinery-derived platform chemicals from macroalgae: a perspective for bioethanol and lactic acid

The extensive growth of energy and plastic demand has raised concerns over the depletion of fossil fuels. Moreover, the environmental conundrums worldwide integrated with global warming and improper plastic waste management have led to the development of sustainable and environmentally friendly biofuel (bioethanol) and biopolymer (lactic acid, LA) derived from biomass for fossil fuels replacement and biodegradable plastic production, respectively. However, the high production cost of bioethanol and LA had limited its industrial-scale production. This paper has comprehensively reviewed the potential and development of third-generation feedstock for bioethanol and LA production, including significant technological barriers to be overcome for potential commercialization purposes. Then, an insight into the state-of-the-art hydrolysis and fermentation technologies using macroalgae as feedstock is also deliberated in detail. Lastly, the sustainability aspect and perspective of macroalgae biomass are evaluated economically and environmentally using a developed cascading system associated with techno-economic analysis and life cycle assessment, which represent the highlights of this review paper. Furthermore, this review provides a conceivable picture of macroalgae-based bioethanol and lactic acid biorefinery and future research directions that can be served as an important guideline for scientists, policymakers, and industrial players.

Graphical abstract

Conceptual Process Design of an Integrated Xylitol Biorefinery With Value-Added Co-Products

This manuscript describes the conceptual process design of an integrated xylitol biorefinery with value-added co-products. Based on an existing three-step framework, the main product of a second-generation integrated biorefinery is chosen in the first stage. Based upon this, other decisions as the feedstock and value-added co-products are made. All relevant unit operations for the process are introduced. An initial superstructure with all potential process alternatives is composed of all introduced models. In the second step of the framework, a global sensitivity analysis is performed, firstly with coarse sampling to determine all viable flowsheet options and secondly with fine sampling to determine the most sensitive operational variables. As a result of the sensitivity analysis, most of the flowsheet options in the initial superstructure are not feasible. Based on these results, flowsheet sampling with the five most sensitive operational variables is performed to create surrogate models. In the scope of this work, three types of surrogate models are benchmarked against each other. Regarding the results of the superstructure optimization, firstly, it becomes apparent that the production of biokerosene does not contribute significantly to the net present value of the biorefinery. Furthermore, reducing the number of unit operations in the downstream processing leads to lower capital expenditures, but it lowers the product yield. Lastly, most flowsheets are economically feasible, indicated by a positive net present value. Based on this result, the most promising candidate process topology is subjected to the third step of the framework, including uncertainty in capital expenditure and operational expenses according to their estimations and uncertainties in the product prices. As a result, the net present value of the flowsheet turns negative, indicating that the high uncertainties for the expenditure and the expenses do not allow for an economically feasible operation. Lastly, the analysis of conceptually designed process flowsheets based on Monte Carlo sampling shows failure rates, with the NPV falling below the break-even point, of around 60% probability or higher. Based on these results, an economically feasible construction and operation of a xylitol biorefinery seems unlikely. Further ways to improve the metrics are elucidated.

Development and Validation of HPLC-DAD Method with Pre-Column PMP Derivatization for Monomeric Profile Analysis of Polysaccharides from Agro-Industrial Wastes

The instrumental analysis of complex mixtures of sugars often requires derivatization to enhance the method’s selectivity and sensitivity. 1-Phenyl-3-methyl-5-pyrazolone (PMP) is a common sugar derivatization agent used in high-performance liquid chromatography (HPLC). Although many C18 column applications for PMP–sugar derivative analysis have been developed, their transferability is not straightforward due to variations in column chemistry and preparation technology. The aim of this study was to develop and validate an application for Zorbax Extend C18 columns for the analysis of 8 neutral and 2 acidic sugars commonly found in plant polysaccharides. The method was further compared to well-established alditol acetates and m-hydroxydiphenyl methods and employed for sugar profiling of selected agro-industrial wastes. The most influential separation factors were the mobile-phase pH and acetonitrile content, optimized at 8.0 and a 12–17% gradient, respectively. The method showed excellent linearity, repeatability and intermediate precision. High sensitivity was achieved, especially for neutral sugars, with an accuracy error range of 5–10% relative standard deviation. The sugar profiling results were highly correlated to the reference method for neutral sugars. The HPLC method was highly applicable for the evaluation of polysaccharides in selected wastes and showed advantages in terms of simplicity, accuracy in acidic sugar determination and suitability for their simultaneous analysis with neutral sugars.

Hydrothermal pretreatment for the production of oligosaccharides: A review

Oligosaccharides are low-molecular-weight carbohydrates with crucial physical, chemical, and physiological properties, which are increasingly important in the fields of food, pharmaceuticals, cosmetics, and biomedicine. Pretreating biomass in a cost-effective way is a significant challenge for oligosaccharides research. Hydrothermal pretreatment is a potentially eco-friendly technology to obtain oligosaccharides by deconstructing biomass. In this work, we compared the differences between hydrothermal pretreatment and the traditional pretreatment method. The fundamentals and classification of hydrothermal pretreatment, as well as the latest studies on hydrothermal preparation of oligosaccharides, were further reviewed and evaluated to provide a theoretical basis for the production and application of oligosaccharides. Some challenges and future trends to develop green and large-scale hydrothermal pretreatment were proposed for the production of oligosaccharides.

Valorisation of food agro-industrial by-products: From the past to the present and perspectives

Food agro-industrial by-products mainly include peels, seeds, stems, bagasse, kernels, and husk, derived during food processing. Due to their overproduction and the lack of sustainable management, such by-products have been conventionally rejected and wasted in landfills, being the principal strategy for their treatment, but nowadays, this strategy has been associated with several environmental, social and economic issues. Hence, we focused on the use of different consolidated biotechnological processes and methodologies as suitable strategies for food by-products management and valorisation, highlighting them as potential bioresources because they still gather high compositional and nutritional value, owing to their richness in functional and bioactive molecules with human health benefits. Food by-products could be utilised for the development of new food ingredients or products for human consumption, promoting their integral valorisation and reincorporation to the food supply chain within the circular bioeconomy concept, creating revenue streams, business and job opportunities. In this review, the main goal was to provide a general overview of the food agro-industrial by-products utilised throughout the years, improving global sustainability and human nutrition, emphasising the importance of biowaste valorisation as well as the methodologies employed for the recovery of value-added molecules.

Hydrothermal pretreatment of lignocellulosic biomass for hemicellulose recovery

The hemicellulosic fraction recovery is of interest for integrated processes in biorefineries, considering the possibility of high economic value products produced from their structural compounds of this polysaccharide. However, to perform an efficient recovery, it is necessary to use biomass fractionation techniques, and hydrothermal pretreatment is highlighted as a valuable technique in the hemicellulose recovery by applying high temperatures and pressure, causing dissolution of the structure. Considering the possibility of this pretreatment technique for current approaches to hemicellulose recovery, this article aimed to explore the relevance of hydrothermal pretreatment techniques (sub and supercritical water) as a strategy for recovering the hemicellulosic fraction from lignocellulosic biomass. Discussions about potential products to be generated, current market profile, and perspectives and challenges of applying the technique are also addressed.

Intelligent self-control of carbon metabolic flux in SecY-engineered Escherichia coli for xylitol biosynthesis from xylose-glucose mixtures

Xylitol is a salutary sugar substitute that has been widely used in the food, pharmaceutical, and chemical industries. Co-fermentation of xylose and glucose by metabolically engineered cell factories is a promising alternative to chemical hydrogenation of xylose for commercial production of xylitol. Here, we engineered a mutant of SecY protein-translocation channel (SecY [ΔP]) in xylitol-producing Escherichia coli JM109 (DE3) as a passageway for xylose uptake. It was found that SecY (ΔP) channel could rapidly transport xylose without being interfered by XylB-catalyzed synthesis of xylitol-phosphate, which is impossible for native XylFGH and XylE transporters. More importantly, with the coaction of SecY (ΔP) channel and carbon catabolite repression (CCR), the flux of xylose to the pentose phosphate (PP) pathway and the xylitol synthesis pathway in E. coli could be automatically controlled in response to glucose, thereby ensuring that the mutant cells were able to fully utilize sugars with high xylitol yields. The E. coli cell factory developed in this study has been proven to be applicable to a broad range of xylose-glucose mixtures, which is conducive to simplifying the mixed-sugar fermentation process for efficient and economical production of xylitol.

Soybean hulls as carbohydrate feedstock for medium to high-value biomolecule production in biorefineries: A review

Soybean is one of the major world crops, with an annual production of 359 million tons. Each ton of processed soybean generates 50-80 kg of soybean hulls (SHs), representing 5-8% of the whole seed. Due to environmental concerns and great economic potential, the search of SHs re-use solutions are deeply discussed. The lignocellulosic composition of SHs has attracted the attention of the scientific and productive sector. Recently, some studies have reported the use of SHs in the production of medium to high value-added molecules, with potential applications in food and feed, agriculture, bioenergy, and other segments. This review presents biotechnological approaches and processes for the management and exploitation of SHs, including pre-treatment methods and fermentation techniques, for the production of different biomolecules. Great potentialities and innovations were found concerning SH exploration and valorisation of the soybean chain under a biorefinery and circular bioeconomy optic.

Waste-to-nutrition: a review of current and emerging conversion pathways

Residual biomass is acknowledged as a key sustainable feedstock for the transition towards circular and low fossil carbon economies to supply whether energy, chemical, material and food products or services. The latter is receiving increasing attention, in particular in the perspective of decoupling nutrition from arable land demand. In order to provide a comprehensive overview of the technical possibilities to convert residual biomasses into edible ingredients, we reviewed over 950 scientific and industrial records documenting existing and emerging waste-to-nutrition pathways, involving over 150 different feedstocks here grouped under 10 umbrella categories: (i) wood-related residual biomass, (ii) primary crop residues, (iii) manure, (iv) food waste, (v) sludge and wastewater, (vi) green residual biomass, (vii) slaughterhouse by-products, (viii) agrifood co-products, (ix) C₁ gases and (x) others. The review includes a detailed description of these pathways, as well as the processes they involve. As a result, we proposed four generic building blocks to systematize waste-to-nutrition conversion sequence patterns, namely enhancement, cracking, extraction and bioconversion. We further introduce a multidimensional representation of the biomasses suitability as potential as nutritional sources according to (i) their content in anti-nutritional compounds, (ii) their degree of structural complexity and (iii) their concentration of macro- and micronutrients. Finally, we suggest that the different pathways can be grouped into eight large families of approaches: (i) insect biorefinery, (ii) green biorefinery, (iii) lignocellulosic biorefinery, (iv) non-soluble protein recovery, (v) gas-intermediate biorefinery, (vi) liquid substrate alternative, (vii) solid-substrate fermentation and (viii) more-out-of-slaughterhouse by-products. The proposed framework aims to support future research in waste recovery and valorization within food systems, along with stimulating reflections on the improvement of resources' cascading use.

Production of xylitol by Saccharomyces cerevisiae using waste xylose mother liquor and corncob residues

Exorbitant outputs of waste xylose mother liquor (WXML) and corncob residue from commercial-scale production of xylitol create environmental problems. To reduce the wastes, a Saccharomyces cerevisiae strain tolerant to WXML was conferred with abilities to express the genes of xylose reductase, a xylose-specific transporter and enzymes of the pentose phosphate pathway. This strain showed a high capacity to produce xylitol from xylose in WXML with glucose as a co-substrate. Additionally, a simultaneous saccharification and fermentation (SSF) process was designed to use corncob residues and cellulase instead of directly adding glucose as a co-substrate. Xylitol titer and the productivity were, respectively, 91.0 g l-1 and 1.26 ± 0.01 g l-1  h-1 using 20% WXML, 55 g DCW l-1 delignified corncob residues and 11.8 FPU gcellulose -1 cellulase at 35° during fermentation. This work demonstrates the promising strategy of SSF to exploit waste products to xylitol fermentation process.

Increasing NADPH Availability for Xylitol Production via Pentose-Phosphate-Pathway Gene Overexpression and Embden-Meyerhof-Parnas-Pathway Gene Deletion in Escherichia coli

Cofactor availability is often a rate-limiting factor in the bioconversion of xylose to xylitol. The overexpression of pentose phosphate pathway genes and the deletion of Embden-Meyerhof-Parnas pathway genes can modulate the glucose metabolic flux and increase the intracellular NADPH supply, enabling Escherichia coli cells to produce xylitol from corncob hydrolysates. The effects of zwf and/or gnd overexpression and pfkA, pfkB, and/or pgi deletion on the intracellular redox environment and xylitol production were examined. The NADPH-enhanced strain 2bpgi produced 162 g/L xylitol from corncob hydrolysates after a 76 h fed-batch fermentation in a 15 L bioreactor, which was 13.3% greater than the 143 g/L xylitol produced by the IS5-d control strain. Additionally, the xylitol productivity and xylitol yield per glucose for 2bpgi were 2.13 g/L/h and 2.50 g/g, respectively. Thus, the genetic modifications in 2bpgi significantly enhanced NADPH regeneration, making 2bpgi a potentially useful strain for the industrial-scale production of xylitol from detoxified corncob hydrolysates.

Stepwise metabolic engineering of Candida tropicalis for efficient xylitol production from xylose mother liquor

Background

Commercial xylose purification produces xylose mother liquor (XML) as a major byproduct, which has become an inexpensive and abundant carbon source. A portion of this XML has been used to produce low-value-added products such as caramel but the remainder often ends up as an organic pollutant. This has become an issue of industrial concern. In this study, a uracil-deficient Candida tropicalis strain was engineered to efficiently convert XML to the commercially useful product xylitol.

Results

The xylitol dehydrogenase gene was deleted to block the conversion of xylitol to xylulose. Then, an NADPH regeneration system was added through heterologous expression of the Yarrowia lipolytica genes encoding 6-phosphate-gluconic acid dehydrogenase and 6-phosphate-glucose dehydrogenase. After process optimization, the engineered strain, C. tropicalis XZX-B4ZG, produced 97.10 g L^− 1 xylitol in 120 h from 300 g L^− 1 XML in a 5-L fermenter. The xylitol production rate was 0.82 g L^− 1 h^− 1 and the conversion rate was 92.40 %.

Conclusions

In conclusion, this study performed a combination of metabolic engineering and process optimizing in C. tropicalis to enhance xylitol production from XML. The use of C. tropicalis XZX-B4ZG, therefore, provided a convenient method to transform the industrial by-product XML into the useful material xylitol.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01596-1.

Characterization of a new bifunctional endo-1,4-β-xylanase/esterase found in the rumen metagenome

Metagenomic data mining of the Nellore cattle rumen microbiota identified a new bifunctional enzyme, endo-1,4-β-xylanase/esterase, which was subsequently overexpressed in E. coli BL21 (DE3). This enzyme was stable at pH intervals of 5 to 6.5 and temperatures between 30 and 45 °C, and under the test conditions, it had a V_max of 30.959 ± 2.334 µmol/min/mg, K_m of 3.6 ± 0.6 mM and k_cat of 2.323 ± 175 s^-1. Additionally, the results showed that the enzyme is tolerant to NaCl and organic solvents and therefore is suitable for industrial environments. Xylanases are widely applicable, and the synergistic activity of endo-1,4-β-xylanase/esterase in a single molecule will improve the degradation efficiency of heteroxylans via the creation of xylanase binding sites. Therefore, this new molecule has the potential for use in lignocellulosic biomass processing and as an animal feed food additive and could improve xylooligosaccharide production efficiency.

Improvement of enzymatic bioxylitol production from sawdust hemicellulose: optimization of parameters

Enzymatic production of bioxylitol from lignocellulosic biomass (LCB) provides a promising alternative to both chemical and fermentative routes. This study aimed to assess the impacts of catalytic variables on bioxylitol production from wood sawdust using xylose reductase (XR) enzyme and to optimize the bioprocess. Enzyme-based xylitol production was carried out in batch cultivation under various experimental conditions to obtain maximum xylitol yield and productivity. The response surface methodology (RSM) was followed to fine-tune the most significant variables such as reaction time, temperature, and pH, which influence the synthesis of bioxylitol from sawdust hydrolysate and to optimize them. The optimum time, temperature, and pH became were 12.25 h, 35 °C, and 6.5, respectively, with initial xylose 18.8 g/L, NADPH 2.83 g/L, XR 0.027 U/mg, and agitation 100 rpm. The maximum xylitol production was attained at 16.28 g/L with a yield and productivity of 86.6% (w/w) and 1.33 g/L·h, respectively. Optimization of catalytic parameters using sequential strategies resulted in 1.55-fold improvement in overall xylitol production. This study explores a novel strategy for using sawdust hemicellulose in bioxylitol production by enzyme technology.

Optimization of activated charcoal detoxification and concentration of chestnut shell hydrolysate for xylitol production

OBJECTIVES

To increase xylose concentration of the chestnut shell hemicellulosic hydrolysate with an acceptable phenolic compound level in order to enhance xylitol production by Candida tropicalis M43.

RESULTS

The xylose concentration and total phenolic compound concentration of the hydrolysate were obtained as 33.68 g/L and 77.38 mg gallic acid equivalent/L, respectively by optimization of detoxification parameters and concentration level (60 °C, 115 min contact time, 5.942% (w/v) dosage of activated charcoal, 120 strokes/min shaking rate and 0.2 volume ratio). Xylitol production was achieved in the hydrolysate by using Candida tropicalis M43. The maximum xylitol concentration was 6.30 g/L and productivity, yield and percentage of substrate conversion were calculated as 0.11 g/L h, 19.13% and 97.79%, respectively. In addition, the chestnut shell hydrolysate fortified with xylose and the maximum xylitol concentration increased to 18.08 g/L in the hydrolysate-based medium containing 80 g/L xylose.

CONCLUSIONS

Optimizing detoxification conditions with concentration level was found to be useful for enhancing xylitol production. In addition, fortification of the hydrolysate caused a three fold increase in maximum xylitol concentration.

Kinetics Study of the Hydrodeoxygenation of Xylitol over a ReOx-Pd/CeO2 Catalyst

In this study, we elucidate the reaction kinetics for the simultaneous hydrodeoxygenation of xylitol to 1,2-dideoxypentitol and 1,2,5-pentanetriol over a ReOx-Pd/CeO2 (2.0 weight% Re, 0.30 weight% Pd) catalyst. The reaction was determined to be a zero-order reaction with respect to xylitol. The activation energy was elucidated through an Arrhenius relationship as well as non-Arrhenius kinetics. The Arrhenius relationship was investigated at 150–170 °C and a constant H2 pressure of 10 bar resulting in an activation energy of 48.7 ± 10.5 kJ/mol. The investigation of non-Arrhenius kinetics was conducted at 120–170 °C and a sub-Arrhenius relation was elucidated with activation energy being dependent on temperature, and ranging from 10.2–51.8 kJ/mol in the temperature range investigated. Internal and external mass transfer were investigated through evaluating the Weisz–Prater criterion and the effect of varying stirring rate on the reaction rate, respectively. There were no internal or external mass transfer limitations present in the reaction.

Process scale-up of an efficient acid-catalyzed steam pretreatment of rice straw for xylitol production by C. Tropicalis MTCC 6192

The present study focuses on operational parameters for the efficient acid catalyzed rice straw pretreatment process for xylitol production. 75.77 % xylose yield was attained when the 24 h presoaked rice straw (≤10 mm or ≤ 15 mm) in 1.5 % (v/v) H₂SO₄ was pretreated in the same reactor at 121 °C for 30 min. Neutralization with barium hydroxide produced insoluble salt and noticeably reduced HMF and furfurals. Xylitol yield of 0.6 g/g of xylose, was achieved by fermenting rice straw hydrolysate medium with C. tropicalis MTCC 6192. This two-step process of production of xylitol from xylose rich hydrolysate is much simpler and produced minimal inhibitors including organic acids such as acetic acid. This process is modified for upscaling at optimized parameters and will simultaneously minimize the pollution problem caused by rice straw and is also promising for commercial scale.

Towards the Green Synthesis of Furfuryl Alcohol in A One-Pot System from Xylose: A Review

In the pursuit of establishing a sustainable biobased economy, valorization of lignocellulosic biomass is increasing its value as a feedstock. Nevertheless, to achieve the integrated biorefinery paradigm, the selective fractionation of its complex matrix to its single constituents must be complete. This review presents and examines the novel catalytic pathways to form furfuryl alcohol (FuOH) from xylose in a one-pot system. This production concept takes on chemical, thermochemical and biochemical transformations or a combination of them. Still, the bulk of the research is targeted to develop heterogeneous catalytic systems to synthesize FuOH from furfural and xylose. The present review includes an overview of the economic aspects to produce this platform chemical in an industrial manner. In the last section of this review, an outlook and summary of catalytic processes to produce FuOH are highlighted.

Hydrolysis of Corncob Hemicellulose by Solid Acid Sulfated Zirconia and Its Evaluation in Xylitol Production

Corncob is an abundant agricultural residue containing high content of hemicellulose. In this paper, the hemicellulosic hydrolysate was prepared from the hydrolysis of corncob using the solid acid sulfated zirconia as a catalyst. According to response surface analysis experiments, the optimum conditions for preparing hemicellulosic hydrolysate catalyzed by sulfated zirconia were determined as follows: solid (sulfated zirconia)-solid (corncob) ratio was 0.33, solid (corncob)-liquid (water) ratio was 0.09, temperature was 153 °C, and time was 5.3 h. Under the optimized conditions, the soluble sugar concentration was 30.12 g/L with a yield of 033 g/g corncob. Subsequently, xylitol production from the resulting hemicellulosic hydrolysate was demonstrated by Candida tropicalis, and results showed that the yield of xylitol from the hemicellulosic hydrolysate could be significantly improved on a basis of decolorization and detoxification before fermentation. The maximum yield of xylitol from the hemicellulosic hydrolysate fermented by C. tropicalis was 0.76 g/g. This study provides a new attempt for xylitol production from the hemicellulosic hydrolysate.

Production and characterization of functional wheat bran hydrolysate rich in reducing sugars, xylooligosaccharides and phenolic acids

The aim was to enhance production of functional hydrolysate from wheat bran (WB). WB was hydrolyzed with 3000 U/mL ɑ-amylase and 1200 U/mL alkaline protease to prepare WB insoluble dietary fibre (WBIDF). Functional hydrolysate production from the extract containing crude xylan of WBIDF by xylanase was optimized by Taguchi method. The optimal condition for xylan degradation and functional substances production was 78.50 U/mL xylanase, pH 10.0, 50 °C, and reaction time 6 h. The maximum yield of reducing sugars was 614.0 μg/mL, xylobiose increased from 12.9 μg/mL to 213.3 μg/mL, xylotriose increased from 34.9 μg/mL to 174.0 μg/mL, ferulic acid 13.1 μg/mL made up 57.5 % of the total identifiable phenolic pool in the hydrolysate. The total antioxidant activity of hydrolysate was 141.8 mg ascorbic acid equivalents g^-1 crude xylan, and the highest 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity reached 92.7 %. The hydrolysate exhibited great potential in agricultural and food industry application.

An improvement in fermentability of acid-hydrolysed hemicellulose from kenaf stem for xylitol production

In recent years, there has been a growing interest in the use of agricultural biomass for fermentation purposes; however, efficient strategies to counter lignocellulose inhibition are warranted to enhance xylitol production performance. Dilute-acid hydrolysis has been studied to selectively release a significant portion of xylose from hemicellulose, while leaving cellulose and lignin intact. The formation of inhibitory compounds, however, could jeopardise the overall performance during fermentation to produce xylitol. In this study, the fermentability of nitric acid-hydrolysed kenaf stem was substantially improved, through either adaptive evolution of the recombinant Escherichia coli BL21 (DE3) or removal of fermentation inhibitors by detoxification with activated carbon. Both methods were compared to evaluate the superiority in fermentative performance. In the fermentation with detoxified hemicellulosic hydrolysate, the non-adapted strain produced the highest xylitol concentration of up to 6.8 g/L, with 61.5% xylose consumption. The yields of xylitol production involving detoxification were successfully enhanced by 22.6% and by 35.7% compared to those involving adaptive evolution and raw hydrolysate, respectively. The results reported herein suggest that the utilization of detoxified kenaf stem hydrolysate could be advantageous.

Identification of genes involved in xylose metabolism of Meyerozyma guilliermondii and their genetic engineering for increased xylitol production

Meyerozyma guilliermondii, a non-conventional yeast that naturally assimilates xylose, is considered as a candidate for biotechnological production of the sugar alternative xylitol. Because the genes of the xylose metabolism were yet unknown, all efforts published so far to increase the xylitol yield of this yeast are limited to fermentation optimization. Hence, this study aimed to genetically engineer this organism for the first time with the objective to increase xylitol production. Therefore, the previously uncharacterized genes of M. guilliermondii ATCC 6260 encoding for xylose reductase (XR) and xylitol dehydrogenase (XDH) were identified by pathway investigations and sequence similarity analysis. Cloning and overexpression of the putative XR as well as knockout of the putative XDH genes generated strains with about threefold increased xylitol yield. Strains that combined both genetic modifications displayed fivefold increase in overall xylitol yield. Enzymatic activity assays with lysates of XR overexpressing and XDH knockout strains underlined the presumed functions of the respective genes. Furthermore, growth evaluation of the engineered strains on xylose as sole carbon source provides insights into xylose metabolism and its utilization for cell growth.