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.     

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.     

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.     

Hydrolysate detoxification with activated charcoal for xylitol production by Candida guilliermondii

A detoxification method using activated charcoal with concentrated rice straw hemicellulosic hydrolysate improved the conversion of xylose to xylitol by the yeast Candida guilliermondii by 22%. This was achieved when the hydrolysate:charcoal ratio was 40 g g^–1, resulting in removal of 27% of phenolic compounds. Under this condition, the xylitol yield factor (0.72 g g^–1) and volumetric productivity (0.61 g l^–1 h^–1) were close to those attained in a semi-defined medium simulating hydrolysate sugars.     

A study on xylitol production from sugarcane bagasse hemicellulosic hydrolysate by Ca-alginate entrapped cells in a stirred tank reactor

Candida guilliermondii FTI 20037 cells were entrapped in Ca-alginate beads and used for xylitol production from sugarcane bagasse hemicellulosic hydrolysate in a stirred tank reactor (STR). Screening design and response surface methodologies were used to determine adequate cultivation conditions for this fermentation system. Quadratic models were fitted to the experimental data by regression analysis, considering the yield (Y_P/S) and the productivity (Q_P) of the xylose-to-xylitol bioconversion as dependent variables. Using a five-fold concentrated hydrolysate, air flowrate of 1.30 l/min, agitation speed of 300 rpm, initial cell concentration of 1.4 g/l and value 6.0 for the initial pH of the fermentation medium resulted in a xylitol production of 47.5 g/l after 120 h of fermentation, corresponding to a Y_P/S of 0.81 g/g and to a Q_P of 0.40 g/l h.     

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     

Batch xylitol production by<Emphasis Type="Italic"> Candida guilliermondii</Emphasis> FTI 20037 from sugarcane bagasse hemicellulosic hydrolyzate at controlled pH values

Batch fermentation of sugarcane bagasse hemicellulosic hydrolyzate by the yeast Candida guilliermondii FTI 20037 was performed using controlled pH values (3.5, 5.5, 7.5). The maximum values of xylitol volumetric productivity (Q _p=0.76 g/l h) and xylose volumetric consumption (Q _s=1.19 g/l h) were attained at pH 5.5. At pH 3.5 and 7.5 the Q _p value decreased by 66 and 72%, respectively. Independently of the pH value, Y _x/s decreased with the increase in Y _p/s suggesting that the xylitol bioconversion improves when the cellular growth is limited. At the highest pH value (7.5), the maximum specific xylitol production value was the lowest (q _pmax=0.085 g/l h.), indicating that the xylose metabolism of the yeast was diverted from xylitol formation to cell growth.List of symbols P _max xylitol concentration (g/l) - Q _x volumetric cell production rate (g/l h) - Q _s volumetric xylose uptake rate (g/l h) - Q _p volumetric xylitol production rate (g/l h) - q _pmax specific xylitol production (g/g h) - q _smax specific xylose uptake rate (g/g h) - _max specific cell growth rate (h^–1) - Y _p/s xylitol yield coefficient, g xylitol per g xylose consumed (g/g) - Y _p/x xylitol yield coefficient, g xylitol per g dry cell mass produced (g/g) - Y _x/s cell yield coefficient, g dry cell mass per g xylose consumed (g/g) - _cell percentage of the cell yield from the theoretical value (%) - _xylitol percentage of xylitol yield from the theoretical value (%)     

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.     

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.     

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.     

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.     

Comparison of different pretreatments in ethanol fermentation using corn cob hemicellulosic hydrolysate with Pichia stipitis and Candida shehatae

A hemicellulosic hydrolysate was prepared with 0.3 M H₂SO₄ at 98 °C for 1 h. The total initial reducing sugar was maintained at 45 g l^–1 by synthetic xylose supplementation. The seven detoxification methods were employed including either the single addition of solid CaO (to pH 10 or 6) or its combinations with zeolite shaking. Over-liming gave the hydrolysate that was most completely fermented by Pichia stipitis and Candida shehatae at 30 °C, pH 6, among the tested methods.     

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.     

Butanol production from wheat straw hydrolysate using <Emphasis Type="Italic">Clostridium beijerinckii</Emphasis>

In these studies, butanol (acetone butanol ethanol or ABE) was produced from wheat straw hydrolysate (WSH) in batch cultures using Clostridium beijerinckii P260. In control fermentation 48.9 g L⁻¹ glucose (initial sugar 62.0 g L⁻¹) was used to produce 20.1 g L⁻¹ ABE with a productivity and yield of 0.28 g L⁻¹ h⁻¹ and 0.41, respectively. In a similar experiment where WSH (60.2 g L⁻¹ total sugars obtained from hydrolysis of 86 g L⁻¹ wheat straw) was used, the culture produced 25.0 g L⁻¹ ABE with a productivity and yield of 0.60 g L⁻¹ h⁻¹ and 0.42, respectively. These results are superior to the control experiment and productivity was improved by 214%. When WSH was supplemented with 35 g L⁻¹ glucose, a reactor productivity was improved to 0.63 g L⁻¹ h⁻¹ with a yield of 0.42. In this case, ABE concentration in the broth was 28.2 g L⁻¹. When WSH was supplemented with 60 g L⁻¹ glucose, the resultant medium containing 128.3 g L⁻¹ sugars was successfully fermented (due to product removal) to produce 47.6 g L⁻¹ ABE, and the culture utilized all the sugars (glucose, xylose, arabinose, galactose, and mannose). These results demonstrate that C. beijerinckii P260 has excellent capacity to convert biomass derived sugars to solvents and can produce over 28 g L⁻¹ (in one case 41.7 g L⁻¹ from glucose) ABE from WSH. Medium containing 250 g L⁻¹ glucose resulted in no growth and no ABE production. Mixtures containing WSH + 140 g L⁻¹ glucose (total sugar approximately 200 g L⁻¹) showed poor growth and poor ABE production. Mention of trade names or commercial products in this article is solely for the purpose of providing scientific information and does not imply recommendation or endorsement by the United States Department of Agriculture.     

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).     

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.     

Fermentation of xylose and rice straw hydrolysate to ethanol byCandida shehatae NCL-3501

Candida shehatae NCL-3501 utilized glucose and xylose efficiently in batch cultures. The specific rate of ethanol production was higher with mixtures of glucose and xylose (0.64–0.83 g g^–1 cells d^–1) compared to that with individual sugars (0.38–0.58 g g^–1 cells d^–1). Although the optimum temperature for growth was 30°C, this strain grew and produced appreciable levels of ethanol at 45°C. A stable ethanol yield (0.40–0.43 g g^–1 substrate utilized) was obtained between 10 g L^–1 and 80 g L^–1 of initial xylose concentration. Conversion efficiency was further improved by immobilization of the cells in calcium alginate beads. Free or immobilized cells ofC. shehatae NCL-3501 efficiently utilized sugars present in rice straw hemicellulose hydrolysate, prepared by two different methods, within 48 h. Ethanol yields of 0.45 g g^–1 and 0.5 g g^–1 from autohydrolysate, and 0.37 g g^–1 from acid hydrolysate were produced by free and immobilized cells, respectively.     

Study of microencapsulated urease in a continuous feed,stirred tank reactor

Urease, (urea amidohydrolase, EC 3.5.1.5) co-encapsulated with haemoglobin in cellulose nitrate membranes was found to exhibit apparent Michaelis-Menten kinetics; however, a steadily increasing apparent Michaelis-Menten constant over the lifetime of the preparation was observed. The activity of the enzyme in a continuous feed stirred tank reactor (CSTR) was investigated and correlated with a mathematical model derived from basic Michaelis-Menten kinetics. Plots relating substrate conversion to feed substrate concentration and tank reactor capacity were constructed and found to be accurate to less than 15% error under the experimental conditions studied.