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.     

Fed-batch culture of Candida guilliermondii FTI 20037 for xylitol production from sugar cane bagasse hydrolysate

A fed-batch culture system was used to study xylitol production by Candida guilliermondii FTI 20037 in a synthetic and a sugar cane bagasse hydrolysate medium. The values achieved for xylitol yield and volumetric productivity were, respectively, 0 · 84 g g⁻¹ and 0 · 64 g l⁻¹ h⁻¹ using the synthetic medium and 0 · 78 g g⁻¹ and 0 · 62 g l⁻¹ h⁻¹ using the hydrolysate medium.     

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.     

Kinetic behavior of Candida guilliermondii yeast during xylitol production from Brewer's spent grain hemicellulosic hydrolysate

Brewer's spent grain, the main byproduct of breweries, was hydrolyzed with dilute sulfuric acid to produce a hemicellulosic hydrolysate (containing xylose as the main sugar). The obtained hydrolysate was used as cultivation medium by Candidaguilliermondii yeast in the raw form (containing 20 g/L xylose) and after concentration (85 g/L xylose), and the kinetic behavior of the yeast during xylitol production was evaluated in both media. Assays in semisynthetic media were also performed to compare the yeast performance in media without toxic compounds. According to the results, the kinetic behavior of the yeast cultivated in raw hydrolysate was as effective as in semisynthetic medium containing 20 g/L xylose. However, in concentrated hydrolysate medium, the xylitol production efficiency was 30.6% and 42.6% lower than in raw hydrolysate and semisynthetic medium containing 85 g/L xylose, respectively. In other words, the xylose-to-xylitol bioconversion from hydrolysate medium was strongly affected when the initial xylose concentration was increased; however, similar behavior did not occur from semisynthetic media. The lowest efficiency of xylitol production from concentrated hydrolysate can be attributed to the high concentration of toxic compounds present in this medium, resulting from the hydrolysate concentration process.     

Evaluation of rice straw hemicellulose hydrolysate in the production of xylitol byCandida guilliermondii

Summary Rice straw was used as a lignocellulosic source to provide rich pentose media. By using a well characterized yeast strain,Candida guilliermondii FTI 20037, the hydrolysate obtained was converted to xylitol with an efficiency of 75% and production of 27 g of xylitol per liter in 48 hours. The satisfactory results reported here can be attributed to the low concentrations of toxic components generated throughout the chemical depolymerization of this raw material.     

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.     

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     

Chemostat study of xylitol production by Candida guilliermondii

The mechanism of production of xylitol from xylose by Candida guilliermondii was studied using chemostat cultures and enzymatic assays. The maximum dilution rate in aerobic conditions was 0.34 1/h. No xylitol was produced. Under oxygen-limited conditions xylose uptake was impaired and glycerol accumulated but no xylitol was detected. Under transient oxygen limitation, caused by a gradual decrease in the agitation rate, onset of xylitol, acetate and residual xylose accumulation occurred simultaneously when q _O2 dropped below 25 mmol/C-mmol cell dry weight (CDW) per hour. Ethanol and glycerol started to accumulate when q _O2 dropped below 20 mmol/C-mmol CDW per hour. The highest in vitro enzyme activities were found at the lowest dilution rate studied (0.091/h) under aerobic conditions. The amount of active enzymes or cofactor availability did not limit the rate of xylose consumption. Our results confirm that a surplus of NADH during transient oxygen limitation inhibited the activity of xylitol dehydrogenase which resulted in xylitol accumulation. Phosphoglucoisomerase (E.C. 5.3.1.9.) and glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49) activities suggest re-shuttling of the metabolites into the pentose phosphate pathway. Received: 7 March 2000 / Received revision: 9 June 2000 / Accepted: 18 June 2000     

Kinetic behavior of Candida guilliermondii yeast during xylitol production from highly concentrated hydrolysate

Rice straw hemicellulosic hydrolysate containing a high xylose concentration was used as fermentation medium to evaluate the kinetic behavior of Candida guilliermondii yeast (FTI 20037) during the bioconversion of xylose into xylitol. Assays were conducted first with detoxified and non-detoxified (raw) hydrolysates and semi-synthetic medium in agitated flasks, and second with detoxified hydrolysate in a stirred-tank bioreactor at a given oxygen transfer rate. The results for the agitated flasks showed that in detoxified hydrolysate the xylose-to-xylitol bioconversion by the yeast was as effective as in synthetic medium and 47% higher than in raw hydrolysate. In the stirred-tank bioreactor, the kinetic behavior of the yeast in detoxified hydrolysate was slower, resulting in smaller values of fermentative parameters, probably due to unsuitability of the oxygen transfer rate employed (K_La=22 h⁻¹).     

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 (%)     

Optimization of the pretreatment of rice straw hemicellulosic hydrolyzates for microbial production of xylitol

Hemicellulosic hydrolyzate obtained from rice straw was evaluated to determine if it was a suitable fementation medium for the production of xylitol byCandida mogii ATCC 18364. To obtain xylose selectively from rice straw, it is important to establish rapid hydrolysis conditions that yield xylose-rich substrates. The results of hydrolysis experiments indicated that the optimal reaction conditions for the recovery of xylose from rice straw hemicellulose were obtained using a sulfuric acid concentration of 1.5%, a reaction temperature of 130°C, a reaction time of 20 min and a solid to liquid ratio of 1∶10. Because the fermentation of concentrated acid hydrolyzates can be inhibited by compounds present in the raw material or produced during the hydrolysis process, various methods were tested to determine if they could detoxify the hydrolyzates and thus improve xylitol production. The greatest xylitol yield (0.53 g/g) and volumetric productivity (0.38 g/L·h) were obtained when an overlimed hydrolyzate was treated with activated charcoal.     

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.     

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.     

The influence of aeration and hemicellulosic sugars on xylitol production by Candida tropicalis

The influence of other hemicellulosic sugars (arabinose, galactose, mannose and glucose), oxygen limitation, and initial xylose concentration on the fermentation of xylose to xylitol was investigated using experimental design methodology. Oxygen limitation and initial xylose concentration had considerable influences on xylitol production by Canadida tropicalis ATCC 96745. Under semiaerobic conditions, the maximum xylitol yield was 0.62 g/g substrate, while under aerobic conditions, the maximum volumetric productivity was 0.90 g/l h. In the presence of glucose, xylose utilization was strongly repressed and sequential sugar utilization was observed. Ethanol produced from the glucose caused 50% reduction in xylitol yield when its concentration exceeded 30 g/l. When complex synthetic hemicellulosic sugars were fermented, glucose was initially consumed followed by a simultaneous uptake of the other sugars. The maximum xylitol yield (0.84 g/g) and volumetric productivity (0.49 g/l h) were obtained for substrates containing high arabinose and low glucose and mannose contents.     

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.     

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.     

Xylitol formation by Candida guilliermondii grown in a cane bagasse hemicellulosic hydrolysate: Effect of aeration and inoculum adaptation

The xylose conversion into by Candida guilliermondii was evaluated in sugar cane bagasse hemicellulosic hydrolysate. The effect of air flow rates of 0.4, 0.6 and 0.8 vvm cn xylitol formation was studied. In addition, inoculum previously adapted to the hydrolysate was also tested in the fermentation carried out at 0.6 vvm. The results showed that xylitol production depends markedly on the aeration rate and on the previous adaptation of the yeast to the hydrolysate. When the highest productivity of xylitol was 0.39 g/l × h. However, during the fermentation carried out at an air flow rate of 0.6 vvm with adapted inoculum, the productivity increased to 0.65 g/l × h. Furthermore, the adapted cells performed quite well in the presencel of acetic concentrations of about 4.5 g/l in the medium.     

Influence of k L a on bioconversion of rice straw hemicellulose hydrolysate to xylitol

The effect of overall oxygen mass transfer coefficient (k_La) on the conversion of xylose to xylitol by Candida guilliermondii FTI 20037 was investigated in batch experiments. Rice straw hemicellulose hydrolysate obtained by acid hydrolysis was employed as a xylose-rich medium. The results showed that this bioconversion strongly depended on the aeration rate. The maximum volumetric productivity (0.52 g/l h^у) and the highest xylitol yield (0.73 g/g) were achieved at an overall oxygen mass transfer coefficient of 15 h^у. Under these conditions 80% efficiency in relation to theoretical yield was attained.     

Mixed inhibitions by methanol, furfural and acetic acid on xylitol production by Candida guilliermondii

Xylose production by Candida guilliermondii FTI 20037 was carried out in a synthetic medium in the presence of 0–100 g methanol l^–1, 0–0.7 g furfural l^–1 or 0–1.3 g acetic acid l^–1. Kinetic results show a mixed inhibition mechanism in all three cases. Maximum specific productivity and saturation constant for product formation were, in the absence of inhibition, 3.6 g_P g_X ^–1 h^–1 and 232 g_S l^–1, respectively, while the inhibition constants, K _i and K _i, were 17 and 50 g methanol l^–1, 0.62 and 7.0 g furfural l^–1, 0.69 and 3.5 g acetic acid l^–1, which suggests the following order of inhibition: furfural > acetic acid > methanol.     

Formation of xylitol in Candida guilliermondii 2581 culture

The yeast strain Candida guilliermondii 2581 was chosen for its ability to produce xylitol in media with high concentrations of xylose. The rate of xylitol production at a xylose concentration of 150 g/l is 1.25 g/l per h; the concentration of xylitol after three days of cultivation is 90 g/l; and the relative xylitol yield is 0.6 g per g substrate consumed. The growth conditions were found that resulted in the maximum relative xylitol yield with complete consumption of the sugar: xylose concentration, 150 g/l; pH 6.0; and shaking at 60 rpm. It was shown that the growth under conditions of limited aeration favors the reduction of xylose.