Biotechnological Production Of Xylitol Research Articles

Fermentative processes for the upcycling of xylose to xylitol by immobilized cells of Pichia fermentans WC1507.

Xylitol is a pentose-polyol widely applied in the food and pharmaceutical industry. It can be produced from lignocellulosic biomass, valorizing second-generation feedstocks. Biotechnological production of xylitol requires scalable solutions suitable for industrial scale processes. Immobilized-cells systems offer numerous advantages. Although fungal pellet carriers have gained attention, their application in xylitol production remains unexplored. In this study, the yeast strain P. fermentans WC 1507 was employed for xylitol production. The optimal conditions were observed with free-cell cultures at pH above 3.5, low oxygenation, and medium containing (NH4)2SO4 and yeast extract as nitrogen sources (xylitol titer 79.4g/L, YP/S 66.3%, and volumetric productivity 1.3g/L/h). Yeast cells were immobilized using inactive Aspergillus oryzae pellet mycelial carrier (MC) and alginate beads (AB) and were tested in flasks over three consecutive production runs. Additionally, the effect of a 0.2% w/v alginate layer, coating the outer surface of the carriers (cMC and cAB, respectively), was examined. While YP/S values observed with both immobilized and free cells were similar, the immobilized cells exhibited lower final xylitol titer and volumetric productivity, likely due to mass transfer limitations. AB and cAB outperformed MC and cMC. The uncoated AB carriers were tested in a laboratory-scale airlift bioreactor, which demonstrated a progressive increase in xylitol production in a repeated batch process: in the third run, a xylitol titer of 63.0g/L, YP/S of 61.5%, and volumetric productivity of 0.52g/L/h were achieved. This study confirmed P. fermentans WC 1507 as a promising strain for xylitol production in both free- and entrapped-cells systems. Considering the performance of the wild strain, a metabolic engineering intervention aiming at further improving the efficiency of xylitol production could be justified. MC and AB proved to be viable supports for cell immobilization, but additional process development is necessary to identify the optimal bioreactor configuration and fermentation conditions.

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

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

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.

Biological production of xylitol by using nonconventional microbial strains.

Xylitol (C5H12O5), an amorphous sugar alcohol of crystalline texture has received great interest on the global market due to its numerous applications in different industries. In addition to its high anticariogenic and sweetening properties, characteristics such as high solubility, stability and low glycemic index confer xylitol its fame in the food and odontological industries. Moreover, it also serves as a building-block in the production of polymers. As a result of the harmful effects of the chemical production of xylitol, the biotechnological means of producing this polyol have evolved over the decades. In contrast to the high consumption of energy, long periods of purification, specialized equipment and high production cost encountered during its chemical synthesis, the biotechnological production of xylitol offers advantages both to the economy and the environment. Non-Saccharomyces yeast strains, also termed as nonconventional, possess the inherent capacity to utilize D-xylose as a sole carbon source, unlike Saccharomyces species.

Mathematical modeling of the biotechnological production of xylitol from hydrolysates of agro-industrial waste

Xylitol is widely used in the pharmaceutical and food industries since it has cariogenic properties and low caloric value compared to sucrose. In recent decades, its demand has increased, which has motivated the generation of alternatives for its production. The biotechnological route is an attractive alternative since in addition to reducing production costs, it uses lignocellulosic hydrolysates from agro-industrial residues as raw material. However, due to the inherent variability in the composition of the waste, the implementation of the process is not a simple task, i. e., the determination of the operating conditions that maximize production yield requires a broad experimental design. In this sense, mathematical modeling can help reduce experimental work, and allows for predicting and evaluating the dynamic behavior of key variables, making it a useful tool for the implementation of process optimization and control schemes. Therefore, the objective of this work is to propose a generalized mathematical model that can be adapted to different study cases (i.e., different substrates and microorganism strains), operating conditions, and fermenter configuration. The results show high determination coefficients R2 > 0.90 for each case study, indicating that the proposed model is easily adaptable to different substrates and operating conditions. In addition, the model allows the incorporation of different effects such as pH, temperature, and agitation.

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

Xylitol production from plant biomass by Aspergillus niger through metabolic engineering

Xylitol is widely used in the food and pharmaceutical industries as a valuable commodity product. Biotechnological production of xylitol from lignocellulosic biomass by microorganisms is a promising alternative option to chemical synthesis or bioconversion from D-xylose. In this study, four metabolic mutants of Aspergillus niger were constructed and evaluated for xylitol accumulation from D-xylose and lignocellulosic biomass. All mutants had strongly increased xylitol production from pure D-xylose, beechwood xylan, wheat bran and cotton seed hulls compared to the reference strain, but not from several other feed stocks. The triple mutant ΔladAΔxdhAΔsdhA showed the best performance in xylitol production from wheat bran and cotton seed hulls. This study demonstrated the large potential of A. niger for xylitol production directly from lignocellulosic biomass by metabolic engineering.

Novel potential yeast strains for the biotechnological production of xylitol from sugarcane bagasse

AbstractThis study aimed to screen the xylitol‐producing strains of Cyberlindnera xylosilytica and Wickerhamomyces rabaulensis for the bioconversion of the hemicellulosic fraction of sugarcane bagasse. Among the 16 strains tested, two of each species (C. xylosilytica UFMG‐CM‐Y309 and UFMG‐CM‐Y409, and W. rabaulensis UFMG‐CM‐Y3716 and UFMG‐CM‐Y3747) were selected on the basis of xylitol yield (Yp/s) and xylitol volumetric productivity (Qp) in YPX broth (yeast extract, 10 g L−1; peptone, 20 g L−1; and d‐xylose, 60 g L−1). These strains were cultivated in sugarcane bagasse hemicellulosic hydrolysate (SCHH) supplemented with ammonium sulfate, rice bran extract, and yeast extract. The xylitol yield was the highest in the strain C. xylosilytica UFMG‐CM‐Y‐309 with a production of 14.06 g L−1; xylitol yield, 0.63 g g−1; xylitol volumetric productivity, 0.20 g L−1 h−1. Xylitol yields ranged from 14% to 74% above those of other xylitol‐production yeast species. This work demonstrates the potential of biotechnological application of these yeasts for xylitol production. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd

Optimized Bioconversion of Xylose Derived from Pre-Treated Crop Residues into Xylitol by Using Candida boidinii

Crop residues can serve as low-cost feedstocks for microbial production of xylitol, which offers many advantages over the commonly used chemical process. However, enhancing the efficiency of xylitol fermentation is still a barrier to industrial implementation. In this study, the effects of oxygen transfer rate (OTR) (1.1, 2.1, 3.1 mmol O2/(L × h)) and initial xylose concentration (30, 55, 80 g/L) on xylitol production of Candida boidinii NCAIM Y.01308 on xylose medium were investigated and optimised by response surface methodology, and xylitol fermentations were performed on xylose-rich hydrolysates of wheat bran and rice straw. High values of maximum xylitol yields (58–63%) were achieved at low initial xylose concentration (20–30 g/L) and OTR values (1.1–1.5 mmol O2/(L × h)). The highest value for maximum xylitol productivity (0.96 g/(L × h)) was predicted at 71 g/L initial xylose and 2.7 mmol O2/(L × h) OTR. Maximum xylitol yield and productivity obtained on wheat bran hydrolysate were 60% and 0.58 g/(L × h), respectively. On detoxified and supplemented hydrolysate of rice straw, maximum xylitol yield and productivity of 30% and 0.19 g/(L × h) were achieved. This study revealed the terms affecting the xylitol production by C. boidinii and provided validated models to predict the achievable xylitol yields and productivities under different conditions. Efficient pre-treatments for xylose-rich hydrolysates from rice straw and wheat bran were selected. Fermentation using wheat bran hydrolysate and C. boidinii under optimized condition is proved as a promising method for biotechnological xylitol production.

Development of a sustainable bioprocess based on green technologies for xylitol production from corn cob

In this work, a sustainable and environmental friendly strategy for the biotechnological production of xylitol was proposed and optimized. For this purpose, corn cob was hydrothermally pretreated at high solid loadings (25%) for an efficient solubilization of xylan in hemicellulose derived compounds, xylooligosaccharides and xylose. Xylose enriched streams were obtained from the enzymatic saccharification of the whole slurry (solid and liquid fraction) resulting from the autohydrolysis pretreatment. The xylitol production in a simultaneous saccharification and fermentation (SSF) process, by the recombinant Saccharomyces cerevisiae PE-2-GRE3 strain, was optimized using different enzyme and substrate (pretreated corn cob solid) loadings by an experimental design. This study demonstrated a significant effect of substrate loading on the production process achieving a maximal concentration of 47 g/L with 6.7 % of pretreated corn cob and 24 FPU/g of enzyme loading, with partial detoxification of the hydrolysate. Furthermore, the 1.42-fold increase in xylitol titer and 1.56-fold increase in productivity achieved in a SSF using an acetic acid free-hydrolysate evidenced the negative effect of acetic acid on the yeast-based xylitol production process. The combination of these green technologies and the optimization of the proposed strategy enhanced the overall xylitol production through the valorization of corn cob.

Biotechnological Production of Xylitol from Oil Palm Empty Fruit Bunches Hydrolysate

This study evaluated the biomass and xylitol production using Oil Palm Empty Fruit Bunches (OPEFB) hydrolysates. <em>Candida tropicalis</em> ATCC 13803, which produces xylitol, was cultivated in the synthetic media Yeast extract Peptone Xylose (YPX), Medium Minimum Xylose (MMX) and hemicellulosic Hydrolysates medium (HR). Biomass concentration was evaluated in the synthetic media to identify the best medium for the biomass production. Hemicellulosic hydrolysates were obtained by dilute acid hydrolysis of OPEFB. All fermentations were performed in 100 mL Erlenmeyer flasks containing 40 mL of medium with 20 g/L xylose, initial pH 5, 6, 119 rpm and 30°C for 72 h. During the fermentations the cellular concentration was determined spectrophotometrically by Optical Density (OD) at 620 nm and the kinetics parameters and xylitol production were evaluated. The best synthetic medium for biomass production was YPX with 2.52 g/L at 30 h. The xylitol yield and yield values for HR media were 0.41 g/g and 0.10 g/L.h, respectively.

XYLITOL ON DENTAL CARIES: A REVIEW

Xylitol is a pentahydroxy sugar-alcohol which exists in a very low quantity in fruits and vegetables (plums, strawberries, cauliflower, and pumpkin). On commercial scale, xylitol can be produced by chemical and biotechnological processes. Chemical production is costly and extensive in purification steps. The precursor xylose is produced from agricultural biomass by chemical and enzymatic hydrolysis and can be converted to xylitol primarily by yeast strain. Hydrolysis under acidic condition is the more commonly used practice influenced by various process parameters. Biotechnological xylitol production is an integral process of microbial species belonging to Candida genus which is influenced by various process parameters such as pH, temperature; time, nitrogen source, and yeast extract level. It is a functional sweetener as it has prebiotic effects which can reduce blood glucose, triglyceride, and cholesterol level. Dental caries is an infectious microbiologic disease of the tooth that results in localized dissolution and destruction of calcified tissues. Xylitol has been shown to reduce dental caries when mixed with food, chewing gums and milk. Dental caries are prevalent in acidic pH where Streptoccocus mutans (MS) ferment resulting in demineralization of tooth, where as Streptococcus mutans cannot ferment xylitol thus it reduces MS by altering their metabolic pathway and enhance remineralization and helps arrest dentinal caries. Reduction in caries rate is greater, when xylitol is used as the sugar substitutes. This review discusses the taste acceptability of xylitol in milk as a step towards measuring the effectiveness for the reduction of dental caries.&#x0D; Keywords: Xylitol, Carbohydrates, Caries, Remineralisation, Demineralization, Streptoccocus mutans

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

BackgroundThe 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.ResultsAcid 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.ConclusionPlant 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.

The yeast Scheffersomyces amazonensis is an efficient xylitol producer.

This study assessed the efficiency of Scheffersomyces amazonensis UFMG-CM-Y493T, cultured in xylose-supplemented medium (YPX) and rice hull hydrolysate (RHH), to convert xylose to xylitol under moderate and severe oxygen limitation. The highest xylitol yields of 0.75 and 1.04gg-1 in YPX and RHH, respectively, were obtained under severe oxygen limitation. However, volumetric productivity in RHH was ninefold decrease than that in YPX medium. The xylose reductase (XR) and xylitol dehydrogenase (XDH) activities in the YPX cultures were strictly dependent on NADPH and NAD+ respectively, and were approximately 10% higher under severe oxygen limitation than under moderate oxygen limitation. This higher xylitol production observed under severe oxygen limitation can be attributed to the higher XR activity and shortage of the NAD+ needed by XDH. These results suggest that Sc. amazonensis UFMG-CM-Y493T is one of the greatest xylitol producers described to date and reveal its potential use in the biotechnological production of xylitol.

Strategies of detoxification and fermentation for biotechnological production of xylitol from sugarcane bagasse

The aim of this study was to evaluate strategies of detoxification and fermentation of the hemicellulosic liquors obtained from sugarcane bagasse autohydrolysis, for the biotechnological production of xylitol. Different sequences of detoxification treatments were performed, and their effects on sugars loss and inhibitors removal were evaluated. Fermentation assays were accomplished with commercial xylose to select the best yeast (C. guilliermondii, C. tropicalis) and the fermentation conditions of the detoxified spent liquors. Detoxification through a sequence of treatments, including Ca(OH)2, IR-120 resin, activated charcoal, and IRA-67 resin, practically removed all inhibitors from the hemicellulosic spent liquors. Maximal concentration of xylitol obtained was 32.0gL⿿1 (C. tropicalis, xylose: 104.1gL⿿1, yield: 0.46gg⿿1, productivity: 0.27gL⿿1h⿿1). The technological parameters to obtain a detoxified spent liquor rich in xylose and its bioconversion to xylitol were determined.

Sugarcane straw as a feedstock for xylitol production by Candida guilliermondii FTI 20037.

Sugarcane straw has become an available lignocellulosic biomass since the progressive introduction of the non-burning harvest in Brazil. Besides keeping this biomass in the field, it can be used as a feedstock in thermochemical or biochemical conversion processes. This makes feasible its incorporation in a biorefinery, whose economic profitability could be supported by integrated production of low-value biofuels and high-value chemicals, e.g., xylitol, which has important industrial and clinical applications. Herein, biotechnological production of xylitol is presented as a possible route for the valorization of sugarcane straw and its incorporation in a biorefinery. Nutritional supplementation of the sugarcane straw hemicellulosic hydrolyzate as a function of initial oxygen availability was studied in batch fermentation of Candida guilliermondii FTI 20037. The nutritional supplementation conditions evaluated were: no supplementation; supplementation with (NH4)2SO4, and full supplementation with (NH4)2SO4, rice bran extract and CaCl2·2H2O. Experiments were performed at pH 5.5, 30 °C, 200 rpm, for 48 h in 125 mL Erlenmeyer flasks containing either 25 or 50 mL of medium in order to vary initial oxygen availability. Without supplementation, complete consumption of glucose and partial consumption of xylose were observed. In this condition the maximum xylitol yield (0.67 g g−1) was obtained under reduced initial oxygen availability. Nutritional supplementation increased xylose consumption and xylitol production by up to 200% and 240%, respectively. The maximum xylitol volumetric productivity (0.34 g L−1 h−1) was reached at full supplementation and increased initial oxygen availability. The results demonstrated a combined effect of nutritional supplementation and initial oxygen availability on xylitol production from sugarcane straw hemicellulosic hydrolyzate.

Biochemical conversion of sugarcane straw hemicellulosic hydrolyzate supplemented with co-substrates for xylitol production

Biotechnological production of xylitol is an attractive route to add value to a sugarcane biorefinery, through utilization of the hemicellulosic fraction of sugarcane straw, whose availability is increasing in Brazil. Herein, supplementation of the sugarcane straw hemicellulosic hydrolyzate (xylose 57gL−1) with maltose, sucrose, cellobiose or glycerol was proposed, and their effect as co-substrates on xylitol production by Candida guilliermondii FTI 20037 was studied. Sucrose (10gL−1) and glycerol (0.7gL−1) supplementation led to significant increase of 8.88% and 6.86% on xylose uptake rate (1.11gL−1h−1 and 1.09gL−1), respectively, but only with sucrose, significant increments of 12.88% and 8.69% on final xylitol concentration (36.11gL−1) and volumetric productivity (0.75gL−1h−1), respectively, were achieved. Based on these results, utilization of complex sources of sucrose, derived from agro-industries, as nutritional supplementation for xylitol production can be proposed as a strategy for improving the yeast performance and reducing the cost of this bioprocess by replacing more expensive nutrients.

Xylitol bioproduction in hemicellulosic hydrolysate obtained from sorghum forage biomass.

This study evaluated the biotechnological production of xylitol from sorghum forage biomass. The yeast Candida guilliermondii was cultivated in hemicellulosic hydrolysates obtained from biomass of three sorghum varieties (A, B, and C). First, the biomass was chemically characterized and subjected to dilute acid hydrolysis to obtain the hemicellulosic hydrolysates which were vacuum-concentrated and detoxified with activated charcoal. The hemicellulosic hydrolysates (initial pH 5.5) were supplemented with nutrients, and fermentations were conducted in 125-mL Erlenmeyer flasks containing 50 mL medium, under 200 rpm, at 30 °C for 96 h. Fermentations were evaluated by determining the parameters xylitol yield (Y P/S ) and productivity (QP), as well as the activities of the enzymes xylose reductase (XR) and xylitol dehydrogenase (XDH). There was no significant difference among the three varieties with respect to the contents of cellulose, hemicellulose, and lignin, although differences were found in the hydrolysate fermentability. Maximum xylitol yield and productivity values for variety A were 0.35 g/g and 0.16 g/L.h(-1), respectively. It was coincident with XR (0.25 U/mg prot) and XDH (0.17 U/mg prot) maximum activities. Lower values were obtained for varieties B and C, which were 0.25 and 0.17 g/g for yield and 0.12 and 0.063 g/L.h(-1) for productivity.

Xylitol production from cashew apple bagasse by Kluyveromyces marxianus CCA510

The production of xylitol, a polyol with high employability in the food and pharmaceutical industry, from cashew apple bagasse hydrolysate (CABH) by a new strain of Kluyveromyces marxianus was studied. Initially, the use of activated charcoal in the detoxification of hydrolysates from CABH was evaluated. Then, the influence of the supplementation of CABH with various nitrogen sources was studied. The activated charcoal reduced the concentration of acid and phenolic compounds. K. marxianus CCA510 was able to produce xylitol using CABH, with the highest yield of 0.36gg−1 and maximum concentration of 12.73gL−1. When it was added urea in the medium the highest xylitol yield was observed, reaching 0.50gg−1, showing that substance as nitrogen source improved the xylitol production. The cashew apple bagasse hydrolysate is a potential medium for biotechnological production of xylitol and detoxification treatments can be employed to reduce potential toxic compounds present in the medium without significant loss of sugars.

Metabolic engineering strategies for improving xylitol production from hemicellulosic sugars

Xylitol is a five-carbon sugar alcohol with potential for use as a sweetener. Industrially, xylitol is currently produced by chemical hydrogenation of D-xylose using Raney nickel catalysts and this requires expensive separation and purification steps as well as high pressure and temperature that lead to environmental pollution. Highly efficient biotechnological production of xylitol using microorganisms is gaining more attention and has been proposed as an alternative process. Although the biotechnological method has not yet surpassed the advantages of chemical reduction in terms of yield and cost, various strategies offer promise for the biotechnological production of xylitol. In this review, the focus is on the most recent developments of the main metabolic engineering strategies for improving the production of xylitol.