Maximum Xylitol Production Research Articles

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

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

Effects of agitation rate and dissolved oxygen on xylose reductase activity during xylitol production at bioreactor scale

The study of xylose reductase (XR) - one of the key enzymes in the production of xylitol - is important in the fermentation process to have maximum efficiency in the bioconversion of xylose to xylitol in lignocellulosic hydrolysate. The aim was to evaluate the effect of agitation rate and dissolved oxygen at 7 L bioreactor scale on the production of xylose reductase (XR) from Candida tropicalis during the bioconversion of xylose into xylitol in the non-detoxified oil palm empty fruit bunch (OPEFB) hydrolysate. The highest xylose consumption (95.5%) and the maximum xylitol production (5.46 g.L-1) were presented under 30% dissolved oxygen and 50 rpm. The maximum XR activity (0.646 U mg-1 protein) was obtained after 144 h of fermentation and at the same conditions of dissolved oxygen and agitation rate mentioned above. The oxygen availability influences the XR activity of C. tropicalis and the xylitol production, observing a xylitol yield factor (YP/S) of 0.27 g.g-1 and volumetric productivity (QP) of 0.33 g.L-1 h-1. At lower dissolved oxygen regardless of the agitation conditions evaluated, an increase in xylitol production was evidenced.

Cyberlindnera dasilvae sp. nov., a xylitol-producing yeast species isolated from rotting wood and frass of cerambycid larva.

Six yeast isolates were obtained from rotting wood samples in Brazil and frass of a cerambycid beetle larva in French Guiana. Sequence analysis of the ITS-5.8S region and the D1/D2 domains of the large subunit rRNA gene showed that the isolates represent a novel species of Cyberlindnera. This novel species is related to Cyberlindnera japonica, Cyberlindnera xylosilytica, Candida easanensis and Candida maesa. It is heterothallic and produces asci with two or four hat-shaped ascospores. The name Cyberlindnera dasilvae sp. nov. is proposed to accommodate the novel species. The holotype of Cy. dasilvae is CBS 16129T and the designated paratype is CBS 16584. The MycoBank number is 838252. All isolates of Cy. dasilvae were able to convert xylose into xylitol with maximum xylitol production within 60 and 72 h. The isolates produced xylitol with values ranging from 12.61 to 31.79 g l-1 in yeast extract-peptone-xylose medium with 5% xylose. When the isolates were tested in sugarcane bagasse hydrolysate containing around 35-38 g l-1d-xylose, isolate UFMG-CM-Y519 showed maximum xylitol production.

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

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

Development of a purification process via crystallization of xylitol produced for bioprocess using a hemicellulosic hydrolysate from the cashew apple bagasse as feedstock.

Xylitol was biotechnologically produced by Kluyveromyces marxianus ATCC36907 using the hemicellulosic hydrolysate ofthe cashew apple bagasse (CABHH). Sequentially, the present study investigated the recovery and purification of xylitol evaluating different antisolvents [ethanol, isopropanol and the ionic liquid 2-hydroxyl-ethylammonium acetate (2-HEAA)], their proportion in the medium (10-90% v/v), and their cooling rate (VC 0.25-0.50°C/min). These processes were contrasted with the crystallization process of commercial xylitol. This study is the first to assess xylitol crystallization using a protic ionic liquid. The hydrolysate obtained from a mild treatment with sulfuric acid contained mainly glucose and xylose at concentrations of 15.7g/L and 11.9g/L, respectively. With this bioprocess, a maximum xylitol production of 4.5g/L was achieved. The performance of the investigated antisolvents was similar in all conditions evaluated in the crystallization process of the commercial xylitol, with no significant difference in yields. For the crystallization processes of the produced xylitol, the best conditions were: 50% (v/v) isopropanol as antisolvent, cooling rate of 0.5°C/min, with a secondary nucleation of yield and purity of 69.7% and 84.8%, respectively. Under the same linear cooling rate, using ethanol, isopropanol or the protic ionic liquid 2-hydroxyl-ethylammonium acetate (2-HEAA), crystallization did not occur, probably due to the presence of carbohydrates not metabolized by the yeast in the broth, which influences the solubility curve of xylitol. With the results of this work, a possible economical and environmentally friendly process of recovery and purification of xylitol from CABHH could be proposed.

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.

Xylitol Production from Exhausted Olive Pomace by Candida boidinii

In this work, the production of xylitol from a hemicellulosic hydrolysate of exhausted olive pomace (EOP), a residue originated in the olive oil production process by Candida boidinii, was assessed. The hydrolysate was obtained by dilute acid pretreatment of EOP at 170 °C and 2% H2SO4 (w/v). A previous detoxification step of the hydrolysate was necessary, and its treatment with activated charcoal and ion-exchange resin was evaluated. Prior to fermentation of the hydrolysate, fermentation tests in synthetic media were performed to determine the maximum xylitol yield and productivity that could be obtained if inhibitory compounds were not present in the medium. In addition, the glucose existing in the media exerted a negative influence on xylitol production. A maximum xylitol yield of 0.52 g/g could be achieved in absence of inhibitor compounds. Fermentation of the hemicellulosic hydrolysate from EOP after detoxification with ion-exchange resin resulted in a xylitol yield of 0.43 g/g.

Immobilization of recombinant Escherichia coli on multi-walled carbon nanotubes for xylitol production

E. coli has been engineered to produce xylitol, but the production faces bottlenecks in terms of production yield and cell viability. In this study, recombinant E. coli (rE. coli) was immobilized on untreated and treated multiwalled carbon nanotubes (MWCNTs) for xylitol production. The immobilized rE. coli on untreated MWCNTs gave the highest xylitol production (5.47 g L−1) and a productivity of 0.22 g L−1 h−1. The doubling time for the immobilized cells increased up to 20.40 h and was higher than that of free cells (3.67 h). Cell lysis of the immobilized cells was reduced by up to 73 %, and plasmid stability improved by up to 17 % compared to those of free cells. Xylitol production using the optimum parameters (pH 7.4, 0.005 mM and 29 °C) achieved a xylitol production and productivity of 6.33 g L−1 and 0.26 g L−1 h−1, respectively. A seven-cycle repeated batch fermentation was carried out for up to 168 h, which showed maximum xylitol production of 7.36 g L−1 during the third cycle. Hence, this new adsorption immobilization system using MWCNTs is an alternative to improve the production of xylitol.

Isolation of xylose-assimilating yeasts and optimization of xylitol production by a new Meyerozyma guilliermondii strain.

Production of xylitol from lignocellulosic biomass is of interest to modern biorefineries, because this biomass should be processed into a spectrum of chemicals (bio-based products) and not only energy. The isolation of new yeast strains capable of efficiently converting xylose into xylitol and withstanding inhibitors released from biomass hydrolysis can contribute to making its production feasible in biorefineries. Forty-three out of 128 yeast strains isolated from the gut of Passalidae beetles were capable of assimilating xylose as the sole carbon source. Meyerozyma guilliermondii UFV-1 was selected due to its ability to grow and ferment D-xylose in a synthetic medium. This yeast assimilated the broad range of sugars present in lignocellulosic biomass hydrolysates, such as xylose, raffinose, cellobiose, rhamnose, arabinose, and glucose. Its optimum growth conditions were pH 8.0 and a temperature of 30 °C. In concentrations of 0.07 mol/Lacetic acid, 0.05 mol/L 5-hydroximethylfurfural, and 0.04 mol/L furfural, M. guilliermondii UFV-1 did not grow. Maximum xylitol production in aerobiosis and hypoxia were 51.88 and 27.73 g/L, respectively. Under aerobic condition, xylose concentration and agitation rate were the factors which were statistically significant, while only the agitation rate was significant in hypoxia. We fitted a response surface (RS) that estimated the best agitation rate (113.33 rpm) and xylose concentration (90 g/L) for maximum xylitol production in aerobiosis. Therefore, M. guilliermondii UFV-1 displays potential for being used for xylitol production in biorefineries.

Multi response optimization for enhanced xylitol production by Debaryomyces nepalensis in bioreactor

In this study, the optimization of different process variables—pH (4–6), aeration rate (200–550 rpm) and agitation rate (0.6–1.8 vvm) were investigated using rotating simplex method and uniform design method to enhance xylitol production from xylose by D. nepalensis in a batch stirred tank bioreactor. Maximum xylitol productivity (0.576 g L−1 h−1) was obtained at pH 4.0, agitation 300 rpm and aeration 1.5 vvm by rotating simplex method. Individual optimum values of pH, agitation and aeration are 4.2, 370 rpm and 1.2 vvm, respectively, for productivity, 4.3, 350 rpm and 1.0 vvm, respectively for xylitol concentration and 4.4, 360 rpm and 0.8 vvm, respectively for yield. Using generalized distance approach, the simultaneous optimal values were found to be—pH 4.3, 370 rpm and 0.9 vvm. After multi-response analysis, batch fermentation at optimal operating conditions resulted in enhanced productivity (0.76 g L−1 h−1), xylitol concentration (59.4 g L−1) and yield (0.58 g g−1) with an increase of 76.74 % of xylitol productivity.

Dual utilization of NADPH and NADH cofactors enhances xylitol production in engineered Saccharomyces cerevisiae.

Xylitol, a natural sweetener, can be produced by hydrogenation of xylose in hemicelluloses. In microbial processes, utilization of only NADPH cofactor limited commercialization of xylitol biosynthesis. To overcome this drawback, Saccharomyces cerevisiae D452-2 was engineered to express two types of xylose reductase (XR) with either NADPH-dependence or NADH-preference. Engineered S. cerevisiae DWM expressing both the XRs exhibited higher xylitol productivity than the yeast strain expressing NADPH-dependent XR only (DWW) in both batch and glucose-limited fed-batch cultures. Furthermore, the coexpression of S. cerevisiae ZWF1 and ACS1 genes in the DWM strain increased intracellular concentrations of NADPH and NADH and improved maximum xylitol productivity by 17%, relative to that for the DWM strain. Finally, the optimized fed-batch fermentation of S. cerevisiae DWM-ZWF1-ACS1 resulted in 196.2 g/L xylitol concentration, 4.27 g/L h productivity and almost the theoretical yield. Expression of the two types of XR utilizing both NADPH and NADH is a promising strategy to meet the industrial demands for microbial xylitol production.

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.

Study of the potential of the air lift bioreactor for xylitol production in fed-batch cultures by Debaryomyces hansenii immobilized in alginate beads

Cell immobilization has shown to be especially adequate for xylitol production. This work studies the suitability of the air lift bioreactor for xylitol production by Debaryomyces hansenii immobilized in Ca-alginate operating in fed-batch cultures to avoid substrate inhibition. The results showed that the air lift bioreactor is an adequate system since the minimum air flow required for fluidization was even lower than that leading to the microaerobic conditions that trigger xylitol accumulation by this yeast, also maintaining the integrity of the alginate beads and the viability of the immobilized cells until 3 months of reuses. Maximum productivities and yields of 0.43 g/l/h and 0.71 g/g were achieved with a xylose concentration of 60 g/l after each feeding. The xylose feeding rate, the air flow, and the biomass concentration at the beginning of the fed-batch operation have shown to be critical parameters for achieving high productivities and yields. Although a maximum xylitol production of 139 g/l was obtained, product inhibition was evidenced in batch experiments, which allowed estimating at 200 and 275 g/l the IC50 for xylitol productivity and yield, respectively. The remarkable production of glycerol in the absence of glucose was noticeable, which could not only be attributed to the osmoregulatory function of this polyol in conditions of high osmotic pressure caused by high xylitol concentrations but also to the role of the glycerol synthesis pathway in the regeneration of NAD(+) in conditions of suboptimal microaeration caused by insufficient aeration or high oxygen demand when high biomass concentrations were achieved.

Fermentation behavior of an osmotolerant yeastD. hanseniifor Xylitol production

Realizing the importance of xylitol as a high-valued compound that serves as a sugar substitute, a new, one step thin layer chromatographic procedure for quick, reliable, and efficient determination of xylose and xylitol from their mixture was developed. Two hundred and twenty microorganisms from the laboratory stock cultures were screened for their ability to produce xylitol from D-xylose. Amongst these, an indigenous yeast isolate no.139 (SM-139) was selected and identified as Debaryomyces hansenii on the basis of morphological and biochemical characteristics and (26S) D1/D2 r DNA region sequencing. Debaryomyces hansenii produced 9.33 gL(-1) of xylitol in presence of 50.0 gL(-1) of xylose in 84 h at pH 5.5, 30°C, 200 rpm. In order to utilize even higher concentrations of xylose for maximum xylitol production, a xylose enrichment technique was developed. The strain of Debaryomyces hansenii was obtained through xylose enrichment technique in a statistically optimized medium containing 0.3% yeast extract, 0.2% peptone, 0.03% MgSO(4) .7H(2) O along with 1% methanol. The culture was inoculated with 6% inoculum and incubated at 30°C and 250 rpm. A yield of 0.6 gg(-1) was obtained with a xylitol volumetric productivity of 0.65 g/L h(-1) in the presence of 200 gL(-1) of xylose although up to 300 gL(-1) of xylose could be tolerated through batch fermentation. Through this technique, even higher concentrations of xylose as substrate could be potentially utilized for maximum xylitol production.

Microbial conversion of bamboo hemicellulose hydrolysate to xylitol

A hemicellulose hydrolysate containing 19 g L−1 xylose was prepared from the culm of bamboo (Phyllostachys pubescens) by hydrolysis with 3 % sulphuric acid with a liquor to solid ratio of 10 (g g−1) at 121 °C for 1 h. After detoxification of the hydrolysate with a commercially available activated char followed by neutralisation with calcium carbonate, the resulting sugar solution was subjected to fermentation using the yeast, Candida magnoliae. The maximum xylitol production (10.5 g L−1) and the maximum xylitol volumetric productivity (0.42 g L−1 h−1) were attained under agitation set at 400 min−1 and aeration rate of 0.67 vvm (volume of air per volume of medium per minute). According to the results, a suitable control of the oxygen supply permits the xylitol formation from bamboo hemicellulose hydrolysate.

Kinetic behavior of Candida tropicalis during xylitol production using semi-synthetic and hydrolysate based media

The effects of the xylose concentration and the air supply on the production of xylitol from D-xylose by the yeast Candida tropicalis (AY 2007) using semi-synthetic and hydrolysate based media were investigated in four fermentation batch runs. The xylitol formation was favored under oxygen-limited conditions (2nd experiment, air flow rate 0.3 vvm) with a maximum xylitol production (36.0gm/l) after 59h as compared with that obtained in the 1st experiment (30.99 g/L) after 66 h under relatively less oxygen limited condition (air flow rate 1.5 vvm). Increasing the initial xylose concentration from 45.5 g/L in the 2nd experiment to 96.9 g/L in the 3rdexperiment led to a great decrease in the xylitol accumulation rate (Qp) from 0.506 to 0.134 g/L, xylitol yield coefficient (Yp/s) from 0.704 to 0.180 g/g and a mild increase in the xylose consumption rate (Qs) from 0.70 to 0.75 g/L/h. The semi-synthetic medium (1st and 2nd experiments) had a maximum xylitol yield coefficient (Yp/s) of 0.701 and 0.704 g/g, respectively which was less than that obtained in the hydrolysate experiment (0.783 g/g). This is probably due to the low hydrolysate initial xylose content (29.8 g/L). Key words: Xylitol, Candida tropicalis, xylose, pH, aeration, kinetics.

Kinetic behavior of Candida tropicalis during xylitol production using semi-synthetic and hydrolysate based media

The effects of the xylose concentration and the air supply on the production of xylitol from D-xylose by the yeast Candida tropicalis (AY 2007) using semi-synthetic and hydrolysate based media were investigated in four fermentation batch runs. The xylitol formation was favored under oxygen-limited conditions (2nd experiment, air flow rate 0.3 vvm) with a maximum xylitol production (36.0gm/l) after 59h as compared with that obtained in the 1st experiment (30.99 g/L) after 66 h under relatively less oxygen limited condition (air flow rate 1.5 vvm). Increasing the initial xylose concentration from 45.5 g/L in the 2nd experiment to 96.9 g/L in the 3rdexperiment led to a great decrease in the xylitol accumulation rate (Qp) from 0.506 to 0.134 g/L, xylitol yield coefficient (Yp/s) from 0.704 to 0.180 g/g and a mild increase in the xylose consumption rate (Qs) from 0.70 to 0.75 g/L/h. The semi-synthetic medium (1st and 2nd experiments) had a maximum xylitol yield coefficient (Yp/s) of 0.701 and 0.704 g/g, respectively which was less than that obtained in the hydrolysate experiment (0.783 g/g). This is probably due to the low hydrolysate initial xylose content (29.8 g/L). Key words: Xylitol, Candida tropicalis, xylose, pH, aeration, kinetics.

Xylitol production by Candida tropicalis under different statistically optimized growth conditions

Nutritional and environmental conditions of the xylose utilizing yeast Candida tropicalis were optimized on a shake-flask scale using a statistical factorial design to maximize the production of xylitol. Effects of the three growth medium components (rice bran, ammonium sulfate and xylose) on the xylitol production were studied, each at three different levels and the highest xylose to xylitol conversion ratio (57.2%) was achieved on using 20 gl-1 of xylose, 15 gl-1 of rice bran and 1 gl-1 of ammonium sulfate. By maintaining the pH of the growth medium at 5.5 using citrate buffer, the xylose to xylitol conversion ratio was greatly enhanced (93.75%) over the use of or unbuffered growth medium and phosphate buffered (88.3 and 83.8%).The least aeration rate was simulated by 40 ml of the growth medium at 250 ml-1. Erlenmeyer flasks result in maximum xylose to xylitol conversion ratio (98%), by using 25 gl-1 of xylose, while maximum xylitol production (36.25 gl-1) with a conversion ratio of 72.5% was achieved by using 50 gl-1 of xylose. Key words: xylitol, Candida tropicalis, xylose, yeast, pH, aeration.

Xylitol production by Candida tropicalis under different statistically optimized growth conditions

Nutritional and environmental conditions of the xylose utilizing yeast Candida tropicalis were optimized on a shake-flask scale using a statistical factorial design to maximize the production of xylitol. Effects of the three growth medium components (rice bran, ammonium sulfate and xylose) on the xylitol production were studied, each at three different levels and the highest xylose to xylitol conversion ratio (57.2%) was achieved on using 20 gl-1 of xylose, 15 gl-1 of rice bran and 1 gl-1 of ammonium sulfate. By maintaining the pH of the growth medium at 5.5 using citrate buffer, the xylose to xylitol conversion ratio was greatly enhanced (93.75%) over the use of or unbuffered growth medium and phosphate buffered (88.3 and 83.8%).The least aeration rate was simulated by 40 ml of the growth medium at 250 ml-1. Erlenmeyer flasks result in maximum xylose to xylitol conversion ratio (98%), by using 25 gl-1 of xylose, while maximum xylitol production (36.25 gl-1) with a conversion ratio of 72.5% was achieved by using 50 gl-1 of xylose. Key words: xylitol, Candida tropicalis, xylose, yeast, pH, aeration.

Fermentation behavior of osmophilic yeast Candida tropicalis isolated from the nectar of Hibiscus rosa sinensis flowers for xylitol production

Eighteen yeast species belonging to seven genera were isolated from ten samples of nectar from Hibiscus rosa sinensis and investigated for xylitol production using D-xylose as sole carbon source. Amongst these isolates, no. 10 was selected as the best xylitol producer and identified as Candida tropicalis on the basis of morphological, biochemical and 26S rDNA sequencing. C. tropicalis produced 12.11 gl(-1) of xylitol in presence of 50 gl(-1) of xylose in 72 h at pH 5, 30°C and 200 rpm. The strain of C. tropicalis obtained through xylose enrichment technique has resulted in a yield of 0.5 gg(-1) with a xylitol volumetric productivity of 1.07 gl(-1)h(-1) in the presence of 300 gl(-1) of xylose through batch fermentation. This organism has been reported for the first time from Hibiscus rosa sinensis flowers. Realizing, the importance of this high valued compound, as a sugar substitute, xylose enrichment technique was developed in order to utilize even higher concentrations of xylose as substrate for maximum xylitol production.