Bioconversion Of Xylose Research Articles

Revalorisation of vine trimming wastes using Lactobacillus acidophilus and Debaryomyces hansenii

AbstractBACKGROUND: Mild acid treatments of vine‐shoot trimmings result in the hydrolysis of hemicellulosic sugars that can be utilised by Lactobacillus acidophilus CECT‐4179 (ATCC 832) and Debaryomyces hansenii NRRL Y‐7426 as carbon sources to obtain food additives. Since the high content of glucose in these hydrolysates reduces the effective bioconversion of xylose into xylitol by D. hansenii, the use of Lactobacillus acidophilus, one of the main probiotic species, allows this problem to be solved by the selective consumption of glucose. In order to use both sugars (glucose and xylose), hemicellulosic vine‐shoot trimming hydrolysates can be sequentially fermented by both micro‐organisms.RESULTS: It was found that, in the first step, L. acidophilus generated almost exclusively lactic acid (32.7 g of lactic acid L−1, QLA = 1.363 g L−1 h−1, YLA/S = 0.72 g g−1) by homofermentative degradation of sugars (mainly glucose), and in the second step, the remaining hemicellulosic sugars were transformed primarily into xylitol by Debaryomyces hansenii (31.3 g of xylitol L−1, QXylitol = 0.708 g L−1 h−1, YXylitol/S = 0.66 g g−1). Furthermore, L. acidophilus proved to be a strong cell‐bounded biosurfactant producer. Cell extracts were able to reduce the surface tension (ST) of PBS in 18 mN m−1 units. Lactobacillus acidophilus cells showed no difference in viability before or after PBS extraction of biosurfactants, achieving values of 0.9 × 109 colony‐forming units (CFU) mL−1 in both cases.CONCLUSIONS: These results have made a serious contribution to the re‐evaluation of a useless and pollutant residue, producing a wide range of natural food additives. Copyright © 2008 Society of Chemical Industry

Inhibitory effect of acetic acid on bioconversion of xylose in xylitol by Candida guilliermondii in sugarcane bagasse hydrolysate

Sugarcane bagasse hydrolysate (initial acetic acid concentration = 3.5g/L), was used as a fermentation medium for conversion of xylose into xylitol by the yeast Candida guilliermondii FTI 20037. Acetic acid (2.0g/L) was added to the medium at different times of fermentation, with the aim of evaluating its effects on the bioconversion process. The addition of acetic acid to the medium after 12h of fermentation resulted in the strongest inhibition of the yeast metabolism. In this case, the xylose consumption and cell growth were, respectively, 23.22 and 11.24% lower than when acid was added to the medium at the beginning of fermentation. As a consequence of the inhibitory effect, lower values of the xylitol yield (0.39g/g) and productivity (0.22g/L.h) were observed, corresponding to a reduction of 36 and 48%, respectively, in relation to the values obtained with the addition of acetic acid after other fermentation times. The results obtained allowed to conclude that, under the experimental conditions employed in this work, the inhibitory effect of acetic acid on the xylose-xylitol bioconversion depends on the fermentation time when this acid was added, and not only on its concentration in the medium.

Avaliação de diferentes tipos de carvão ativo na destoxificação de hidrolisado de palha de arroz para produção de xilitol

O hidrolisado hemicelulósico de palha de arroz foi tratado com cinco tipos de carvão ativo (pó e granulado) com o objetivo de remover, por adsorção, compostos tóxicos que podem agir como inibidores no processo de bioconversão de xilose em xilitol, por Candida guilliermondii. Os valores máximos de fator de rendimento em xilitol (Y P/S = 0,67g g-1) e produtividade volumétrica (Q P = 0,61g L-1 h-1) foram atingidos quando o hidrolisado foi tratado com carvão ativo em pó de partículas de tamanho pequeno (0,043mm), baixa granulometria (32% retidos em peneira de 325mesh) e grande área superficial (860m² g-1), características as quais favoreceram a adsorção dos compostos tóxicos.

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 L a=22 h −1).

Use of response surface methodology to evaluate the optimum environmental conditions for the bioconversion of xylose into xylitol in eucalyptus hemicellulosic hydrolysate

AbstractCandida guilliermondii fermented a eucalyptus hemicellulosic hydrolysate that had been treated with calcium oxide, phosphoric acid and activated charcoal. The influences of different xylose and ammonium sulfate concentrations on the production of xylitol were studied. A response surface was obtained with value R2 > 0.97 and p < 0.01. The model estimated a maximum xylitol production of 20.82 g dm−3 in the culture medium supplemented with ammonium sulfate (1.8 g dm−3) and xylose (45.5 g dm−3). Copyright © 2003 Society of Chemical Industry

Xylitol production from high xylose concentration: evaluation of the fermentation in bioreactor under different stirring rates.

To investigate the production of xylitol by the yeast Candida guilliermondii FTI 20037, in a bioreactor, from rice straw hemicellulosic hydrolysate with a high xylose concentration. Batch fermentation was carried out with rice straw hemicellulosic hydrolysate containing about 85 g xylose l(-1), in a stirred-tank bioreactor at 30 degrees C, under aeration of 1.3 vvm (volume of air per volume of medium per min) and different stirring rates (200, 300 and 500 rev min(-1)). The bioconversion of xylose into xylitol by the yeast depended on the stirring rate, the maximum xylitol yield (YP/S = 0.84 g g(-1)) being achieved at 300 rev min-1, with no need to pretreat the hydrolysate for purification. To determine the most adequate oxygen transfer rate is fundamental to improving the xylose-to-xylitol bioconversion by C. guilliermondii. For the microbial production of xylitol to be economically viable, the initial concentration of xylose in the lignocellulosic hydrolysate should be as high as possible, as with high substrate concentrations it is possible to increase the final product concentration. Nevertheless, there are few reports on the use of high xylose concentrations. Considering a process in bioreactor, from rice straw hemicellulosic hydrolysate, this is an innovator work.

Response surface methodology for xylitol production from sugarcane bagasse hemicellulosic hydrolyzate using controlled vacuum evaporation process variables

An effective bioconversion of xylose into xylitol by Candida guilliermondii FTI 20037 requires sugarcane bagasse hydrolyzate with a high level of xylose and a low concentrations of toxic compounds. The composition of the hydrolyzate depends on the vacuum evaporation process variables (pH, temperature, xylose concentration degree and treatment with activated charcoal before or after the process). The influence of these variables on xylitol volumetric productivity was measured using a 2 5−1 fractional factorial central composite design followed by a response surface methodology. The model predicted a xylitol volumetric productivity of 0.350 g/l h in hydrolyzate treated with charcoal (pH 0.92) and subsequently evaporated under vacuum to reach a xylose concentration of 51.0 g/l. Under these conditions, the xylose, acetic acid and arabinose consumption levels were 93, 57 and 13.62%, respectively, and xylitol yield and production were 0.54 g/g and 24.33 g/l, respectively. The final cell concentration was fivefold, the initial value (0.278×10 8 cell/ml). The temperature levels (43–80 °C) tested during vacuum evaporation did not influence xylitol production significantly.

Application of factorial design to the study of xylitol production from eucalyptus hemicellulosic hydrolysate.

This study deals with the bioconversion of xylose into xylitol by Candida guilliermondii FTI 20037 using eucalyptus hemicellulosic hydrolysate obtained by acid hydrolysis. The influence of various parameters (ammonium sulfate, rice bran, pH, and xylose concentration) on the production of xylitol was evaluated. The experiments were based on multivariate statistical concepts, with the application of factorial design techniques to identify the most important variables in the process. The levels of these variables were quantified by the response surface methodology, which permitted the establishment of a significant mathematical model with a coefficient determination of R2 = 0.92. The best results (xylitol = 10.0 g/L, yield factor = 0.2 g/g, and productivity = 0.1 g/[L x h]) were attained with hydrolysate containing ammonium sulfate (1.1 g/L), rice bran (5.0 g/L), and xylose (initial concentration of 60.0 g/L), after 72 h of fermentation. The pH of fermentation was adjusted to 8.0 and the inoculum level utilized was 3 g/L.

Ethanol production from xylose with the yeast Pichia stipitis and simultaneous product recovery by gas stripping using a gas-lift loop fermentor with attached side-arm (GLSA).

The bioconversion of xylose into ethanol with the yeast Pichia stipitis CBS 5773 is inhibited when 20 g/L of ethanol are present in the fermentation broth. In order to avoid this limitation, the fermentation was carried out with simultaneous recovery of product by CO(2) stripping. The fermentation was also improved by attaching a side-arm to the main body of a classical gas-lift loop fermentor. This side-arm increases the liquid circulation, mass transfer, and gas distribution, reducing the amount of oxygen in the inlet gas necessary to perform the fermentation of xylose under microaerobic conditions (K(L)a approximately 16 h(-1)). The continuous stripping of ethanol from the fermentation broth in this new bioreactor system allowed the consumption of higher xylose concentrations than using Erlenmeyer shaker flasks, improved significantly the process productivity and provided a clean ethanol solution by using an ice-cooled condenser system. Finally, a fed-batch fermentation was carried out with a K(L)a = 15.8 h(-1). Starting with 248.2 g of xylose, 237.6 g of xylose was consumed to produce 88.1 g of ethanol which represents 72.6% of the theoretical yield (47.2 g/L of ethanol was recovered in the condenser, while 9.6 g/L remained in the fermentation broth).

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

Xylitol, a functional sweetener, was produced from xylose using Candida tropicalis ATCC 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 .

Simultaneous Bioconversion of Cellulose and Hemicellulose to Ethanol

ABSTRACT: Lignocellulosic materials containing cellulose, hemicellulose, and lignin as their main constituents are the most abundant renewable organic resource present on Earth. The conversion of both cellulose and hemicellulose for production of fuel ethanol is being studied intensively with a view to develop a technically and economically viable bioprocess. The fermentation of glucose, the main constituent of cellulose hydrolyzate, to ethanol can be carried out efficiently. On the other hand, although bioconversion of xylose, the main pentose sugar obtained on hydrolysis of hemicellulose, to ethanol presents a biochemical challenge, especially if it is present along with glucose, it needs to be fermented to make the biomass-to-ethanol process economical. A lot of attention therefore has been focussed on the utilization of both glucose and xylose to ethanol. Accordingly, while describing the advancements that have taken place to get xylose converted efficiently to ethanol by xylose-fermenting organisms, the review deals mainly with the strategies that have been put forward for bioconversion of both the sugars to achieve high ethanol concentration, yield, and productivity. The approaches, which include the use of (1) xylose-fermenting yeasts alone, (2) xylose isomerase enzyme as well as yeast, (3) immobilized enzymes and cells, and (4) sequential fermentation and co-culture process are described with respect to their underlying concepts and major limitations. Genetic improvements in the cultures have been made either to enlarge the range of substrate utilization or to channel metabolic intermediates specifically toward ethanol. These contributions represent real significant advancements in the field and have also been adequately dealt with from the point of view of their impact on utilization of both cellulose and hemicellulose sugars to ethanol.

Xylitol production from Eucalyptus wood hydrolysates extracted with organic solvents

Overliming and extraction of acid hydrolysates of Eucalyptus wood were assayed as pretreatments to improve the bioconversion of xylose into xylitol by Debaryomyces hansenii. Chloroform, trichloroethylene, ethyl acetate, diethyl ether and hexane were utilized for extraction, directly or after overliming of hydrolysates. Under selected conditions, charcoal adsorption of extracted hydrolysates was assayed as a means of enhancing bioconversion. The best pretreatments assayed were extraction of overlimed hydrolysates with ethyl ether, with or without further charcoal adsorption. With these processing schemes, volumetric productivities of xylitol in the region of 1·1 g litre −1 h −1 and product yields near 0·7 g xylitol g −1 xylose were obtained.

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.

Selective removal of acetic acid from hardwood-spent sulfite liquor using a mutant yeast

The hexoses in spent sulfite liquor can be converted to ethanol by yeasts, but conversion to ethanol of the pentose d-xylose in lignocellulosic hydrolysates is inhibited generally by the presence of acetic acid. The feasibility of a yeast-based process for selective removal of the acetic acid in hardwood-spent sulfite liquor was demonstrated. The process depends on the use of a mutant of Saccharomyces cerevisiae that grows on acetic acid but not on d-xylose, d-glucose, d-mannose, or d-fructose. The process could be used to decrease the concentration of acetic acid in hardwood liquor within 24 h to levels that no longer inhibit bioconversion of xylose to ethanol. Indications of the conversion of d-xylose to ethanol in the acetic acid-depleted liquor were the ability of several d-xylose-fermenting yeasts to use essentially all of the sugars and produce as much as 73% of the theoretical amount of ethanol within 24 h. The process might be generally applicable to obviation of acetic acid inhibition effects in ethanol production from hemicellulose hydrolysates.

Influence of media composition on xylitol fermentation by Candida guiliiermondii using response surface methodology

The bioconversion of xylose to xylitol by the yeast Candida guilliermondii FTI 20037 was evaluated under different nutritional conditions using rice straw hemicellulose hydrolysate. Statistical designs were used to determine the fermentation medium composition. Ammonium sulfate and rice bran have been identified as required nutrients in the hydrolysate since there was a significant interaction between them. In the presence of both nutrients, the xylitol yield factor (Yp/s) and volumetric productivities (Qp) were 0.68 g/g and 0.54 g/L.h, respectively.

Fed-batch xylitol production with recombinantXYL-1-expressingSaccharomyees cerevisiae using ethanol as a co-substrate

The bioconversion of xylose into xylitol in fed-batch fermentation with a recombinantSaccharomyces cerevisiae strain, transformed with the xylose-reductase gene ofPichia stipitis, was studied. When only xylose was fed into the fermentor, the production of xylitol continued until the ethanol that had been produced during an initial growth phase on glucose, was depleted. It was concluded that ethanol acted as a redox-balance-retaining co-substrate. The conversion of high amounts of xylose into xylitol required the addition of ethanol to the feed solution. Under O2-limited conditions, acetic acid accumulated in the fermentation broth, causing poisoning of the yeast at low extracellular pH. Acetic acid toxicity could be avoided by either increasing the pH from 4.5 to 6.5 or by more effective aeration, leading to the further metabolism of acetic acid into cell mass. The best xylitol/ethanol yield, 2.4 gg−1 was achieved under O2-limited conditions. Under anaerobic conditions ethanol could not be used as a co-substrate, because the cell cannot produce ATP for maintenance requirements from ethanol anaerobically. The specific rate of xylitol production decreased with increasing aeration. The initial volumetric productivity increased when xylose was added in portions rather than by continuous feeding, due to a more complete saturation of the transport system and the xylose reductase enzyme.