Fermentation of <Emphasis Type="SmallCaps">d</Emphasis>-glucose and <Emphasis Type="SmallCaps">d</Emphasis>-xylose mixtures by <Emphasis Type="Italic">Candida tropicalis</Emphasis> NBRC 0618 for xylitol production

The fermentation of d-glucose and d-xylose mixtures by the yeast Candida tropicalis NBRC 0618 has been studied under the most favourable operation conditions for the culture, determining the most adequate initial proportion in these sugars for xylitol production. In all the experiments a synthetic culture medium was used, with an initial total substrate concentration of 25 g L⁻¹, a constant pH of 5.0 and a temperature of 30 °C. From the experimental results, it was deduced that the highest values of specific rates of production and of overall yield in xylitol were achieved for the mixtures with the highest percentage of d-xylose, specifically in the culture with the initial d-glucose and d-xylose concentrations of 1 and 24 g L⁻¹, respectively, with an overall xylitol yield of 0.28 g g⁻¹. In addition, the specific rates of xylitol production declined over the time course of the culture and the formation of this bioproduct was favoured by the presence of small quantities of d-glucose. The sum of the overall yield values in xylitol and ethanol for all the experiments ranged from 0.26 to 0.56 g bioproduct/g total substrate.     

Xylose utilisation by recombinant strains of Saccharomyces cerevisiae on different carbon sources

Autoselective xylose-utilising strains of Saccharomyces cerevisiae expressing the xylose reductase (XYL1) and xylitol dehydrogenase (XYL2) genes of Pichia stipitis were constructed by replacing the chromosomal FUR1 gene with a disrupted fur1::LEU2 allele. Anaerobic fermentations with 80 g l⁻¹ d-xylose as substrate showed a twofold higher consumption of xylose in complex medium compared to defined medium. The xylose consumption rate increased a further threefold when 20 g l⁻¹ d-glucose or raffinose was used as co-substrate together with 50 g l⁻¹ d-xylose. Xylose consumption was higher with raffinose as co-substrate than with glucose (85% versus 71%, respectively) after 82 h fermentations. A high initial ethanol concentration and moderate levels of glycerol and acetic acid accompanied glucose as co-substrate, whereas the ethanol concentration gradually increased with raffinose as co-substrate with no glycerol and much less acetic acid formation. Received: 12 March 1999 / Received revision: 31 June 1999 / Accepted: 5 July 1999     

Screening of yeasts for production of xylitol fromd-xylose and some factors which affect xylitol yield inCandida guilliermondii

Summary The ability to convertd-xylose to xylitol was screened in 44 yeasts from five genera. All but two of the strains produced some xylitol with varying rates and yields. The best xylitol producers were localized largely in the speciesCandida guilliermondii andC. tropicalis. Factors affecting xylitol production by a selectedC. guilliermondii strain, FTI-20037, were investigated. The results showed that xylitol yield by this strain was affected by the nitrogen source. Yield was highest at 30–35°C, and could be increased with decreasing aeration rate. Using high cell density and a defined medium under aerobic conditions, xylitol yield byC. guilliermondii FTI-20037 from 104 g/ld-xylose was found to be 77.2 g/l. This represented a yield of 81% of the theoretical value, which was computed to be 0.9 mol xylitol per mold-xylose.Issued as NRCC publication No. 28798.     

Xylose reductase activity in <Emphasis Type="Italic">Debaryomyces hansenii</Emphasis> UFV-170 cultivated in semi-synthetic medium and cotton husk hemicellulose hydrolyzate

To develop a new enzymatic xylose-to-xylitol conversion, deeper knowledge on the regulation of xylose reductase (XR) is needed. To this purpose, a new strain of Debaryomyces hansenii (UFV-170), which proved a promising xylitol producer, was cultivated in semi-synthetic media containing different carbon sources, specifically three aldo-hexoses (d-glucose, d-galactose and d-mannose), a keto-hexose (d-fructose), a keto-pentose (d-xylose), three aldo-pentoses (d-arabinose, l-arabinose and d-ribose), three disaccharides (maltose, lactose and sucrose) and a pentitol (xylitol). The best substrate was lactose on which cell concentration reached about 20 g l⁻¹ dry weight (DW), while the highest specific growth rates (0.58–0.61 h⁻¹) were detected on lactose, d-mannose, d-glucose and d-galactose. The highest specific activity of XR (0.24 U mg⁻¹) was obtained in raw extracts of cells grown on d-xylose and harvested in the stationary growth phase. When grown on cotton husk hemicellulose hydrolyzates, cells exhibited XR activities five to seven times higher than on semi-synthetic media.     

Supplementation requirements of brewery’s spent grain hydrolysate for biomass and xylitol production by Debaryomyces hansenii CCMI 941

The effect of nutrient supplementation of brewery’s spent grain (BSG) hydrolysates was evaluated with respect to biomass and xylitol production by Debaryomyces hansenii. For optimal biomass production, supplementation of full-strength BSG hydrolysates required only phosphate (0.5 g l⁻¹ KH₂PO₄), leading to a biomass yield and productivity of 0.60 g g⁻¹ monosaccharides and 0.55 g l⁻¹ h⁻¹, respectively. Under the conditions studied, no metabolic products other than CO₂ and biomass were identified. For xylitol production, fourfold and sixfold concentrated hydrolysate-based media were used to assess the supplementation effects. The type of nutrient supplementation modulated the ratio of total polyols/total extracellular metabolites as well as the xylitol/arabitol ratio. While the former varied from 0.8 to 1, the xylitol/arabitol ratio reached a maximum value of 2.6 for yeast extract (YE)-supplemented hydrolysates. The increase in xylitol productivity and yield was related to the increase of the percentage of consumed xylose induced by supplementation. The best xylitol yield and productivity were found for YE supplementation corresponding to 0.55 g g⁻¹ and 0.36 g l⁻¹ h⁻¹, respectively. In sixfold concentrated hydrolysates, providing that the hydrolysate was supplemented, the levels of xylitol produced were similar or higher than those for arabitol. Xylitol yield exhibited a further increase in the sixfold hydrolysate supplemented with trace elements, vitamins and minerals to 0.65 g g⁻¹, albeit the xylitol productivity was somewhat lower. The effect of using activated charcoal detoxification in non-supplemented versus supplemented sixfold hydrolysates was also studied. Detoxification did not improve polyols formation, suggesting that the hemicellulose-derived inhibitor levels present in concentrated BSG hydrolysates are well tolerated by D. hansenii.     

Simultaneous utilization of D-cellobiose, D-glucose, and D-xylose by recombinant Corynebacterium glutamicum under oxygen-deprived conditions

Corynebacterium glutamicum R was metabolically engineered to broaden its sugar utilization range to d-xylose and d-cellobiose contained in lignocellulose hydrolysates. The resultant recombinants expressed Escherichia coli xylA and xylB genes, encoding d-xylose isomerase and xylulokinase, respectively, for d-xylose utilization and expressed C. glutamicum R bglF ^317A and bglA genes, encoding phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) β-glucoside-specific enzyme IIBCA component and phospho-β-glucosidase, respectively, for d-cellobiose utilization. The genes were fused to the non-essential genomic regions distributed around the C. glutamicum R chromosome and were under the control of their respective constitutive promoter trc and tac that permitted their expression even in the presence of d-glucose. The enzyme activities of resulting recombinants increased with the increase in the number of respective integrated genes. Maximal sugar utilization was realized with strain X5C1 harboring five xylA–xylB clusters and one bglF ^317A –bglA cluster. In both d-cellobiose and d-xylose utilization, the sugar consumption rates by genomic DNA-integrated strain were faster than those by plasmid-bearing strain, respectively. In mineral medium containing 40 g l⁻¹ d-glucose, 20 g l⁻¹ d-xylose, and 10 g l⁻¹ d-cellobiose, strain X5C1 simultaneously and completely consumed these sugars within 12 h and produced predominantly lactic and succinic acids under growth-arrested conditions.     

Betaine Tripled the Volumetric Productivity of d(-)-lactate by Escherichia coli Strain SZ132 in Mineral Salts Medium

Osmotic stress restricts glycolytic flux, growth (rate and yield), d-lactate productivity, and d-lactate tolerance in Escherichia coli B strain SZ132 during batch fermentation in mineral salts medium with 10% (w/v) sugar. Addition of 1 mm betaine, a non-metabolized protective osmolyte, doubled cell yield, increased specific productivity of d-lactate and glycolytic flux by 50%, and tripled volumetric productivity (from 8.6 to 25.7 mmol l⁻¹ h⁻¹; 0.8 to 2.3 g l⁻¹ h⁻¹). Glycolytic flux and specific productivity in mineral salts medium with betaine exceeded that in Luria broth, substantially eliminating the need for complex nutrients during d-lactate production. In mineral salts medium supplemented with betaine, SZ132 produced approximately 1 mol d-lactate (90 g) per 100 g sugar (glucose or sucrose). Revisions requested 17 January 2006; Revisions received 7 February 2006     

Characterization of ribose-5-phosphate isomerase of <Emphasis Type="Italic">Clostridium thermocellum</Emphasis> producing <Emphasis Type="SmallCaps">d</Emphasis>-allose from <Emphasis Type="SmallCaps">d</Emphasis>-psicose

The rpiB gene, encoding ribose-5-phosphate isomerase (RpiB) from Clostridium thermocellum, was cloned and expressed in Escherichia coli. RpiB converted d-psicose into d-allose but it did not convert d-xylose, l-rhamnose, d-altrose or d-galactose. The production of d-allose by RpiB was maximal at pH 7.5 and 65°C for 30 min. The half-lives of the enzyme at 50°C and 65°C were 96 h and 4.7 h, respectively. Under stable conditions of pH 7.5 and 50°C, 165 g d-allose l⁻¹ was produced without by-products from 500 g d-psicose l⁻¹ after 6 h.     

L-Arabinose metabolism in Candida arabinofermentans PYCC 5603T and Pichia guilliermondii PYCC 3012: influence of sugar and oxygen on product formation

l-Arabinose utilization by the yeasts Candida arabinofermentans PYCC 5603^T and Pichia guilliermondii PYCC 3012 was investigated in aerobic batch cultures and compared, under similar conditions, to d-glucose and d-xylose metabolism. At high aeration levels, only biomass was formed from all the three sugars. When oxygen became limited, ethanol was produced from d-glucose, demonstrating a fermentative pathway in these yeasts. However, pentoses were essentially respired and, under oxygen limitation, the respective polyols accumulated—arabitol from l-arabinose and xylitol from d-xylose. Different l-arabinose concentrations and oxygen conditions were tested to better understand l-arabinose metabolism. P. guilliermondii PYCC 3012 excreted considerably more arabitol from l-arabinose (and also xylitol from d-xylose) than C. arabinofermentans PYCC 5603^T. In contrast to the latter, P. guilliermondii PYCC 3012 did not produce any traces of ethanol in complex l-arabinose (80 g/l) medium under oxygen-limited conditions. Neither sustained growth nor active metabolism was observed under anaerobiosis. This study demonstrates, for the first time, the oxygen dependence of metabolite and product formation in l-arabinose-assimilating yeasts.     

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.     

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     

Xylitol conversion by fermentation using five yeast strains and polyelectrolyte-assisted ultrafiltration

Batch fermentations for xylitol production were conducted using Candida boidinii (BCRC 21432), C. guilliermondii (BCRC 21549), C. tropicalis (BCRC 20520), C. utilis (BCRC 20334), and P. anomala (BCRC 21359) together with a mixture of sugars simulating lignocellulosic hydrolysates as the carbon source. C. tropicalis had the highest bioconversion yield (Y_P/S) of 0.79 g g⁻¹ (g xylitol·g xylose⁻¹) over 48 h. Additional fermentations with C. tropicalis achieved Y_P/S values of 0.6 and 0.39 g g⁻¹ after 96 and 72 h using urea and soybean meal as the nitrogen sources, respectively. Ethanol and arabitol were also produced in all fermentation. Xylitol in the fermentation broth was recovered by cross-flow ultrafiltration. With prior application of 2 mg polydiallyl dimethylammonium chloride l⁻¹ on the membrane surface, protein in the permeate was reduced from 7.1 to 1.5 mg l⁻¹ after 2 h.     

Production of <Emphasis Type="SmallCaps">d</Emphasis>-lactic acid from sugarcane molasses,sugarcane juice and sugar beet juice by <Emphasis Type="Italic">Lactobacillus delbrueckii</Emphasis>

Lactobacillus delbrueckii was grown on sugarcane molasses, sugarcane juice and sugar beet juice in batch fermentation at pH 6 and at 40°C. After 72 h, the lactic acid from 13% (w/v) sugarcane molasses (119 g total sugar l⁻¹) and sugarcane juice (133 g total sugar l⁻¹) was 107 g l⁻¹ and 120 g l⁻¹, respectively. With 10% (w/v) sugar beet juice (105 g total sugar l⁻¹), 84 g lactic acid l⁻¹ was produced. The optical purities of d-lactic acid from the feedstocks ranged from 97.2 to 98.3%.     

Characterization of a xylitol dehydrogenase and a d-arabitol dehydrogenase from the thermo- and acidophilic red alga Galdieria sulphuraria

Galdieria sulphuraria (Galdieri) Merola can grow heterotrophically on at least ten different polyols. We investigated their metabolic path to glycolysis/gluconeogenesis and identified two NAD-dependent polyol dehydrogenases. Activity of other enzymes metabolizing mannitol or sorbitol could not be detected. The two dehydrogenases had a broad substrate specificity and were termed xylitol dehydrogenase (EC 1.1.1.14; substrate specificity: xylitol > d-sorbitol > d-mannitol > l-arabitol) and d-arabitol dehydrogenase (EC 1.1.1.11; substrate specificity: d-arabitol > l-fucitol > d-mannitol > d-threitol) according to the substrate with the lowest K _m value. The xylitol dehydrogenase was stable during purification. In contrast, the d-arabitol dehydrogenase was thermolabile and depended on divalent ions for stability and activity, preferentially Mn²⁺ and Ni²⁺. The molecular mass of the xylitol dehydrogenase was estimated to be 295 kDa by size-exclusion chromatography and 220 kDa by rate-sedimentation centrifugation. The d-arabitol dehydrogenase had a molecular mass of 105 kDa as determined by rate-sedimentation centrifugation. The specific activity of both enzymes increased about fourfold when cells were transferred from autotrophic to heterotrophic conditions regardless of whether sugars or polyols were supplied as substrates. The significance of polyol metabolism in Galdieria sulphuraria with regard to the natural habitat of the alga is discussed. Received: 15 January 1997 / Accepted: 12 February 1997     

Microbial production of xylitol from D-xylose using Candida tropicalis

Candida tropicalis DSM 7524 was used to produce xylitol from d-xylose. The fermentation conditions were optimized during continuous cultivation. The strain employed showed no great dependence upon temperature in a range between 30° C and 37° C. It achieved its best yield of xylitol from d-xylose at a pH value of 2.5. Such low pH values allow non sterile cultivation, which is a major economic factor. With an oxygen uptake rate of 0.8–1 ml oxygen per litre culture medium, the C. tropicalis produce xylitol at a yield of between 77% and 80% of the theoretical value. Higher yeast extract concentrations prevent the conversion of d-xylose into xylitol. d-xylose acts as a growth inhibitor in higher concentrations. The maximum xylitol yield was reached at a d-xylose concentration of around 100 g/l. In a non sterile batch culture with substrate shift 220 g/l xylitol were produced from 300 g/l d-xylose at a xylitol productivity rate of 0.37 g/(lh). In order to increase the specific yield, C. tropicalis was immobilised on porous glass and cultivated in a fluidized bed reactor. In a continuous non sterile cultivation with immobilised cells 155 g/l d-xylose produced 90–95% g/l xylitol with a productivity of 1.35 g/(lh).Mr. S. S. da Silva was a visiting scientist to the GBF. He was supported by a scholarship from the National Council of Scientific and Technological Development, Brasilia, Brazil (CNPq).We also would like to gratefully acknowledge the support of Prof. Dr. Michele Vitolo of the University of Sao Paulo, and the Centre for Biotechnology and Chemistry, Lorena, S. P. Brazil, in particular the Department of Fermentative Process.We are grateful to Prof. Rainer Jonas, head of the International Cooperation between Germany/Brazil for the helpful discussions and Dr. Heinrich Lönsdorf (GBF) for the Scanning electron micrographs.Dedicated to the 65th birthday of Prof. Dr. Fritz Wagner.     

The production of xylitol from d-xylose by fermentation with Hansenula polymorpha

We have analysed the influence of the initial pH of the medium and the quantity of aeration provided during the batch fermentation of solutions of d-xylose by the yeast Hansenula polymorpha (34438 ATCC). The initial pH was altered between 3.5 and 6.5 whilst aeration varied between 0.0 and 0.3 vvm. The temperature was kept at 30 °C during all the experiments. Hansenula polymorpha is known to produce high quantities of xylitol and low quantities of ethanol. The most favourable conditions for the growth of xylitol turned out to be: an initial pH of between 4.5 and 5.5 and the aeration provided by the stirring vortex alone. Thus, at an initial pH of 5.5, the maximum specific production rate (μ_m) was 0.41 h⁻¹, the overall biomass yield (Y _x/s ^G) was 0.12 g g⁻¹, the specific d-xylose-consumption rate (q _s ) was 0.075 g g⁻¹ h⁻¹ (for t = 75 h), the specific xylitol-production rate (q _Xy ) was 0.31 g g⁻¹ h⁻¹ (for t = 30 h) and the overall yields of ethanol (Y _E/s ^G) and xylitol (Y _Xy/s ^G) were 0.017 and 0.61 g g⁻¹ respectively. Both q _s and q _Xy decreased during the course of the experiments once the exponential growth phase had finished. Received: 26 March 1998 / Received revision: 30 June 1998 / Accepted: 2 July 1998     

The oxygen requirements of yeasts for the fermentation of d-xylose and d-glucose to ethanol

Summary The effect of oxygen availability on d-xylose and D-glucose metabolism by Pichia stipitis, Candida shehatae and Pachysolen tannophilus was investigated. Oxygen was not required for fermentation of d-xylose or d-glucose, but stimulated the ethanol production rate from both sugars. Under oxygen-limited conditions, the highest ethanol yield coefficient (Y_e/s) of 0.47 was obtained on d-xylose with. P. stipitis, while under similar conditions C. shehatae fermented d-xylose most rapidly with a specific productivity (q_pmax) of 0.32 h^-1. Both of these yeasts fermented d-xylose better and produced less xylitol than. P. tannophilus. Synthesis of polyols such as xylitol, arabitol, glycerol and ribitol reduced the ethanol yield in some instances and was related to the yeast strain, carbon source and oxygen availability. In general, these yeasts fermented d-glucose more rapidly than d-xylose. By contrast Saccharomyces cerevisiae fermented d-glucose at least three-fold faster under similar conditions.Nomenclature q_pmax maximum specific rate of ethanol production (g ethanol per g dry biomass per hour) - Y_e/s ethanol yield (g ethanol per g substrate utilized) - Y_p/s polyol yield (g polyol per g substrate utilized) - Y_x/s biomass yield (g dry biomass per g substrate utilized) - _max maximum specific growth rate (per hour)     

Yeast Immobilization in LentiKats®: A New Strategy for Xylitol Bioproduction from Sugarcane Bagasse

Summary A new PVA-hydrogel matrix for yeast cell immobilization for xylitol bioproduction from sugarcane bagasse was studied. Five repeated-batch fermentation runs were carried out in medium based on sugarcane bagasse hemicellulosic hydrolysate with reuse of the entrapped biocatalyst. The system performance as well as the metabolic behaviour of cells entrapped into the matrix were evaluated. The biocatalyst remained stable and exhibited a similar fermentative profile in all the successive batches, demonstrating the viability of the system. At the end of the run, an average xylitol production was observed of 35.1 g l⁻¹ and average xylitol yield and productivity of 0.58 g g⁻¹ and 0.49 g l⁻¹ h⁻¹, respectively.     

Alcoholic fermentation of xylose and mixed sugars using recombinant <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> engineered for xylose utilization

Previously, a Saccharomyces cerevisiae strain was engineered for xylose assimilation by the constitutive overexpression of the Orpinomyces xylose isomerase, the S. cerevisiae xylulokinase, and the Pichia stipitis SUT1 sugar transporter genes. The recombinant strain exhibited growth on xylose, under aerobic conditions, with a specific growth rate of 0.025 h⁻¹, while ethanol production from xylose was achieved anaerobically. In the present study, the developed recombinant yeast was adapted for enhanced growth on xylose by serial transfer in xylose-containing minimal medium under aerobic conditions. After repeated batch cultivations, a strain was isolated which grew with a specific growth rate of 0.133 h⁻¹. The adapted strain could ferment 20 g l⁻¹ of xylose to ethanol with a yield of 0.37 g g⁻¹ and production rate of 0.026 g l⁻¹ h⁻¹. Raising the fermentation temperature from 30°C to 35°C resulted in a substantial increase in the ethanol yield (0.43 g g⁻¹) and production rate (0.07 g l⁻¹ h⁻¹) as well as a significant reduction in the xylitol yield. By the addition of a sugar complexing agent, such as sodium tetraborate, significant improvement in ethanol production and reduction in xylitol accumulation was achieved. Furthermore, ethanol production from xylose and a mixture of glucose and xylose was also demonstrated in complex medium containing yeast extract, peptone, and borate with a considerably high yield of 0.48 g g⁻¹.     

Induction of aldose reductase activity inCandida guilliermondii by pentose sugars

Summary Cell extracts ofCandida guilliermondii grown ind-xylose,l-arabinose,d-galactose,d-glucose,d-mannose and glycerol as sole carbon sources possessed NADPH-dependent aldose reductase activity, but no NADH-dependent activity was detected.d-xylose andl-arabinose were the best inducers of aldose reductase activity. The highest enzyme activity ind-xylose orl-arabinose-grown cells was observed first withl-arabinose followed byd-xylose as substrates of the enzymatic reaction. However, only low activity was found ind-glucose,d-mannose andd-galactose-grown cells, indicating that these carbon sources cause catabolite repression. Enzyme activities induced ind-xylose-grown cells were twice as high as those obtained from the cells under resting conditions. Furthermore, the level of induction of aldose reductase activity depended on the initial concentration ofd-xylose. The present study shows that aldose reductase activity may be efficiently induced by pentose sugars of hemicellulosic hydrolysates and weakly by hemicellulosic hexoses.