Xylitol production from aspenwood hemicellulose hydrolysate by Candida guilliermondii

The production of xylitol by the yeast Candida guilliermondii was investigated in batch fermentations with aspenwood hemicellulose hydrolysate and compared with results obtained in semi-defined media with a mixture of glucose and xylose. The hemicellulose hydrolysate had to be supplemented by yeast extract and the maximum xylitol yield (0.8 g g^–1) and productivity (0.6 g l^–1 h^–1) were reached by controlling oxygen input.     

Effect of inoculum level on xylitol production from rice straw hemicellulose hydrolysate byCandida guilliermondii

The effect of inoculum level on xylitol production byCandida guilliermondii was evaluated in a rice straw hemicellulose hydrolysate. High initial cell density did not show a positive effect in this bioconversion since increasing the initial cell density from 0.67 g L^–1 to 2.41 g L^–1 decreased both the rate of xylose utilization and xylitol accumulation. The maximum xylitol yield (0.71 g g^–1) and volumetric productivity (0.56 g L^–1 h^–1) were reached with an inoculum level of 0.9 g L^–1. These results show that under appropriate inoculum conditions rice straw hemicellulose hydrolysate can be converted into xylitol by the yeastC. guilliermondii with efficiency values as high as 77% of the theoretical maximum.     

Batch xylitol production from wheat straw hemicellulosic hydrolysate using Candida guilliermondii in a stirred tank reactor

Batch production of xylitol from the hydrolysate of wheat straw hemicellulose using Candida guilliermondii was carried out in a stirred tank reactor (agitation speed of 300 rpm, aeration rate of 0.6 vvm and initial cell concentration of 0.5 g l^–1). After 54 h, xylitol production from 30.5 g xylose l^–1 reached 27.5 g l^–1, resulting in a xylose-to-xylitol bioconversion yield of 0.9 g g^–1 and a productivity of 0.5 g l^–1 h^–1.     

Eucalyptus hydrolysate detoxification with activated charcoal adsorption or ion-exchange resins for xylitol production

Eucalyptus hemicellulosic hydrolysate used for xylitol production by Candida guilliermondii FTI20037 was previously treated either with ion-exchange resins or with activated charcoal adsorption combined with pH adjustment, in order that acetic acid, furfural and hydroxymethylfurfural could be removed. The best results for xylitol yield factor (0.76 g/g) and volumetric productivity (0.68 g/(l h) were attained when a three-fold concentrated hydrolysate was treated with ion-exchange resins. Using activated charcoal combined with pH adjustment for treating a three-fold concentrated hydrolysate resulted in a xylitol yield factor of 0.40 g/g and a volumetric productivity of 0.30 g/(l h). This same treatment applied to a six-fold concentrated hydrolysate resulted in a xylitol yield factor of 0.66 g/g and a volumetric productivity of 0.50 g/(l h).     

Statistical screening method for selection of important variables on xylitol biosynthesis from rice straw hydrolysate by Candida guilliermondii FTI 20037

Fourteen assays were conducted to study the influence of different variables, namely xylose concentration, inoculum level, agitation speed and nutrient supplementation, on xylitol biosynthesis by Candida guilliermondii FTI 20037. The maximum predicted values for xylitol yield (0.65 g g^–1) and xylitol productivity (0.66 g l^–1 h^–1) can be attained with rice straw hydrolysate containing 60 g xylose l^–1 without supplementation of ammonium sulfate, calcium chloride and rice bran extract, using 5 g inoculum l^–1, at 250 rpm. Xylose concentration and inoculum level were selected for further optimization studies.     

Effect of phosphate buffer concentration on the batch xylitol production by Candida guilliermondii

AIMS: To evaluate the effect of phosphate buffer concentration on growth and xylitol production by Candida guilliermondii FTI 20037. METHODS AND RESULTS: Fermentations runs were carried out in batch mode employing semisynthetic medium supplemented with phosphate buffer at different concentrations (from 200 to 600 mmol l(-1)). The xylitol yield (Y(P/S)) and volumetric productivity (Q(P)) were improved when the fermentation medium was supplemented with phosphate buffer at concentration of 600 mmol l(-1). Under this condition (Y(P/S)) and (Q(P)) values were 0.75 g g(-1) and 0.66 g l(-1) h(-1), respectively, whereas in the absence of the phosphate buffer these values decreased to 0.52 g g(-1) and 0.44 g l(-1)h(-1) respectively. CONCLUSIONS: The use of phosphate buffer at 600 mmol l(-1) promoted an easier pH control during shake flasks fermentation of C. guilliermondii. In addition the xylitol yield and productivity were significantly improved in response to the supplementation of potassium phosphate in the medium. The increase in these parameters could be related to both osmotic effect and pH control. SIGNIFICANCE AND IMPACT OF THE STUDY: This approach provided a method for improving the xylitol production from semisynthetic medium by C. guilliermondii, being possible their use as a simple strategy to achieve efficient fermentation processes employing complex medium such as lignocellulosic hydrolysates.     

Influence of Medium Composition on Xylitol Bioproduction from Wheat Straw Hemicellulosic Hydrolysate

Summary Xylose-to-xylitol batch bioconversions from wheat straw hemicellulosic hydrolysate were carried out in Erlenmeyer flasks in order to assess the influence of medium composition (hydrolysate concentration, supplementation with ammonium sulphate, calcium chloride and rice bran extract, and initial pH) on xylitol production, productivity and yield. By using the screening design and the response surface methodologies, the statistically significant variables influencing the bioconversion were selected and linear models were fitted to the experimental data. According to the results, the best conditions to perform the bioconversion consisted in using a threefold concentrated hydrolysate supplemented with ammonium sulphate (1.0 g/l) and rice bran extract (5.0 g/l), whose pH was adjusted to 6.0 prior to inoculation. Under these conditions, a xylitol production of 24.17 g/l was observed after 72 h of fermentation, resulting in a productivity of 0.34 g/l h and in a bioconversion yield of 0.49 g/g.     

Evaluation of xylitol production from corn cob hemicellulose hydrolysate by Candida parapsilosis

Candida parapsilosis was grown for 59 h in a medium containing corn cob hydrolysate consisting of 50 g xylose l^–1, 3.0 g glucose l^–1, 2.0 g arabinose l^–1, and 0.9 g acetic acid l^–1. A biomass of 9.1 g l^–1 was produced with 36 g xylitol l^–1 and 2.5 g ethanol l^–1. In a medium containing 50 g xylose l^–1 instead of corn cob hydrolysate, the concentrations of cells, xylitol, and ethanol were 8.6 g l^–1, 33 g l^–1, and 0.2 g l^–1, respectively. The differences between two cultures were due to the glucose and arabinose in the corn cob hydrolysate stimulating growth and the low concentration of acetic acid stimulating xylitol production.     

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. Journal of Industrial Microbiology & Biotechnology (2002) 29, 16–19 doi:10.1038/sj.jim.7000257 Received 15 October 2001/ Accepted in revised form 30 March 2002     

Environmental parameters affecting xylitol production from sugar cane bagasse hemicellulosic hydrolyzate by Candida guilliermondii

The bioconversion of xylose to xylitol by Candida guilliermondii FTI 20037 cultivated in sugar cane bagasse hemicellulosic hydrolyzate was influenced by cell inoculum level, age of inoculum and hydrolyzate concentration. The maximum xylitol productivity (0.75 g L⁻¹ h⁻¹) occurred in tests carried out with hydrolyzate containing 54.5 g L⁻¹ of xylose, using 3.0 g L⁻¹ of a 24-h-old inoculum. Xylitol productivity and cell concentration decreased with hydrolyzate containing 74.2 g L⁻¹ of xylose. Received 02 February 1996/ Accepted in revised form 15 November 1996     

Increased xylitol production rate during long-term cell recycle fermentation of Candida tropicalis

Long-term cell recycle fermentations of Candida tropicalis were performed over 14 rounds of fermentation. The average xylitol concentrations, fermentation times, volumetric productivities and product yields for 14 rounds were 105 g l^–1, 333 h, 4.4 g l^–1 h^–1 and 78%, respectively, in complex medium; and 110 g l^–1, 284 h, 5.4 g l^–1 h^–1 and 81%, respectively, in a chemically defined medium. These productivities were 1.7 and 2.4 times those with batch fermentation in the complex and chemically defined media, respectively. The xylitol yield from xylose with cell recycle fermentation using the chemically defined medium was 81% (w/w), which was 7% greater than the xylitol yield with batch fermentation (74%); both modes of fermentation gave the same yield using the complex medium. These results suggest that the chemically defined medium is more suitable for production of xylitol than complex medium.     

Glucose repression of xylitol production in Candida tropicalis mixed-sugar fermentations

Glucose repressed xylose utilization inCandida tropicalis pre-grown on xylose until glucose reached approximately 0–5 g l^–1. In fermentations consisting of xylose (93 g l^–1) and glucose (47 g l^–1), xylitol was produced with a yield of 0.65 g g^–1 and a specific rate of 0.09 g g^–1 h^–1, and high concentrations of ethanol were also produced (25 g l^–1). If the initial glucose was decreased to 8 g l^–1, the xylitol yield (0.79 g g^–1) and specific rate (0.24 g g^–1 h^–1) increased with little ethanol formation (<5 g l^–1). To minimize glucose repression, batch fermentations were performed using an aerobic, glucose growth phase followed by xylitol production. Xylitol was produced under O₂ limited and anaerobic conditions, but the specific production rate was higher under O₂ limited conditions (0.1–0.4 vs. 0.03 g g^–1 h^–1). On-line analysis of the respiratory quotient defined the time of xylose reductase induction.     

Effects of environmental conditions on production of xylitol byCandida boidinii

Candida boidinii NRRL Y-17213 produced more xylitol thanC. magnolia (NRRL Y-4226 and NRRL Y-7621),Debaryomyces hansenii (C-98 M-21, C-56 M-9 and NRRL Y-7425), orPichia (Hansenula) anomala (NRRL Y-366). WithC. boidinii, highest xylitol productivity was at pH 7 but highest yield was at pH 8, using 5 g urea and 5 g Casamino acids/I. Decreasing the aeration rate decreased xylose consumption and cell growth but increased the xylitol yield. When an initial cell density of 5.1 g/l was used instead of 1.3 g/l, xylitol yield and the specific xylitol production rate doubled. Substrate concentration had the greatest effect on xylitol production; increasing xylose concentration 7.5-fold (to 150 g/l) gave a 71-fold increase in xylitol production (53 g/l) and a 10-fold increase in xylitol/ethanol ratio. The highest xylitol yield (0.47 g/g), corresponding to 52% of the theoretical yield, was obtained with 150 g xylose/l after 14 days. Xylose at 200 g/l inhibited xylitol production.E. Vandeska and S. Kuzmanova were and S. Amartey and T. Jeffries are with the Forest Products Laboratory, Institute for Microbial and Biochemical Technology, 1 Gifford Pinchot Drive, Madison, WI 53703, USA. E. Vandeska and S. Kuzmanova are now with the Faculty of Technology and Metallurgy, Rudjer Boskovic 16, 91000 Skopje, Macedonia     

Production of xylitol from Candida tropicalis by using an oxidation-reduction potential-stat controlled fermentation

An on-line device, ORP (oxidation-reduction potential)-stat, was used to control glucose-feeding for enhancing xylitol conversion from D-xylose during an oxygen-limited fermentation by Candida tropicalis. The fermentation was carried out in a 5 l jar fermenter. After glucose in the medium was depleted, a switching to a limited aeration and feeding glucose controlled by ORP-stat was performed. The maximum xylitol yield was obtained under a condition at an ORP of -180 mV and at an aeration rate of 0.2 l min(-1).     

A model of xylitol production by the yeast Candida mogii

Production of xylitol from xylose in batch fermentations of Candida mogii ATCC 18364 is discussed in the presence of glucose as the cosubstrate. Various initial ratios of glucose and xylose concentrations are assessed for their impact on yield and rate of production of xylitol. Supplementation with glucose at the beginning of the fermentation increased the specific growth rate, biomass yield and volumetric productivity of xylitol compared with fermentation that used xylose as the sole carbon source. A mathematical model is developed for eventual use in predicting the product formation rate and yield. The model parameters were estimated from experimental observations, using a genetic algorithm. Batch fermentations, which were carried out with xylose alone and a mixture of xylose and glucose, were used to validate the model. The model fitted well with the experimental data of cell growth, substrate consumption and xylitol production.     

Immobilized cells cultivated in semi-continuous mode in a fluidized bed reactor for xylitol production from sugarcane bagasse

Summary Xylitol production from sugarcane bagasse hemicellulosic hydrolyzate was evaluated in a fluidized bed reactor operated in semi-continuous mode, using cells immobilized on porous glass. The fermentative process was performed during five successive cycles of 72 h each one. The lowest xylitol production occurred in the first cycle, where a high cell concentration (12 g l⁻¹) was observed. In the subsequent cycles the xylitol concentration was ever increasing due to the cells adaptation to the medium. In the last one, 18 g xylitol l⁻¹ was obtained with a yield factor of 0.44 g g⁻¹ and volumetric productivity of 0.32 g l⁻¹ h⁻¹.     

Production of xylitol by Candida peltata

Candida peltata NRRL Y-6888 to ferment xylose to xylitol was evaluated under different fermentation conditions such as pH, temperature, aeration, substrate concentration and in the presence of glucose, arabinose, ethanol, methanol and organic acids. Maximum xylitol yield of 0.56 g g⁻¹ xylose was obtained when the yeast was cultivated at pH 6.0, 28°C and 200 rpm on 50 g L⁻¹ xylose. The yeast produced ethanol (0.41 g g⁻¹ in 40 h) from glucose (50 g L⁻¹) and arabitol (0.55 g g⁻¹ in 87 h) from arabinose (50 g L⁻¹). It preferentially utilized glucose > xylose > arabinose from mixed substrates. Glucose (10 g L⁻¹), ethanol (7.5 g L⁻¹) and acetate (5 g L⁻¹) inhibited xylitol production by 61, 84 and 68%, respectively. Arabinose (10 g L⁻¹) had no inhibitory effect on xylitol production. Received 24 December 1998/ Accepted in revised form 18 March 1999     

The behavior of key enzymes of xylose metabolism on the xylitol production by <Emphasis Type="Italic">Candida guilliermondii</Emphasis> grown in hemicellulosic hydrolysate

A variety of raw materials have been used in fermentation process. This study shows the use of rice straw hemicellulosic hydrolysate, as the only source of nutrient, to produce high added-value products. In the present work, the activity of the enzymes xylose reductase (XR); xylitol dehydrogenase (XD); and glucose-6-phosphate dehydrogenase (G6PD) during cultivation of Candida guilliermondii on rice straw hemicellulosic hydrolysate was measured and correlated with xylitol production under different pH values (around 4.5 and 7.5) and initial xylose concentration (around 30 and 70 g l(-1)). Independent of the pH value and xylose concentration evaluated, the title of XD remained constant. On the other hand, the volumetric activity of G6PD increased whereas the level of XR decreased when the initial xylose concentration was increased from 30 to 70 g l(-1). The highest values of xylitol productivity (Q (P) approximately 0.40 g l(-1)) and yield factor (Y (P/S) approximately 0.60 g g(-1)) were reached at highest G6PD/XR ratio and lowest XR/XD ratio. These results suggest that NADPH concentrations influence the formation of xylitol more than the activity ratios of the enzymes XR and XD. Thus, an optimal rate between G6PD and XR must be reached in order to optimize the xylitol production.     

Analysis and optimization of a two-substrate fermentation for xylitol production using Candida tropicalis

Xylitol, a functional sweetener, was produced from xylose using Candida tropicalisATCC 13803. A two-substrate fermentation was designed in order to increase xylitol yield and volumetric productivity. Glucose was used initially for cell growth followed by conversion of xylose to xylitol without cell growth and by-product formation after complete depletion of glucose. High glucose concentrations increased volumetric productivity by reducing conversion time due to high cell mass, but also led to production of ethanol, which, in turn, inhibited cell growth and xylitol production. Computer simulation was undertaken to optimize an initial glucose concentration using kinetic equations describing rates of cell growth and xylose bioconversion as a function of ethanol concentration. Kinetic constants involved in the equations were estimated from the experimental results. Glucose at 32 g L⁻¹ was estimated to be an optimum initial glucose concentration with a final xylose concentration of 86 g L⁻¹ and a volumetric productivity of 5.15 g-xylitol L⁻¹ h⁻¹. The two-substrate fermentation was performed under optimum conditions to verify the computer simulation results. The experimental results were in good agreement with the predicted values of simulation with a xylitol yield of 0.81 g-xylitol g-xylose⁻¹ and a volumetric productivity of 5.06 g-xylitol L⁻¹ h⁻¹. Received 16 June 1998/ Accepted in revised form 28 February 1999