Xylitol Production Research Articles

Techno-economic and environmental impact assessment of an olive tree pruning waste multiproduct biorefinery

This work focuses on the economic performance and environmental assessment of two biorefinery schemes using 100 t/day of olive tree pruning waste as feedstock. The first scheme is based on a multiproduct biorefinery producing 6363.64 m3/yr of ethanol, 999.31 t/yr of xylitol, and 1221.51 t/yr of antioxidant, obtaining a positive investment balance with a net present value (NPV) of 11.56 M€ in a 20-year period with an initial investment of 39.5 M€ (antioxidants account for 60.2% of revenues). In addition, it shows lower environmental impacts: 0.435 kg CO2 eq in global warning, 0.007 kg 1,4-DCB in human carcinogenic toxicity, 0.148 kg oileq in fossil resource scarcity for 1 kg OTP consumed, in comparison to the business-as-usual production of ethanol, xylitol and antioxidants (7.96 kg CO2 eq, 0.444 kg 1,4-DCB and 2.228 kg oileq). The second scheme considered is the standalone production of antioxidants, showing a similar economic performance (NPV = 11.9 M€) to the previous multiproduct biorefinery with lower initial investment requirements (24.9 M€). This process configuration also reduces some important environmental impacts such as global warming impact (0.55 kg CO2 eq) and freshwater consumption (0.067 m3), with respect to the multiproduct scheme. Thus, it has been demonstrated that antioxidants production from OTP is a promising alternative in terms of both process economics and environmental performance.

Comparative techno-economic assessment of sugarcane biorefineries producing glutamic acid, levulinic acid and xylitol from sugarcane

In this study, six alternative biorefinery scenarios for the production of levulinic acid (LA), glutamic acid (GA) and/or xylitol- or a combination of these chemicals- annexed to an existing sugar mill in 1G-only (molasses feedstock) or integrated 1G2G (molasses and sugar cane lignocellulose feedstock) configurations were simulated in AspenPlus®. Then, mass and energy balances obtained from the simulations were used for the subsequent equipment sizing and costing and to perform the economic performance assessments of the scenarios. All the investigated scenarios were identified as profitable scenarios with the internal rates of return (IRRs) ranging from 21.3%−59.8%. Generally, GA scenarios showed better economic outcomes compared to LA counterparts, attributed to the factors such as higher market prices, higher product throughput and less complicated processing technology which lowers the cost of production. Considering the market size and economic outcome, the 1G GA scenario producing 11.1 t/h GA showed the best economic performance with 59.8% IRR and a minimum product selling price (MSP) of $1687/t. However, a multiproduct facility (1G2G GA-xylitol) can result in more robust economic performance, as a result of implementing xylitol mature technology. Although all investigated scenarios showed profitability potential, life cycle analysis (LCA), social impact assessment and market analysis are required to find out the most sustainable biorefinery option.

Dental Practitioners’ Knowledge, Attitude, and Practice in Caries Risk Assessment and Management: A Cross-sectional Survey in Kampala Metropolitan, Uganda

The objective of this study was to determine the knowledge, attitude, and practices regarding caries risk assessment (CRA) and management among dental practitioners in Kampala Metropolitan, Uganda. This cross-sectional study was conducted among 270 dental practitioners in Kampala Metropolitan, Uganda, in May 2021. The participants were dental surgeons and public health dental officers. A self-administered structured questionnaire was used to collect data. The questionnaire included items about participants' sociodemographic characteristics, knowledge, attitude, and practices in CRA and management. Attitude and practices were rated using different Likert scales. Descriptive statistics, Chi-square/Fisher's exact and one-way analysis of variance (ANOVA) with post-hoc Bonferroni tests were used to analyze the data. The significance level was set at p <0.05. About 60.7% of the participants were public health dental officers with a median age of 30 years (interquartile range [IQR], 27-60). Overall, the participants were familiar with the current concepts regarding CRA and management. More than 70% of the participants correctly identified risk factors and indicators of dental caries. Most participants (98.5%) had a positive attitude toward performing CRA. However, their practices regarding caries management were inadequate as majority (>75%) of participants reported that they never or occasionally recommended evidence-based products like topical fluoride, probiotics, or xylitol products in the prevention and management of dental caries. Dental surgeons had significantly better knowledge and practices than public health dental officers (p <0.05). In the present study, the participants were familiar with the current concepts about CRA and had a positive attitude toward CRA. However, their practices regarding caries prevention and management were inadequate. The study provided baseline data about knowledge, attitude, and practices regarding CRA and caries management among dental practitioners in Uganda. It is recommended to design training courses in evidence-based protocols in the prevention and management of dental caries for dental practitioners in Uganda.

Production of Xylitol from Corn Biomass using Candida sp. As Microbial Agent

Xylitol, C5H12O5, is a white and odorless crystalline powder of sweetening agents that included as low-calorie sweetener. It could be used as a healthy ingredient for food and pharmaceutical. Natural sources of xylitol are fruit and vegetable, even in minute quantities. At industrial scale, xylitol is produced through hydrolysis and hydrogenation process of lignocellulosic materials. The aim of this research was to study the production of xylitol from corn biomass using Candida sp. as a microbial agent. The research was conducted using different species of Candida sp. (C. guilliermondii and C. tropicalis) and supplement media growth (with or without glucose). Xylitol concentration was examined after fermentation for 3 and 5 days. The results showed that corncob is a promising material to use in producing xylitol from lignocellulosic biomass. The longer fermentation time, the higher xylitol concentration, ranged from 0.049 to 0.088 g/L. However, compare to another microbe species, the treatment using Candida tropicalis showed that long fermentation resulted in a lower xylitol concentration. The addition of glucose as co-substrate increased xylose consumption rate and xylitol productivity. These results provide useful information to develop further study about xylitol production using agricultural biomass.

Microbial xylitol production from corn cob using Candida guilliermondii

Xylitol is a polyol that is widely used in the food industries as an alternative sweetener due to some health benefits. The microbial production of xylitol based on corn cobs was potential to be developed due to the abundant availability of corn cobs in Indonesia. In addition, it is an alternative process that is a higher yield and environmentally friendly. This study was conducted to assign the optimum process in the xylitol production based on corn cobs using Candida guilliermondii. The hydrolysis of corn cobs used sulfuric acid 1% at a temperature of 121°C for 60 minutes. The hydrolysate was then added with calcium hydroxide and activated charcoal to remove inhibitors. The design of experimental used was a factorial completely randomized design with three factors included corn cobs hydrolysate concentration (50% and 66%), glucose (0 g l-1 and 5 g l-1), and incubation period (120 h and 168 h). The result showed that concentration of hydrolysate, glucose, and incubation period had an effect on the xylitol produced. The highest xylitol was 144.09 ppm that was produced by the hydrolysate of 66%, glucose of 5 g l-1 and incubation period of 168 h, meanwhile the residual xylose was 2081.87 ppm.

Sugarcane bagasse valorization to xylitol: Techno‐economic and life cycle assessment

AbstractA detailed techno‐economic analysis and life cycle assessment (LCA) of a novel bio‐refinery that produces xylitol from sugarcane bagasse are provided. The proposed process includes dilute acid pretreatment in pressurized conditions followed by fermentation (upstream section). The fermentation broth is then sent for separation and purification to the downstream section. Calculations are performed for a plant with 4 t/h of dry bagasse throughput. With a fermentation yield of 0.54 g xylitol per g of xylose, the plant produced 437.4 kg/h of xylitol. Upstream data are adapted from experimental studies, while ASPEN PLUS® flowsheet simulation is used to obtain data for the downstream section. The xylitol production facility is assumed to be annexed to an existing sugar mill in India. The total utility requirement in the process is reduced using heat integration strategies. Cradle‐to‐gate scope is considered for the LCA and 1 kg of xylitol is taken as the functional unit. The product cost of xylitol is calculated to be 230 INR/kg (US$3.17/kg). For a 4 year payback period, the selling price of xylitol must be 450 INR/kg (US$6.2/kg). The fermentation and pretreatment sections are the major components of the product cost. The LCA results show that the life cycle greenhouse gas emissions are 2.759 kg CO2 eq. per kg xylitol. The electricity requirement within the plant is identified as the major source of greenhouse gas emissions, and reduction of fermentation duration is identified as a key factor. The results identify opportunities to improve the process from an economic as well as an environmental standpoint. © 2022 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Effect of exogenous additives and stress inducers on xylitol production by Debaryomyces nepalensis in batch culture

AbstractThe present study investigates various strategies to improve the production of xylitol, a platform chemical, by batch cultivation of Debaryomyces nepalensis. Enhancement of xylitol production via triggerring its biosynthetic pathway in yeast cells was carried out under varied initial phosphate concentrations of the medium and by the addition of stress inducers and co‐substrates. Among the tested phosphate concentrations, the xylitol production was found to be increased by 1.62‐fold at limited initial phosphate concentration (1.5 g L−1). Similarly, among all of the studied co‐substrates, the xylitol production was found to be enhanced by adding 10% maltose at the start of fermentation. Among the different stress inducers tested, addition of hydrogen peroxide and sodium hypochlorite at low concentration (10 ppm) resulted in improved xylitol yields by 28 and 34%, respectively, when compared with the control. De facto stress inducers and nutrient limitations may be worthy of consideration to enhance the production of xylitol in batch culture. We envision that the addition of stress inducers at minimum dosage will enhance xylitol productivity. © 2022 Society of Chemical Industry and John Wiley &amp; Sons, Ltd

Xylose and yeasts: A story beyond xylitol production

Six different yeasts were used to study their metabolism of glucose and xylose, and mainly their capacity to produce ethanol and xylitol. The strains used were Candida guilliermondii, Debaryomyces hansenii, Saccharomyces cerevisiae, Kluyveromyces marxianus, Meyerozyma guilliermondii and Clavispora lusitaniae, four isolated from a rural mezcal fermentation facility. All of them produced ethanol when the substrate was glucose. When incubated in a medium containing xylose instead of glucose, only K. marxianus and M. guilliermondii were able to produce ethanol from xylose. On the other hand, all of them could produce some xylitol from xylose, but the most active in this regard were K. marxianus, M. guilliermondii, C. lusitaniae, and C. guilliermondii with the highest amount of xylitol produced. The capacity of all strains to take up glucose and xylose was also studied. Xylose, in different degrees, produced a redox imbalance in all yeasts. Respiration capacity was also studied with glucose or xylose, where C. guilliermondii, D. hansenii, K. marxianus and M. guilliermondii showed higher cyanide resistant respiration when grown in xylose. Neither xylose transport nor xylitol production were enhanced by an acidic environment (pH 4), which can be interpreted as the absence of a proton/sugar symporter mechanism for xylose transport, except for C. lusitaniae. The effects produced by xylose and their magnitude depend on the background of the studied yeast and the conditions in which these are studied.

METABOLIC PATHWAY MODIFICATION FOR PRODUCTION OF XYLITOL FROM GLUCOSE IN ESCHERICHIA COLI

Glucose is a cheap and readily available substrate for production of large-scale chemicals. Synthesis of xylitol, a high demand chemical in global market is currently done by using xylose, which contributes to its high operational cost. Studies on production of xylitol from glucose have explored several approaches, from sequential fermentation to multiple and single gene expression. Xylitol-5-phosphate dehydrogenase (XPDH), is an enzyme that enables conversion of glucose to xylitol in a single step fermentation. This study explores conversion of xylitol from glucose in E. coli by the expression of xpdh from Clostridium difficile with modifications in metabolic pathways to enhance xylitol production. The xpdh gene was carried by pACYC-Duet-1 expression vector and induced by the addition of IPTG. Initial screening of E. coli expressing xpdh (NA116) was done by shake-flask fermentation for 24 hours and its metabolites were analyzed by HPLC. NA116 was able to produce 0.273 g/L xylitol from 4.33 g/L consumed glucose in 24 hours. Further metabolic pathway modification to eliminate competing pathways yielded four mutants, NA207 (∆rpiA), NA208 (∆rpiB), NA209 (∆pgi) and NA211 (∆rpi∆Apgi). Screening of mutants for xylitol production showed that highest xylitol production from glucose was achieved by NA211 with almost double the amount of the original strain, 0.585 g/L. This showed successful xylitol conversion from glucose in a single fermentation in E. coli with improved yield through metabolic pathway modification.

Xylitol production by Pseudomonas gessardii VXlt-16 from sugarcane bagasse hydrolysate and cost analysis.

Xylitol is a well-known sugar alcohol with exponentially rising market demand due to its diverse industrial applications. Organic agro-industrial residues (OAIR) are economic alternative for the cost-effective production of commodity products along with addressing environmental pollution. The present study aimed to design a process for xylitol production from OAIR via microbial fermentation with Pseudomonas gessardii VXlt-16. Parametric analysis with Taguchi orthogonal array approach resulted in a conversion factor of 0.64g xylitol/g xylose available in untreated sugarcane bagasse hydrolysate (SBH). At bench scale, the product yield increased to 71.98/100g (0.66g/Lh). 48.49g of xylitol crystals of high purity (94.56%) were recovered after detoxification with 2% activated carbon. Cost analysis identified downstream operations as one of the cost-intensive parts that can be countered by adsorbent recycling. Spent carbon, regenerated with acetic acid washing can be reused for six cycles effectively and reduced downstream cost by about ≈32%. The strategy would become useful in the cost-effective production of several biomass-dependent products like proteins, enzymes, organic acids, as well.

Activation of cryptic xylose metabolism by a transcriptional activator Znf1 boosts up xylitol production in the engineered Saccharomyces cerevisiae lacking xylose suppressor BUD21 gene

BackgroundXylitol is a valuable pentose sugar alcohol, used in the food and pharmaceutical industries. Biotechnological xylitol production is currently attractive due to possible conversion from abundant and low-cost industrial wastes or agricultural lignocellulosic biomass. In this study, the transcription factor Znf1 was characterised as being responsible for the activation of cryptic xylose metabolism in a poor xylose-assimilating S. cerevisiae for xylitol production.ResultsThe results suggest that the expression of several xylose-utilising enzyme genes, encoding xylose reductases for the reduction of xylose to xylitol was derepressed by xylose. Their expression and those of a pentose phosphate shunt and related pathways required for xylose utilisation were strongly activated by the transcription factor Znf1. Using an engineered S. cerevisiae strain overexpressing ZNF1 in the absence of the xylose suppressor bud21Δ, xylitol production was maximally by approximately 1200% to 12.14 g/L of xylitol, corresponding to 0.23 g/g xylose consumed, during 10% (w/v) xylose fermentation. Proteomic analysis supported the role of Znf1 and Bud21 in modulating levels of proteins associated with carbon metabolism, xylose utilisation, ribosomal protein synthesis, and others. Increased tolerance to lignocellulosic inhibitors and improved cell dry weight were also observed in this engineered bud21∆ + pLJ529-ZNF1 strain. A similar xylitol yield was achieved using fungus-pretreated rice straw hydrolysate as an eco-friendly and low-cost substrate.ConclusionsThus, we identified the key modulators of pentose sugar metabolism, namely the transcription factor Znf1 and the suppressor Bud21, for enhanced xylose utilisation, providing a potential application of a generally recognised as safe yeast in supporting the sugar industry and the sustainable lignocellulose-based bioeconomy.Graphical

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.

Process development for crystalline xylitol production from corncob biomass by Pichia caribbica

Xylitol, a commodity chemical, is widely used in nutraceutical and pharmaceutical formulations. Microbial xylitol production is a promising alternative to mitigate current industrial practice issues and offers an environment-friendly sustainable conversion route. This study demonstrates a bioprocess for xylitol production from corncob using a mesophilic yeast Pichia caribbica. Corncob was hydrolyzed by dilute acid and steam explosion to recover fermentable xylose and used as the feed for xylitol production. Activated carbon treatment (3% w/v) completely removed the biomass-derived inhibitors furfural and hydroxymethyl furfural from the liquid hydrolysate. The fermenting yeast Pichia caribbica produced 124.1 ± 0.45 g/L xylitol from the detoxified and concentrated corncob hydrolysate with a high yield of 0.80 ± 0.02 g/g. The crystallized xylitol with 96.5% purity demonstrated no harmful effects on the cell line used as a control for the in-vitro toxicity studies. This proof of concept can be applied to help scale up for bio-refinery-based large-scale production of xylitol from corncob biomass.

Valorization of Pinus taeda hemicellulosic hydrolysate for the production of value-added compounds in an ethanol biorefinery

Production of cellulosic ethanol from lignocellulosic biomass leads to the generation of a hemicellulosic hydrolysate during the feedstock pretreatment. This hydrolysate is rich in sugars, but also contains inhibitory compounds (mainly acetic acid and phenolic compounds) in concentrations that may be toxic to microbial growth. Currently, this side-stream of the cellulosic ethanol production process is processed as a waste, due to the lack of feasible alternatives for tackling the complexity of wood hemicellulosic hydrolysate. Thus, this work evaluated the ability of six microorganisms to metabolize the raw and detoxified hemicellulosic hydrolysate produced from Pinus taeda for the production of lactic acid, ethanol, xylitol, single-cell protein, lipids and carotenoids, with the aim of selecting a potential alternative for valorization of this side stream generated during the production of cellulosic ethanol contributing to the implementation of a sustainable advanced biorefinery. The tested microorganisms included a lactic acid bacterium, Bacillus coagulans; a probiotic bacterium, Lactobacillus salivarius; two oleaginous yeasts, Rhodosporidium toruloides and Saitoella coloradoensis; a thermotolerant yeast, Kluyveromyces marxianus; and a methylotrophic yeast, Hansenula polymorpha. L. salivarius, K. marxianus, and H. polymorpha showed promising ability to metabolize the partially detoxified hydrolysate (composed of (g/L): mannose, 29.27; glucose, 17.25; galactose, 6.18; xylose, 4.94; arabinose, 1.23; acetic acid, 7.99; formic acid, 4.86; levulinic acid, 4.04; 5-hydroxymethylfurfural, 0.74; total phenolic compounds, 0.40). On the other hand, the oleaginous yeasts and B. coagulans presented high sensitivity to the inhibitory compounds. L. salivarius produced lactic acid with high yield (1.1 g/g), which was limited by product inhibition. K. marxianus produced xylitol at 0.37 g/g xylose and ethanol at 0.19 g/g hexoses. Finally, H. polymorpha converted hexoses and acetic acid into single-cell protein with yield of 0.27 g/g. The production of lactic acid by L. salivarius proved to be a promising alternative for valorization of Pinus hemicellulosic hydrolysate in an ethanol biorefinery.

Xylitol: Bioproduction and Applications-A Review

Xylitol, a natural compound classified as a sugar alcohol, is found diversely in fruits and vegetables in small quantities. Commercial production of xylitol has expanded due to its health benefits and wide applications as an alternative sweetener in food and pharmaceutical products. Production of xylitol on large scale is industrially being achieved by the chemical method. However, the biotechnological method offers the possibilities of lowered cost and energy compared to the chemical methods. It involves the conversion of xylose to xylitol by microbes or enzymes which is environmentally safe. This review highlights the prospects of the biotechnological method of xylitol production. Various microorganisms that have been used to produce xylitol, the bioprocess parameters, and genetic modifications to increase xylitol yield have been reviewed. In addition, the applications, benefits, and safety concerns to health have been discussed.

Process Design and Techno-Economic Feasibility Analysis of an Integrated Pineapple Processing Waste Biorefinery

This study assesses the techno-economic feasibility of an integrated biorefinery based on pineapple processing waste. Xylooligosaccharides, ethanol, xylitol, bromelain, and silage are among the key products of the biorefinery. The economic performance of the processes involved in generating the biorefinery products was assessed based on calculations performed in ASPEN Plus. Seven different scenarios were designed with individual and multiple products and were further evaluated for a plant capacity of 10 tons per hour as the base case. Sensitivity analysis showed that plant capacity and selling price of value-added products were the most important factors that influenced plant economics. The plant capacity twice the base capacity often made the venture economically feasible as in the case of scenarios 1 (production of xylitol and silage) and 7 (production of bromelain, xylitol, and silage) with an NPV of $9.2 million and $8.9 million, respectively. Increasing the selling price of the products by 25% of the base case made scenarios 1 and 6 (production of bromelain, xylitol, ethanol, and silage) economically viable (NPV > 0). A decrease in the price for procurement of pineapple waste from $25/ton to $10/ton made scenario 4 (production of bromelain and silage) profitable with an NPV of $3.3 million and IRR of 42%.

Microbial xylitol production

Xylitol is pentahydroxy sugar alcohol, existing in very trace amount in fruits and vegetables, and finds varied application in industries like food, pharmaceuticals, confectionaries, etc. and is of prime importance to health. Owing to its trace occurrence in nature and considerable increase in market demand that exceeds availability, alternate production through biotechnological and chemical approach is in process. Biochemical production involves substrates like lignocellulosic biomasses and industrial effluents and is an eco-friendly process with high dependency on physico-chemical parameters. Although the chemical processes are faster, high yielding and economical, they have a great limitation as usage of toxic chemicals and thus need to be regulated and replaced by an environment friendly approach. Microbes play a major role in xylitol production through a biotechnological process towards the development of a sustainable system. Major microbes working on assimilation of xylose for production of xylitol include Candida tropicalis, Candida maltose, Bacillus subtilis, Debaromyces hansenii, etc. The present review reports all probable microbial xylitol production biochemical pathways encompassing diverse bioprocesses involved in uptake and conversion of xylose sugars from agricultural residues and industrial effluents. A comprehensive report on xylitol occurrence and biotechnological production processes with varied substrates has been encompassed. KEY POINTS: • Xylitol from agro-industrial waste • Microbial xylose assimilation.

Corn stover conversion into bioethanol and xylitol through an integral bioprocess: Kinetic study and modelling

BackgroundCorn stover residue is an important lignocellulosic material with a biochemical composition that arouses an interest in transforming into bioethanol and high added value product, such as e.g., xylitol. MethodsHydrolysis experiments were carried out in a semi-pilot reactor of 12 L capacity. Fermentation experiments were conducted in a 1 L batch bioreactor capacity. Detailed kinetic study of the Candida guilliermondii NRC 5578 growth was carried out. Fermentation and the xylitol production process were modelled. Significant FindingsThe used crude corn stover residue was composed of 11.1 % of lignin, 27.1 % of hemicellulose, 32.6 % cellulose, 24.5 % pentosanes, and 33.0 % of α-cellulose. Reconditioning liquid fractions of prehydrolysate and hydrolysate at 1.4 % (w/v) of H2SO4 were determined for the prehydrolysate 5.31 g/L, 9.07 g/L, 3.07 g/L, 11.2 g/L, 3.24 g/L, 3.89 and 2.56 g/L for pentoses, hexoses, arabinose, xylose, mannose, glucose, and fructose, respectively, and for hydrolysate 15.2 g/L of glucose. The higher values of maximum specific growth rate were registered for control experiments and compared to prehydrolysated and hydrolysated cultures. The highest ethanol formation rates of 2.29 g/(g L) and 0.117 g/(g L) in synthetic and hydrolysated cultures, respectively, were registered when 7 g/L glucose was used as carbon source.

Sequential detoxification of oil palm fronds hydrolysate with coconut shell activated charcoal and pH controlled in bioreactor for xylitol production

Acid hydrolysis pre-treatment on biomass produces furan derivatives, phenolics, and organic acids, which are harmful to the fermenting microorganisms. Removal of inhibitor compounds from the hydrolysate using coconut shell activated charcoal (CSAC) adsorption is essential to improve the quality of detoxified hydrolysate as a feedstock for xylitol production. This study investigated adsorption parameters of different pH (pH 1–7), temperature (17.5–92.5 °C), CSAC-to-hydrolysate loading (0.5–9.5% (w/v)) and contact time (30–142.5 min) using a central composite design (CCD) to evaluate the interactive effect of the adsorption process parameters on response variables. Under optimal adsorption process (pH 2.5, 55.6 °C, 5.3% (w/v) and 74 min), the results demonstrated loss of 10.4% sugar and 12.8 % of detoxified hydrolysate volume with 79.9% of furfural removal. Inhibition by acetic acid could be alleviated by pH control in bioreactor since the concentration of acetic acid was decreased by up to 73.7% at the end of OPF hydrolysate fermentation. The fermentability test using detoxified OPF hydrolysate by Kluyveromyces marxianus ATCC 36907 demonstrated 78% higher xylitol production compared to non-detoxified OPF hydrolysate. This finding corroborates that this sequential detoxification is a promising method to enhance the xylitol production using OPF hydrolysate.

Improved xylitol production from olive stones hydrolysates by biological detoxification

Olive stones, currently used as a biosource of heat and power, is gaining research interest as a feedstock for a biorefinery based on olive derived wastes. In this work, pure xylitol crystals were obtained and fully characterized as one of the possibilities of taking full advantage of this by-product of olive oil production. The proposed process includes a two-step pretreatment (water and acid extractions), detoxification of the liquid fractions and fermentation of sugars. After the water extraction, a liquid fraction with antioxidant capacity was recovered; the second step was done with dilute sulfuric acid for solubilization of sugars, resulting in a solution containing more than 60 g/l of xylose as the main sugar. This liquor contained also up to 16 g/l acetic acid, which rendered it not fermentable. Also other compounds, potentially inhibitors in downstream operations, were produced during the pretreatment, requiring a detoxification procedure. In this work, a chemical-free, biological treatment with Saccharomyces cerevisiae proved to be the best detoxification method, as glucose, furfural, hydroxymethylfurfural and acetic acid were consumed. The subsequent inoculation of Candida boidinii produced xylitol concentrations above 38 g/l after 90 h fermentation time and a yield as high as 63%, equivalent to 12 g xylitol/100 g olive stones.