Development of a yeast strain for xylitol production without hydrolysate detoxification as part of the integration of co-product generation within the lignocellulosic ethanol process

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.

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.

Fungal biorefinery for sustainable resource recovery from waste

Depletion of natural resources and negative impact of fossil fuels on environment are becoming a global concern. The concept of biorefinery is one of the alternative platforms for the production of biofuels and chemicals. Valorisation of biological resources through complete utilization of waste, reusing secondary products and generating energy to power the process are the key principles of biorefinery. Agricultural residues and biogenic municipal solid wastes are getting importance as a potential feedstock for the generation of bioproducts. This communication reviews and highlights the scope of yeast and fungi as a potent candidate for the synthesis of gamut of bioproducts in an integrated approach addressing sustainability and circular bioeconomy. It also provides a close view on importance of microbes in biorefinery, feedstock pretreatment strategies for renewable sugar production, cultivation systems and yeast and fungi based products. Integrated closed loop approach towards multiple product generation with zero waste discharge is also discussed.

An overview of the metabolically engineered strains and innovative processes used for the value addition of biomass derived xylose to xylitol and xylonic acid

Xylose, the most abundant pentose sugar of the hemicellulosic fraction of lignocellulosic biomass, has to be utilized rationally for the commercial viability of biorefineries. An effective pre-treatment strategy for the release of xylose from the biomass and an appropriate microbe of the status of an Industrial strain for the utilization of this pentose sugar are key challenges which need special attention for the economic success of the biomass value addition to chemicals. Xylitol and xylonic acid, the alcohol and acid derivatives of xylose are highly demanded commodity chemicals globally with plenty of applications in the food and pharma industries. This review emphasis on the natural and metabolically engineered strains utilizing xylose and the progressive and innovative fermentation strategies for the production and subsequent recovery of the above said chemicals from pre-treated biomass medium.

Enhanced xylitol production using non-detoxified xylose rich pre-hydrolysate from sugarcane bagasse by newly isolated Pichia fermentans

BACKGROUND

Integrated management of hemicellulosic fraction and its economical transformation to value-added products is the key driver towards sustainable lignocellulosic biorefineries. In this aspect, microbial cell factories are harnessed for the sustainable production of commercially viable biochemicals by valorising C5 and C6 sugars generated from agro-industrial waste. However, in the terrestrial ecosystem, microbial systems can efficiently consume glucose. On the contrary, pentose sugars are less preferred carbon source as most of the microbes lack metabolic pathway for their utilization. The effective utilization of both pentose and hexose sugars is key for economical biorefinery.

RESULTS

Bioprospecting the food waste and selective enrichment on xylose-rich medium led to screening and isolation of yeast which was phylogenetically identified as Pichia fermentans. The newly isolated xylose assimilating yeast was explored for xylitol production. The wild type strain robustly grew on xylose and produced xylitol with > 40% conversion yield. Chemical mutagenesis of isolated yeast with ethyl methanesulphonate (EMS) yielded seven mutants. The mutant obtained after 15 min EMS exposure, exhibited best xylose bioconversion efficiency. This mutant under shake flask conditions produced maximum xylitol titer and yield of 34.0 g/L and 0.68 g/g, respectively. However, under the same conditions, the control wild type strain accumulated 27.0 g/L xylitol with a conversion yield of 0.45 g/g. Improved performance of the mutant was attributed to 34.6% activity enhancement in xylose reductase with simultaneous reduction of xylitol dehydrogenase activity by 22.9%. Later, the culture medium was optimized using statistical design and validated at shake flask and bioreactor level. Bioreactor studies affirmed the competence of the mutant for xylitol accumulation. The xylitol titer and yield obtained with pure xylose were 98.9 g/L and 0.67 g/g, respectively. In comparison, xylitol produced using non-detoxified xylose rich pre-hydrolysate from sugarcane bagasse was 79.0 g/L with an overall yield of 0.54 g/g.

CONCLUSION

This study demonstrates the potential of newly isolated P. fermentans in successfully valorising the hemicellulosic fraction for the sustainable xylitol production.

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.

Comparison of Different Lactobacilli Regarding Substrate Utilization and Their Tolerance Towards Lignocellulose Degradation Products

Fermentative lactic acid production is currently impeded by low pH tolerance of the production organisms, the successive substrate consumption of the strains and/or the requirement to apply purified substrate streams. We identified Lactobacillus brevis IGB 1.29 in compost, which is capable of producing lactic acid at low pH values from lignocellulose hydrolysates, simultaneously consuming glucose and xylose. In this study, we compared Lactobacillus brevis IGB 1.29 with the reference strains Lactobacillus brevis ATCC 367, Lactobacillus plantarum NCIMB 8826 and Lactococcus lactis JCM 7638 with regard to the consumption of C5- and C6-sugars. Simultaneous conversion of C5- and C6-monosaccharides was confirmed for L. brevis IGB 1.29 with consumption rates of 1.6 g/(L h) for glucose and 1.0 g/(L h) for xylose. Consumption rates were lower for L. brevis ATCC 367 with 0.6 g/(L h) for glucose and 0.2 g/(L h) for xylose. Further trials were carried out to determine the sensitivity towards common toxic degradation products in lignocellulose hydrolysates: acetate, hydroxymethylfurfural, furfural, formate, levulinic acid and phenolic compounds from hemicellulose fraction. L. lactis was the least tolerant strain towards the inhibitors, whereas L. brevis IGB 1.29 showed the highest tolerance. L. brevis IGB 1.29 exhibited only 10% growth reduction at concentrations of 26.0 g/L acetate, 1.2 g/L furfural, 5.0 g/L formate, 6.6 g/L hydroxymethylfurfural, 9.2 g/L levulinic acid or 2.2 g/L phenolic compounds. This study describes a new strain L. brevis IGB 1.29, that enables efficient lactic acid production with a lignocellulose-derived C5- and C6-sugar fraction.

Integrated wood biorefinery: Improvements and tailor-made two-step strategies on hydrolysis techniques

This study categorized different pretreatment methods into mild (below 120 °C), normal (120-200 °C) and extreme conditions (above 200 °C) for selective approach with efficient wood hydrolysis for direct market applications. The model two-step strategy of selective normal-hydrolysis: steam explosion (170 °C for 30 min) with concentrating normal-hydrolysis: organosolv at (160 °C for 20 min) on hard/softwood will delivery individual fractions of hemicellulose, lignin, and cellulose with recovery rate above 95%. The first step releases C5 sugars with a recovery rate of 80% followed by the second step for C6 sugars with 95% rate and direct use of reduced sugars into C5 and C6 value-added products. The categorized conditions will ease the selection of the pretreatment method for the wood type and model strategy will increase the hydrolysis rate with greater simplicity and validity. The integrated wood biorefinery with two-step treatment is an in-house and closed-loop with endless industrial applications.

Production of bioalcohols and antioxidant compounds by acid hydrolysis of lignocellulosic wastes and fermentation of hydrolysates with Hansenula polymorpha

The effect of the H₂SO₄ concentration in the hydrolysis of sunflower-stalk waste, at 95ºC and using a liquid/solid relation of 20, was studied. In a later stage, the hydrolysates were fermented at different temperatures with the aim of ethanol and xylitol production. A total conversion of the hemicellulose at the acid concentration of 0.5 mol/L was achieved; whereas an acid concentration of 2.5 mol/L was needed to reach the maximum value in the conversion of the cellulose fraction. The analysis of the hydrolysis kinetics has enabled to determine the apparent reaction order, which was 1.3. The hydrolysates from hydrolysis process with H₂SO₄ 0.5 mol/L, once detoxified, were fermented at pH 5.5, temperatures 30, 40, and 50ºC with the yeast Hansenula polymorpha (ATCC 34438), resulting in a sequential uptake of sugars. In relation to ethanol and xylitol yields, the best results were observed at 50°C ( Y E / s O  = 0.11 g/g;  Y X y / s O  = 0.12 g/g). Instantaneous xylitol yields were higher than in ethanol, at the three temperatures essayed. Different phenolic compounds were analyzed in the hydrolysates; hydroxytyrosol was the most abundant (3.79 mg/L). The recovery of these compounds entails the elimination of inhibitors in the fermentation process and the production of high value-added antioxidant products.

Release of glucose repression on xylose utilization in Kluyveromyces marxianus to enhance glucose-xylose co-utilization and xylitol production from corncob hydrolysate

BACKGROUND

Lignocellulosic biomass is one of the most abundant materials for biochemicals production. However, efficient co-utilization of glucose and xylose from the lignocellulosic biomass is a challenge due to the glucose repression in microorganisms. Kluyveromyces marxianus is a thermotolerant and efficient xylose-utilizing yeast. To realize the glucose-xylose co-utilization, analyzing the glucose repression of xylose utilization in K. marxianus is necessary. In addition, a glucose-xylose co-utilization platform strain will facilitate the construction of lignocellulosic biomass-utilizing strains.

RESULTS

Through gene disruption, hexokinase 1 (KmHXK1) and sucrose non-fermenting 1 (KmSNF1) were proved to be involved in the glucose repression of xylose utilization while disruption of the downstream genes of cyclic AMP-protein kinase A (cAMP-PKA) signaling pathway or sucrose non-fermenting 3 (SNF3) glucose-sensing pathway did not alleviate the repression. Furthermore, disruption of the gene of multicopy inhibitor of GAL gene expression (KmMIG1) alleviated the glucose repression on some nonglucose sugars (galactose, sucrose, and raffinose) but still kept glucose repression of xylose utilization. Real-time PCR analysis of the xylose utilization related genes transcription confirmed these results, and besides, revealed that xylitol dehydrogenase gene (KmXYL2) was the critical gene for xylose utilization and stringently regulated by glucose repression. Many other genes of candidate targets interacting with SNF1 were also evaluated by disruption, but none proved to be the key regulator in the pathway of the glucose repression on xylose utilization. Therefore, there may exist other signaling pathway(s) for glucose repression on xylose consumption. Based on these results, a thermotolerant xylose-glucose co-consumption platform strain of K. marxianus was constructed. Then, exogenous xylose reductase and xylose-specific transporter genes were overexpressed in the platform strain to obtain YHY013. The YHY013 could efficiently co-utilized the glucose and xylose from corncob hydrolysate or xylose mother liquor for xylitol production (> 100 g/L) even with inexpensive organic nitrogen sources.

CONCLUSIONS

The analysis of the glucose repression in K. marxianus laid the foundation for construction of the glucose-xylose co-utilizing platform strain. The efficient xylitol production strain further verified the potential of the platform strain in exploitation of lignocellulosic biomass.

Efficient Xylitol Production from Cornstalk Hydrolysate Using Engineered Escherichia coli Whole Cells

Economic transformation of lignocellulose hydrolysate into valued-added products is of particular importance for energy and environmental issues. In this study, xylose reductase and glucose dehydrogenase were cloned into plasmid pETDuet-1 and then simultaneously expressed in Escherichia coli BL21(DE3), which was used as whole-cell catalyst for the first time to convert xylose into xylitol coupled with gluconate production. When tested with reconstituted xylose and glucose solution, 0.1 g/mL cells could convert 1 M xylose and 1 M glucose completely and produced 145.81 g/L xylitol with a yield of 0.97 (g/g) and 184.85 g/L gluconic acid with a yield of 1.03 (g/g) in 24 h. Subsequently, the engineered cells were applied in real cornstalk hydrolysate, which generated 30.88 g/L xylitol and 50.89 g/L gluconic acid. The cells were used without penetration treatment, and CaCO₃ was used to effectively regulate the pH during the production, which further saved costs.

Production of xylitol and bio-detoxification of cocoa pod husk hemicellulose hydrolysate by Candida boidinii XM02G

The use of cocoa pod husk hemicellulose hydrolysate (CPHHH) was evaluated for the production of xylitol by Candida boidinii XM02G yeast isolated from soil of cocoa-growing areas and decaying bark, as an alternative means of reusing this type of waste. Xylitol was obtained in concentrations of 11.34 g.L^-1, corresponding to a yield (Y_p/s) of 0.52 g.g^-1 with a fermentation efficiency (ε) of 56.6%. The yeast was tolerant to inhibitor compounds present in CPHHH without detoxification in different concentration factors, and was able to tolerate phenolic compounds at approximately 6 g.L^-1. The yeast was also able to metabolize more than 99% (p/v) of furfural and hydroxymethylfurfural present in the non-detoxified CPHHH without extension of the cell-growth lag phase, showing the potential of this microorganism for the production of xylitol. The fermentation of cocoa pod husk hydrolysates appears to provide an alternative use which may reduce the impact generated by incorrect disposal of this waste.

Oxygen-radical pretreatment promotes cellulose degradation by cellulolytic enzymes

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Acidic and enzymatic saccharification of waste agricultural biomass for biotechnological production of xylitol

Background

The plant biomass and agro-industrial wastes show great potential for their use as attractive low cost substrates in biotechnological processes. Wheat straw and corn cob as hemicellulosic substrates were acid hydrolyzed and enzymatically saccharified for high xylose production. The hydrolysate was concentrated and fermented by using Saccharomyces cerevisiae and Kluyveromyces for production of xylitol.

Results

Acid hydrolysis of wheat straw and corn cob in combination with enzymatic hydrolysis showed great potential for production of free sugars from these substrates. Kluyveromyces produced maximum xylitol from acid treated wheat straw residues with enzymatic saccharification. The percentage xylitol yield was 89.807 g/L and volumetric productivity of 0.019 g/L/h. Kluyveromyces also produced maximum xylitol from corn cob acid hydrolyzed liquor with xylitol yield 87.716 g/L and volumetric productivity 0.018 g/L/h.

Conclusion

Plant and agro-industrial biomass can be used as a carbohydrate source for the production of xylitol and ethanol after microbial fermentation. This study revealed that wheat straw acid and enzyme hydrolyzed residue proved to be best raw material for production of xylitol with S. cerevisiae. The xylitol produced can be utilized in pharmaceuticals after purification on industrial scale as pharmaceutical purposes.

Ethanol production from dilute-acid steam exploded lignocellulosic feedstocks using an isolated multistress-tolerant Pichia kudriavzevii strain

Renewable and low‐cost lignocellulosic wastes have attractive applications in bioethanol production. The yeast Saccharomyces cerevisiae is the most widely used ethanol‐producing microbe; however, its fermentation temperature (30–35°C) is not optimum (40–50°C) for enzymatic hydrolysis in the simultaneous saccharification and fermentation (SSF) process. In this study, we successfully performed an SSF process at 42°C from a high solid loading of 20% (w/v) acid‐impregnated steam explosion (AISE)‐treated rice straw with low inhibitor concentrations (furfural 0.19 g l⁻¹ and acetic acid 0.95 g l⁻¹) using an isolate Pichia kudriavzevii SI, where the ethanol titre obtained (33.4 g_p l⁻¹) was nearly 39% greater than that produced by conventional S. cerevisiae BCRC20270 at 30°C (24.1 g_p l⁻¹). In addition, P. kudriavzevii SI exhibited a high conversion efficiency of > 91% from enzyme‐saccharified hydrolysates of AISE‐treated plywood chips and sugarcane bagasse, although high concentrations of furaldehydes, such as furfural 1.07–1.21 g l⁻¹, 5‐hydroxymethyl furfural 0.20−0.72 g l⁻¹ and acetic acid 4.80–7.65 g l⁻¹, were present. This is the first report of ethanol fermentation by P. kudriavzevii using various acid‐treated lignocellulosic feedstocks without detoxification or added nutrients. The multistress‐tolerant strain SI has greater potential than the conventional S. cerevisiae for use in the cellulosic ethanol industry.

Sustainable biobutanol production from pineapple waste by using Clostridium acetobutylicum B 527: Drying kinetics study

Present investigation explores the use of pineapple peel, a food industry waste, for acetone-butanol-ethanol (ABE) production using Clostridium acetobutylicum B 527. Proximate analysis of pineapple peel shows that it contains 35% cellulose, 19% hemicellulose, and 16% lignin on dry basis. Drying experiments on pineapple peel waste were carried out in the temperature range of 60-120°C and experimental drying data was modeled using moisture diffusion control model to study its effect on ABE production. The production of ABE was further accomplished via acid hydrolysis, detoxification, and fermentation process. Maximum total sugar release obtained by using acid hydrolysis was 97g/L with 95-97% and 10-50% removal of phenolics and acetic acid, respectively during detoxification process. The maximum ABE titer obtained was 5.23g/L with 55.6% substrate consumption when samples dried at 120°C were used as a substrate (after detoxification).

Enhanced xylitol production using immobilized Candida tropicalis with non-detoxified corn cob hemicellulosic hydrolysate

This study reports an industrially applicable non-sterile xylitol fermentation process to produce xylitol from a low-cost feedstock like corn cob hydrolysate as pentose source without any detoxification. Different immobilization matrices/mediums (alginate, polyvinyl alcohol, agarose gel, polyacrylamide, gelatin, and κ-carrageenan) were studied to immobilize Candida tropicalis NCIM 3123 cells for xylitol production. Amongst this calcium alginate, immobilized cells produced maximum amount of xylitol with titer of 11.1 g/L and yield of 0.34 g/g. Hence, the process for immobilization using calcium alginate beads was optimized using a statistical method with sodium alginate (20, 30 and 40 g/L), calcium chloride (10, 20 and 30 g/L) and number of freezing–thawing cycles (2, 3 and 4) as the parameters. Using optimized conditions (calcium chloride 10 g/L, sodium alginate 20 g/L and 4 number of freezing–thawing cycles) for immobilization, xylitol production increased significantly to 41.0 g/L (4 times the initial production) with corn cob hydrolysate as sole carbon source and urea as minimal nutrient source. Reuse of immobilized biomass showed sustained xylitol production even after five cycles.

Xylitol production from waste xylose mother liquor containing miscellaneous sugars and inhibitors: one-pot biotransformation by Candida tropicalis and recombinant Bacillus subtilis

Background

The process of industrial xylitol production is a massive source of organic pollutants, such as waste xylose mother liquor (WXML), a viscous reddish-brown liquid. Currently, WXML is difficult to reuse due to its miscellaneous low-cost sugars, high content of inhibitors and complex composition. WXML, as an organic pollutant of hemicellulosic hydrolysates, accumulates and has become an issue of industrial concern in China. Previous studies have focused only on the catalysis of xylose in the hydrolysates into xylitol using one strain, without considering the removal of other miscellaneous sugars, thus creating an obstacle to subsequent large-scale purification. In the present study, we aimed to develop a simple one-pot biotransformation to produce high-purity xylitol from WXML to improve its economic value.

Results

In the present study, we developed a procedure to produce xylitol from WXML, which combines detoxification, biotransformation and removal of by-product sugars (purification) in one bioreactor using two complementary strains, Candida tropicalis X828 and Bacillus subtilis Bs12. At the first stage of micro-aerobic biotransformation, the yeast cells were allowed to grow and metabolized glucose and the inhibitors furfural and hydroxymethyl furfural (HMF), and converted xylose into xylitol. At the second stage of aerobic biotransformation, B. subtilis Bs12 was activated and depleted the by-product sugars. The one-pot process was successfully scaled up from shake flasks to 5, 150 L and 30 m³ bioreactors. Approximately 95 g/L of pure xylitol could be obtained from the medium containing 400 g/L of WXML at a yield of 0.75 g/g xylose consumed, and the by-product sugars glucose, l-arabinose and galactose were depleted simultaneously.

Conclusions

Our results demonstrate that the one-pot procedure is a viable option for the industrial application of WXML to produce value-added chemicals. The integration of complementary strains in the biotransformation of hemicellulosic hydrolysates is efficient under optimized conditions. Moreover, our study of one-pot biotransformation also provides useful information on the combination of biotechnological processes for the biotransformation of other compounds.

Aerobic and sequential anaerobic fermentation to produce xylitol and ethanol using non-detoxified acid pretreated corncob

BACKGROUND

For economical bioethanol production from lignocellulosic materials, the major technical challenges to lower the production cost are as follows: (1) The microorganism should use efficiently all glucose and xylose in the lignocellulose hydrolysate. (2) The microorganism should have high tolerance to the inhibitors present in the lignocellulose hydrolysate. The aim of the present work was to combine inhibitor degradation, xylitol fermentation, and ethanol production using a single yeast strain.

RESULTS

A new process of integrated aerobic xylitol production and anaerobic ethanol fermentation using non-detoxified acid pretreated corncob by Candida tropicalis W103 was proposed. C. tropicalis W103 is able to degrade acetate, furfural, and 5-hydromethylfurfural and metabolite xylose to xylitol under aerobic conditions, and the aerobic fermentation residue was used as the substrate for ethanol production by anaerobic simultaneous saccharification and fermentation. With 20% substrate loading, furfural and 5-hydroxymethylfurfural were degraded totally after 60 h aerobic incubation. A maximal xylitol concentration of 17.1 g l(-1) was obtained with a yield of 0.32 g g(-1) xylose. Then under anaerobic conditions with the addition of cellulase, 25.3 g l(-1) ethanol was produced after 72 h anaerobic fermentation, corresponding to 82% of the theoretical yield.

CONCLUSIONS

Xylitol and ethanol were produced in Candida tropicalis W103 using dual-phase fermentations, which comprise a changing from aerobic conditions (inhibitor degradation and xylitol production) to anaerobic simultaneous saccharification and ethanol fermentation. This is the first report of integrated xylitol and ethanol production from non-detoxified acid pretreated corncob using a single microorganism.

Simultaneous saccharification and co-fermentation of un-detoxified rice hull hydrolysate by Saccharomyces cerevisiae ICV D254 and Spathaspora arborariae NRRL Y-48658 for the production of ethanol and xylitol

Co-fermentation and simultaneous saccharification of rice hull hydrolysate (RHH) were investigated for the production of ethanol and xylitol by Saccharomyces cerevisiae, Spathaspora arborariae, or the combination of both. In bioreactor cultures under oxygen limitation, S. cerevisiae was capable of metabolizing glucose from RHH, which contained small amounts of acetic acid, furfural, and hydroxymethylfurfural, achieving ethanol yields of 0.45 and concentrations of 10.5 g L(-1). In the co-culture of S. cerevisiae and S. arborariae pentoses and hexoses from RHH, were converted to ethanol and xylitol, with yields of 0.48 and 0.39, and concentrations of 11 g L(-1) and 3 g L(-1), respectively. The simultaneous saccharification and co-fermentation using both yeasts produced ethanol and xylitol to final concentrations of 14.5 g L(-1) and 3 g L(-1), respectively. Results showed good prospects to use co-cultures of S. cerevisiae and S. arborariae for the bioconversion of RHH into ethanol and xylitol without further detoxification.

Optimization of CDT-1 and XYL1 expression for balanced co-production of ethanol and xylitol from cellobiose and xylose by engineered Saccharomyces cerevisiae

Production of ethanol and xylitol from lignocellulosic hydrolysates is an alternative to the traditional production of ethanol in utilizing biomass. However, the conversion efficiency of xylose to xylitol is restricted by glucose repression, causing a low xylitol titer. To this end, we cloned genes CDT-1 (encoding a cellodextrin transporter) and gh1-1 (encoding an intracellular β-glucosidase) from Neurospora crassa and XYL1 (encoding a xylose reductase that converts xylose into xylitol) from Scheffersomyces stipitis into Saccharomyces cerevisiae, enabling simultaneous production of ethanol and xylitol from a mixture of cellobiose and xylose (main components of lignocellulosic hydrolysates). We further optimized the expression levels of CDT-1 and XYL1 by manipulating their promoters and copy-numbers, and constructed an engineered S. cerevisiae strain (carrying one copy of PGK1p-CDT1 and two copies of TDH3p-XYL1), which showed an 85.7% increase in xylitol production from the mixture of cellobiose and xylose than that from the mixture of glucose and xylose. Thus, we achieved a balanced co-fermentation of cellobiose (0.165 g/L/h) and xylose (0.162 g/L/h) at similar rates to co-produce ethanol (0.36 g/g) and xylitol (1.00 g/g).

Coupled production of single cell oil as biodiesel feedstock, xylitol and xylanase from sugarcane bagasse in a biorefinery concept using fungi from the tropical mangrove wetlands

This work evaluates sugarcane bagasse (SCB) conversion, in a biorefinery approach, to coproduce biodiesel and high value products using two novel mangrove fungi. On acid pre-treatment, sugarcane bagasse hydrolysate (SCBH) resulted in a xylitol yield of 0.51 g/g xylose consumed in 72 h by Williopsis saturnus. After SCB pretreatment, sugarcane bagasse residue (SCBR) was utilized using Aspergillus terreus for production of xylanase (12.74 U/ml) and cell biomass (9.8 g/L) which was extracted for single cell oil (SCO; 0.19 g/g) and transesterified to biodiesel. The FAME profile exhibited long chain SFAs and PUFAs with predicted biodiesel properties lying within the range specified by international standards. This biorefining approach of SCB utilization for co-production of xylitol, xylanase and SCO gains importance in terms of sustainability and eco-friendliness.

Evaluation of hardboard manufacturing process wastewater as a feedstream for ethanol production

Waste streams from the wood processing industry can serve as feedstream for ethanol production from biomass residues. Hardboard manufacturing process wastewater (HPW) was evaluated on the basis of monomeric sugar recovery and fermentability as a novel feedstream for ethanol production. Dilute acid hydrolysis, coupled with concentration of the wastewater resulted in a hydrolysate with 66 g/l total fermentable sugars. As xylose accounted for 53 % of the total sugars, native xylose-fermenting yeasts were evaluated for their ability to produce ethanol from the hydrolysate. The strains selected were, in decreasing order by ethanol yields from xylose (Y p/s, based on consumed sugars), Scheffersomyces stipitis ATCC 58785 (CBS 6054), Pachysolen tannophilus ATCC 60393, and Kluyveromyces marxianus ATCC 46537. The yeasts were compared on the basis of substrate utilization and ethanol yield during fermentations of the hydrolysate, measured using an HPLC. S. stipitis, P. tannophilus, and K. marxianus produced 0.34, 0.31, and 0.36 g/g, respectively. The yeasts were able to utilize between 58 and 75 % of the available substrate. S. stipitis outperformed the other yeast during the fermentation of the hydrolysate; consuming the highest concentration of available substrate and producing the highest ethanol concentration in 72 h. Due to its high sugar content and low inhibitor levels after hydrolysis, it was concluded that HPW is a suitable feedstream for ethanol production by S. stipitis.

Xylitol production from D-xylose and horticultural waste hemicellulosic hydrolysate by a new isolate of Candida athensensis SB18

This paper describes the production of xylitol from d-xylose and horticultural waste hemicellulosic hydrolysate by a new strain of Candida athensensis SB18. Strain SB18 completely consumed 250 and 300 g L(-1) D-xylose and successful converted it to xylitol in the respective yield of 0.83 and 0.87 g g(-1), resulting in 207.8 and 256.5 g L(-1) of xylitol, respectively. The respective volumetric productivity were 1.15 and 0.97 g L(-1) h(-1). Approximately 100.1 g L(-1) of xylitol was obtained from the bioconversion of detoxified horticultural waste hemicellulosic hydrolysate using strain SB18. The yield and productivity were 0.81 g g(-1) xylose and 0.98 g L(-1) h(-1), respectively. Strain C. athensensis SB18 was able to completely utilize glucose, mannose, xylose and partially arabinose. This work demonstrates that stain C. athensensis SB18 is a promising strain for high-titer and high-yield xylitol production and it has great potential in bioconversion of hemicellulosic hydrolysate.

Pretreatment of rice straw using an extrusion/extraction process at bench-scale for producing cellulosic ethanol

A combination of a twin-screw extrusion and an acid-catalyzed hot water extraction process performed at a bench-scale was used to prepare high monomeric xylose hydrolysate for cellulosic production. The influences of the screw speed (30-150 rpm), barrel temperature (80-160 °C) and corresponding specific mechanical energy of the extruder on the structural properties of the pretreated rice straw, sugar concentration and conversion were investigated. The optimal condition for the extrusion step was determined to be 40 rpm with 3% H2SO4 at 120 °C; the optimal condition for the extraction step was determined to be 130 °C for 20 min. After the pretreatment at the optimal condition, 83.7% of the xylan was converted to monomeric xylose, and the concentration reached levels of 53.7 g/L. Finally, after the subsequent enzymatic hydrolysis, an 80% yield of the total saccharification was obtained.

Oil production by oleaginous yeasts using the hydrolysate from pretreatment of wheat straw with dilute sulfuric acid

This paper explores the use of the hydrolysate from the dilute sulfuric acid pretreatment of wheat straw for microbial oil production. The resulting hydrolysate was composed of pentoses (24.3g/L) and hexoses (4.9 g/L), along with some other degradation products, such as acetic acid, furfural, and hydroxymethylfurfural (HMF). Five oleaginous yeast strains, Cryptococcus curvatus, Rhodotorula glutinis, Rhodosporidium toruloides, Lipomyces starkeyi, and Yarrowia lipolytica, were evaluated by using this hydrolysate as substrates. The results showed that all of these strains could use the detoxified hydrolysate to produce lipids while except R. toruloides non-detoxified hydrolysate could also be used for the growth of all of the selective yeast strains. C. curvatus showed the highest lipid concentrations in medium on both the detoxified (4.2g/L) and non-detoxified (5.8 g/L) hydrolysates. And the inhibitory effect studies on C. curvatus indicated HMF had insignificant impacts at a concentration of up to 3g/L while furfural inhibited cell growth and lipid content by 72.0% and 62.0% at 1g/L, respectively. Our work demonstrates that lipid production is a promising alternative to utilize hemicellulosic sugars obtained during pretreatment of lignocellulosic materials.