Xylitol Concentration Research Articles

Evaluation of porous glass and zeolite as cells carriers for xylitol production from sugarcane bagasse hydrolysate

Adsorbing carriers for immobilisation of Candida guilliermondii cells to use for xylitol production from sugarcane bagasse hemicellulose hydrolysate were tested. Biomass was immobilised in situ by natural adsorption, i.e. through direct contact between cells for the inoculum and carrier particles at the beginning of fermentations. The carriers employed were: Syrane porous glass beads with 2.53 mm diameter in average and pore diameter in the range 60–300 μm, supplied by Bioengineering (Wald, Switzerland), and NaX zeolite UOP WE 894 purchased from Plury Química S.A. (Diadema, SP, Brazil). At the end of the run with free cells taken as a reference test, xylitol concentration ( P f) achieved 35.5 g/l, corresponding to a xylose-to-xylitol yield factor ( Y P/S) of 0.72 g/g and a volumetric productivity ( Q P) of 0.49 g/l h, while final cell concentration ( X f) and productivity ( Q x) were only 5.32 g/l and 0.048 g/l h, respectively. Both systems with immobilised cells exhibited lower xylitol productions ( P f = 28.8–29.5 g/l, Y P/S = 0.52–0.53 g/g, Q P = 0.32–0.33 g/l h) and higher cell growth, with particular concern to porous glass ( X f = 10.5 g/l, Q x = 0.10 g/l h). Electronic microscopy observations demonstrated that the excellent performance of porous glass as cell support was due to the development of a thick extracellular matrix either within the large pores or on the surface of this material. As a consequence, almost 50% of the cells resulted to be adsorbed to the carrier at the end of the run. This growth was also responsible for a decrease in the fraction of xylose available for xylitol production. Employing zeolite, a material with pore size smaller than cell size, immobilised cells represented only 30% of the final population and immobilisation was just observed on the carrier surface. The low cell attachment on this material can be explained by the stress exerted on the outer immobilised cells by the friction among beads.

Production of xylitol by recombinant Saccharomyces cerevisiae containing xylose reductase gene in repeated fed-batch and cell-recycle fermentations

Xylitol is a well-known sugar substitute with low-calorie and anti-cariogenic characteristics. An effort of biological production of xylitol from xylose was made in repeated fed-batch and cell-recycle fermentations of recombinant Saccharomyces cerevisiae BJ3505/δXR harboring the xylose reductase gene from Pichia stipitis. Batch fermentation with 20 g/l xylose and 18 g/l glucose resulted in 9.52 g/l dry cell mass, 20.1 g/l xylitol concentration and approximately 100% conversion yield. Repeated fed-batch operation to remove 10% of culture broth and to supplement an equal volume of 200 g/l xylose was designed to improve xylitol production. In spite of a sudden drop of cell concentration, an increase in dry cell mass led to high accumulation of xylitol at 48.7 g/l. To overcome loss of xylitol-producing biocatalysts in repeated fed-batch fermentation, cell-recycle equipment of hollow fiber membrane was implemented into a xylitol production system. Cell-recycle operation maintained concentration of the recombinant cells high inside a bioreactor. Final dry cell mass of 22.0 g/l, 116 g/l xylitol concentration, 2.34 g/l h overall xylitol productivity were obtained in cell-recycle fermentation supplemented with xylose and yeast extract solution, which were equivalent to 2.3-, 5.8- and 3.8-fold increases compared with the corresponding values of batch-type xylitol production parameters.

The electrochemical oxidation of xylitol on Pt (1 1 1) in acid medium

A voltammetric and spectroscopic study of the oxidation of xylitol, in acid medium, on a platinum single crystal, Pt (1 1 1) and on polycrystalline platinum, Pt (poly), electrodes is presented in this paper. The superior electrocatalytic activity of the Pt (1 1 1) electrode surface in comparison with that of the Pt (poly) was clearly demonstrated. The oxidation of xylitol on the Pt (1 1 1) electrode surface has been studied by varying the scan potential rate and the xylitol concentration. The CVs show a shoulder and a well-defined anodic peak. It was concluded that both peaks are due to an irreversible one-electron transfer reaction and that the most positive peak presents a diffusion control. The reactions are of the order of 1/2 with respect to the initial polyol concentration. FTIRS data of the system Pt (1 1 1) in 0.1 M HClO 4 + 0.01 M xylitol, either in water or D 2O, have shown the existence of bands corresponding to adsorbed CO, bridge and linearly bonded, both involved in the surface poisoning in the range from 0.325 to 0.625 V vs. reversible hydrogen electrode (RHE). At E>0.625 V vs. RHE, bands, which may be attributed to acid groups and to a lactone, have been observed. Thus, the rather probable reaction products are, besides CO 2, xylonic acid (and the corresponding lactone, xylonolactone) and other short-chain aliphatic carboxylic acids resulting from the breaking of the initial molecular carbon skeleton.

Statistical investigation on the effects of starting xylose concentration and oxygen mass flowrate on xylitol production from rice straw hydrolyzate by response surface methodology

A 3 2 full-factorial design combined with response surface methodology was used to investigate the simultaneous effects of starting xylose concentration ( S 0) and oxygen mass flowrate ( q O 2 ) on xylitol production from rice straw hydrolyzate. Fermentations were performed at 30 °C, using Debaryomyces hansenii NRRL Y-7426 as xylitol producer and varying S 0 between 50 and 150 g/l and q O 2 between 2.5 and 5.9 mg O 2 /s. At the lowest starting xylose concentration and an oxygen mass flowrate of 4.2 mg O 2 /s, volumetric productivity and xylitol yield reached maximum values ( Q p=0.70 g P/l h and Y P/S=0.73 g P/g S, respectively), but xylitol concentration was quite low (35.9 g P/l). The results were in close agreement with the model prediction. The statistical model also allowed identifying the optimum operating conditions ( S 0=71 g/l and q O 2 =4.1 mg O 2 /s) able to simultaneously maximize volumetric productivity (0.53 g P/l h), xylitol yield (0.71 g P/g S) and final xylitol concentration (42.2 g P/l).

Influence of aeration rate and carrier concentration on xylitol production from sugarcane bagasse hydrolyzate in immobilized-cell fluidized bed reactor

A new approach to the biotechnological production of xylitol from sugarcane bagasse using Candida guilliermondii cells immobilized on porous glass in a fluidized bed reactor (FBR) is presented. The influence of aeration rate (AR) and carrier concentration ( C s) on the production process was evaluated. Seven batch fermentation runs were conducted according to a 2 2 factorial design. The results obtained showed that C s had a negative influence on xylitol yield ( Y p/s) and volumetric productivity ( Q p), whereas AR had a positive influence on Q p and a negative influence on Y p/s. With an AR=0.093 min −1 and C s=62.5 g/l, the Y p/s value was low (0.25 g/g), but the Q p was highest (0.44 g/l h), probably because cell metabolism was faster when more oxygen was available to the yeast. When the highest levels of the AR and C s were used, the xylitol yield and concentration were the lowest and the cell concentration was the highest, suggesting that cell metabolism was directed preferentially towards biomass production instead of xylitol accumulation.

Xylitol production by Ca-alginate entrapped cells: comparison of different fermentation systems

Candida guilliermondii cells were entrapped in Ca-alginate beads and used for xylitol production from concentrated sugarcane bagasse hemicellulosic hydrolysate in three different fermentation systems, namely 125-ml Erlenmeyer flasks (EF), 2.4-l stirred tank reactor (STR) and 2.4-l basket-type stirred tank reactor (BSTR). The EF system provided a xylitol production of 21.0 g P l −1, a product yield based on xylose consumption of 0.54 g P g S −1, and an overall production rate of 0.44 g P l −1 h −1 after 48 h of fermentation. By the STR system, 23.5 g P l −1 was produced after 60 h of fermentation, corresponding to a yield of 0.58 g P g S −1 and a production rate of 0.39 g P l −1 h −1. As the average volume of the beads decreased by 10.7% ( p<0.05) during this fermentation, a basket was fitted into the reactor vessel to prevent the beads abrasion and thus maintain their integrity. However, the xylitol yield (0.46 g P g S −1), production rate (0.21 g P l −1 h −1) and concentration (15.0 g P l −1) attained by this system were affected by mass-transfer limitations.

Optimization of fed-batch fermentation for xylitol production by Candida tropicalis.

Xylitol, a functional sweetener, was produced from xylose by biological conversion using Candida tropicalis ATCC 13803. Based on a two-substrate fermentation using glucose for cell growth and xylose for xylitol production, fed-batch fermentations were undertaken to increase the final xylitol concentration. The effects of xylose and xylitol on xylitol production rate were studied to determine the optimum concentrations for fed-batch fermentation. Xylose concentration in the medium (100 g l(-1)) and less than 200 g l(-1) total xylose plus xylitol concentration were determined as optimum for maximum xylitol production rate and xylitol yield. Increasing the concentrations of xylose and xylitol decreased the rate and yield of xylitol production and the specific cell growth rate, probably because of an increase in osmotic stress that would interfere with xylose transport, xylitol flux to secretion to cell metabolism. The feeding rate of xylose solution during the fed-batch mode of operation was determined by using the mass balance equations and kinetic parameters involved in the equations in order to increase final xylitol concentration without affecting xylitol and productivity. The optimized fed-batch fermentation resulted in 187 g l(-1) xylitol concentration, 0.75 g xylitol g xylose(-1) xylitol yield and 3.9 g xylitol l(-1) h(-1) volumetric productivity.

Metabolic behavior of immobilized Candida guilliermondii cells during batch xylitol production from sugarcane bagasse acid hydrolyzate.

Candida guilliermondii cells, immobilized in Ca-alginate beads, were used for batch xylitol production from concentrated sugarcane bagasse hydrolyzate. Maximum xylitol concentration (20.6 g/L), volumetric productivity (0.43 g/L. h), and yield (0.47 g/g) obtained after 48 h of fermentation were higher than similar immobilized-cell systems but lower than free-cell cultivation systems. Substrates, products, and biomass concentrations were used in material balances to study the ways in which the different carbon sources were utilized by the yeast cells under microaerobic conditions. The fraction of xylose consumed to produce xylitol reached a maximum value (0.70) after glucose and oxygen depletion while alternative metabolic routes were favored by sub-optimal conditions.

Improvement in xylitol production from sugarcane bagasse hydrolysate achieved by the use of a repeated-batch immobilized cell system.

Candida guilliermondii cells were immobilized in Ca-alginate beads and used for xylitol production from concentrated sugarcane bagasse hydrolysate during five successive fermentation batches, each lasting 48 hours. The bioconversion efficiency of 53.2%, the productivity of 0.50 g/l x h and the final xylitol concentration of 23.8 g/l obtained in the first batch increased to 61.5%, 0.59 g/l x h and 28.4 g/l, respectively, in the other four batches (mean values), with variation coefficients of up to 2.3%.

Influence of temperature and pH on xylitol production from xylose by Debaryomyces hansenii.

The production of xylitol from concentrated synthetic xylose solutions (S(o) = 130-135 g/L) by Debaryomyces hansenii was investigated at different pH and temperature values. At optimum starting pH (pH(o) = 5.5), T = 24 degrees C, and relatively low starting biomass levels (0.5-0.6 g(x)/L), 88% of xylose was utilized for xylitol production, the rest being preferentially fermented to ethanol (10%). Under these conditions, nearly 70% of initial carbon was recovered as xylitol, corresponding to final xylitol concentration of 91.9 g(P)/L, product yield on substrate of 0.81 g(P)/g(S), and maximum volumetric and specific productivities of 1.86 g(P)/L x h and 1.43 g(P)/g(x) x h, respectively. At higher and lower pH(o) values, respiration also became important, consuming up to 32% of xylose, while negligible amounts were utilized for cell growth (0.8-1.8%). The same approach extended to the effect of temperature on the metabolism of this yeast at pH(o) = 5.5 and higher biomass levels (1.4-3.0 g(x)/L) revealed that, at temperatures ranging from 32-37 degrees C, xylose was nearly completely consumed to produce xylitol, reaching a maximum volumetric productivity of 4.67 g(P)/L x h at 35 degrees C. Similarly, both respiration and ethanol fermentation became significant either at higher or at lower temperatures. Finally, to elucidate the kinetic mechanisms of both xylitol production and thermal inactivation of the system, the related thermodynamic parameters were estimated from the experimental data with the Arrhenius model: activation enthalpy and entropy were 57.7 kJ/mol and -0.152 kJ/mol x K for xylitol production and 187.3 kJ/mol and 0.054 kJ/mol x K for thermal inactivation, respectively.

Optimization of xylitol production by Candida guilliermondii FTI 20037 using response surface methodology

The combined effects of initial xylose concentration and inoculum level on xylitol production by Candida guilliermondii from rice straw hydrolysate were studied using response surface methodology. A 2 2 full-factorial central composite design was employed for experimental design and analysis of the results. The optimum xylose concentration and inoculum level were found to be 82 and 3 g/l, respectively. In these conditions, xylitol concentration of 52 g/l with a production rate of 0.54 g/l and a yield factor of 0.65 g/g was attained. These results are in close agreement with the model predictions.

Production of ethanol and xylitol from corn cobs by yeasts

Saccharomyces cerevisiae and Candida tropicalis were used separately and as co-culture for simultaneous saccharification and fermentation (SSF) of 5–20% (w/v) dry corn cobs. A maximal ethanol concentration of 27, 23, 21 g/l (w/v) from 200 g/l (w/v) dry corn cobs was obtained by S. cerevisiae, C. tropicalis and the co-culture, respectively, after 96 h of fermentation. However, theoretical yields of 82%, 71% and 63% were observed from 50 g/l dry corn cobs for the above cultures, respectively. Maximal xylitol concentration of 21, 20 and 15 g/l from 200 g/l (w/v) dry corn cobs was obtained by C. tropicalis, co-culture, and S. cerevisiae, respectively. Maximum theoretical yields of 79.0%, 77.0% and 58% were observed from 50 g/l of corn cobs, respectively. The volumetric productivities for ethanol and xylitol increased with the increase in substrate concentration, whereas, yield decreased. Glycerol and acetic acid were formed as minor by-products. S. cerevisiae and C. tropicalis resulted in better product yields (0.42 and 0.36 g/g) for ethanol and (0.52 and 0.71 g/g) for xylitol, respectively, whereas, the co-culture showed moderate level of ethanol (0.32 g/g) and almost maximal levels of xylitol (0.69 g/g).

Wood Hydrolysis and Hydrolyzate Detoxification for Subsequent Xylitol Production

This study deals with two different aspects of the transformation of lignocellulosics into xylitol: the optimization of conditions for wood hydrolysis and the setting-up of an adequate hydrolyzate detoxification procedure necessary to obtain high xylitol yields in the successive fermentation process. A comparison between the processes of wood autohydrolysis (steam explosion) and pre-hydrolysis with dilute sulfuric acid, carried out batch-wise in laboratory scale, shows comparable yields, either in terms of final concentrations of xylose and pentose sugars in the hydrolyzate or of solubilised fraction of wood. On the other hand, notwithstanding the longer time required, the pre-hydrolysis with dilute sulfuric acid produced acid hydrolyzates with lower contents of inhibiting substances (furfural, acetic acid, etc.). In order to obtain satisfactory xylitol yields from the hydrolysate produced by steam explosion, samples of this hydrolyzate were submitted to different detoxification techniques and then fermented batch-wise by a Pachysolen tannophilus strain previously adapted to this substrate. The best detoxification was performed by adding to the traditional overliming with Ca(OH)2 and sulfite reduction, three steps of a) filtration to remove insoluble substances, b) stripping of acetic acid and furfural, and c) lignin-derived compounds removal by adsorption on charcoal. The fermentation of this hydrolyzate was very effective, achieving a final xylitol concentration of 39.5 g/l from 89.0 g/l xylose after 96 h, corresponding to a volumetric productivity of 0.41 g/lh and a product yield of 0.63 g/g.

Characterization of two-substrate fermentation processes for xylitol production using recombinant Saccharomyces cerevisiae containing xylose reductase gene

Fermentation characteristics of recombinant Saccharomyces cerevisiae containing a xylose reductase gene from Pichia stipitis were investigated in an attempt to convert xylose to xylitol, a natural five-carbon sugar alcohol used as a sweetener. Xylitol was produced with a maximum yield of 0.95 g g −1 xylitol xylose consumed in the presence of glucose used as a co-substrate for co-factor regeneration. Addition of glucose caused inhibition of xylose transport and accumulation of ethanol. Such problems were solved by adopting glucose-limited fed-batch fermentations where a high ratio of xylose to glucose was maintained during the bioconversion phase. The optimized two-substrate fed-batch fermentation carried out with S. cerevisiae EH13.15:pY2XR at 30°C resulted in 105.2 g l −1 xylitol concentration with 1.69 g l −1 h −1 productivity.

Production of xylitol in cell recycle fermentations of Candida tropicalis

Xylitol was produced a in two-substrate, batch fermentation with cell recycling of Candida tropicalis ATCC 13803. A series of cell-recycle experiments showed that the feeding of xylose, glucose and yeast extract in the xylitol production phase was most effective in enhancing xylitol productivity. The optimized cell recycle fermentation resulted in 0.82 g xylitol/g xylose yield, 4.94 g xylitol l−1 h−1 productivity, and final xylitol concentration of 189 g l−1. These results were 1.3 times higher in volumetric xylitol productivity and 2.2 times higher in final product concentration compared with the corresponding values of the optimized two-substrate batch culture.

Production of highly concentrated xylitol by Candida magnoliae under a microaerobic condition maintained by simple fuzzy control

Microbial production of xylitol from xylose was investigated using Candida magnoliae. In particular, the effect of the oxygenation condition on the xylitol production yield was examined and the significance of maintaining a microaerobic condition was demonstrated. A simple system of fuzzy logic control (FLC) was devised to maintain the microaerobic condition in the xylitol production phase by regulating the proportion of air (air flow rate) supplied to the fermentor. The input variables to the fuzzy control system were the dissolved oxygen (DO) concentration in the culture broth and the CO 2 concentration in the exit gas. A batch cultivation test using the FLC system confirmed the importance of maintaining a constant microaerobic condition throughout the xylitol production phase, and indicated it would be advantageous for this phase to be prolonged. An intermittent fed-batch culture was therefore carried out. The FLC system allowed a constant microaerobic condition to be maintained, resulting in minimal cell mass production and constant xylitol accumulation in the culture medium. As a consequence, a very high xylitol concentration of 356 g/dm 3 could be attained. The xylitol yield in the fed-batch culture was 0.75, which corresponded to 82% of the theoretical yield.

Screening of microorganisms for xylitol production and fermentation behavior in high concentrations of xylose

Microorganisms with the ability to produce xylitol from high concentrations of xylose were screened from soils by enrichment culture using xylose as a sole carbon source. The selected strain was classified and determined as Candida sp. according to a taxonomic identification. In this strain, various conditions for xylitol production were investigated. Organic nutrients such as peptone and yeast extract were essential for xylitol production, and the optimal initial pH and k L a were between 4.0 and 6.0, and 5.2/h, respectively. Under the optimal condition, xylitol production from 200 g/l of xylose was 173 g/l after 5 days incubation, a 99.3% yield of the theoretical value. In order to produce a higher concentration of xylitol, a fed-batch culture of xylose was carried out by feeding 2.0 g of xylose per 10 ml of culture medium. The production from 4.0 g of xylose was 2.9 g of xylitol (concentration of 256 g/l) after 11.5 days which is a yield of 85.0%.

A preliminary information about continuous fermentation using cell recycling for improving microbial xylitol production rates(scientific note).

Xylitol is a sugar-alcohol with important technological properties, such as anticariogenicity, low caloric value, and negative dissolution heat. It can be used successfully in food formulations and pharmaceutical industries. Its production is therefore in great demand. Biotechnological xylitol production has several economic advantages in comparison with the conventional process based on the chemical reduction of xylose. The efficiency and the productivity of this fermentation chiefly depends on the microorganism and the process conditions employed. In this article a simple continuous culture with cell recycling was evaluated to enhance the capability of Candida guilliermondii FTI 20037 to produce xylitol. The fermentation was initiated batchwise by directly inoculating the grown seed culture in a 2-L bench-scale fermentor. Continuous feeding was begun at a dilution rate (D) of 0.060/h after the xylose concentration had completely consumed and the cell concentration was about 4.0 g/L. At a dilution rate of 0.060/h the xylitol concentration was about 15 g/L and increased by about 35%, whereas the dilution rate decreased by about 58%. Furthermore, the volumetric productivity, Qp, markedly depended on the dilution rate, diminishing by about 37% as D was changed from 0.060 to 0.025/h. These preliminary results show us that continuous fermentation with cell recycling is a good way to study the xylitol production by xylose-fermenting yeasts.

Increase of xylitol yield by feeding xylose and glucose in Candida tropicalis.

Candida tropicalis, a strain isolated from the sludge of a factory manufacturing xylose, produced a high xylitol concentration of 131 g/l from 150 g/l xylose at 45 h in a flask. Above 150 g/l xylose, however, volumetric xylitol production rates decreased because of a lag period in cell growth. In fed-batch culture, the volumetric production rate and xylitol yield from xylose varied substantially with the controlled xylose concentration and were maximum at a controlled xylose concentration of 60 g/l. To increase the xylitol yield from xylose, feeding experiments using different ratios of xylose and glucose were carried out in a fermentor. The maximum xylitol yield from 300 g/l xylose was 91% at a glucose/xylose feeding ratio of 15%, while the maximum volumetric production rate of xylitol was 3.98 g l-1 h-1 at a glucose/xylose feeding ratio of 20%. Xylitol production was found to decrease markedly as its concentration rose above 250 g/l. In order to accumulate xylitol to 250 g/l, 270 g/l xylose was added in total, at a glucose/xylose feeding ratio of 15%. Under these conditions, a final xylitol production of 251 g/l, which corresponded to a yield of 93%, was obtained from 270 g/l xylose in 55 h.

Using response-surface methodology to evaluate xylitol production by Candida guilliermondii by fed-batch process with exponential feeding rate

Xylitol production by Candida guilliermondii FTI 20037 from sugar cane bagasse hydrolyzate by fed-batch process using exponential feeding rate was evaluated. A factorial design was employed to study the effects of the xylose concentration in the fermenter ( S 0) and the xylose concentration in the feeding solution ( S i ) upon the xylitol/xylose yield coefficient and the xylitol concentration. It was found that none of the variables had a significant effect on the xylose-to-xylitol conversion factor. However, their effects on the xylitol concentration were significant. A response-surface methodology permitted us to obtain a polynomial model for xylitol production ( C P) at 95% confidence level: C P=15.896−1410 S 0-0.487 S i −0.032 S 0 2. The best experimental results achieved for xylitol production was 44 g l −1, with a xylitol yield coefficient of 0.78 g g −1 and a volumetric productivity of 0.62 g l −1 h −1.