Production of Substituted Oligosaccharides by Hydrolytic Processing

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Ind. Eng. Chem. Res. 2004, 43, 1608-1614

KINETICS, CATALYSIS, AND REACTION ENGINEERING Production of Substituted Oligosaccharides by Hydrolytic Processing of Barley Husks Gil Garrote, Herminia Domı´nguez, and Juan Carlos Parajo´ * Department of Chemical Engineering, University of Vigo (Campus Ourense), Polytechnical Building, As Lagoas, 32004 Ourense, Spain

Barley husks have been subjected to nonisothermal treatments in aqueous media in order to cause the selective degradation of their hemicellulose fraction. The concentration profiles of araban, xylan, oligosaccharides, monosaccharides, furfural, furfural-degradation products, acetyl groups, and acetic acid have been experimentally measured. Kinetic models describing the time course of hemicellulose-decomposition products have been developed. On the basis of the model predictions, operational conditions leading to maximal concentrations of oligosaccharides (suitable as prebiotic components of foods) have been identified, and the hemicellulose-degradation products obtained under these conditions have been quantified. Additional experiments have been carried out to identify nonsaccharide byproducts present in autohydrolysis liquors. Introduction The hemicellulose fraction of many agricultural products and wastes (including barley husks) is made up of xylopiranose units (native xylan) with a variety of substituents (arabinose and uronic acid substituents units linked through glycosidic bonds and/or ferulic acid, cinnamic acid, and acetic acid units linked through ester bonds).1,2 When xylan-containing raw materials are treated in aqueous media at mild temperatures (about 200 °C), the hemicellulosic polymers are broken down and solubilized, leading to liquors containing a variety of hemicellulose-degradation products, including sugar oligomers with acetyl and uronic substituents, monosaccharides, acetic acid, furfural generated by pentose dehydration and furfural-degradation products. The rest of the structural components (cellulose and lignin) suffer little alteration during this kind of treatment: cellulose is almost untouched, whereas the acid-soluble, ligninderived products are solubilized. The solid phase from treatments is enriched in cellulose, enabling its further utilization for making cellulose pulps (following a philosophy similar to that of the prehydrolysis-kraft process) or as a substrate for glucose production by enzymatic or acid hydrolysis. When the hydrolytic stage is carried out under appropriate conditions, oligosaccharides account for a substantial part of initial hemicelluloses.3-6 However, autohydrolysis is nonspecific, and other effects different from hemicelulose depolymerization (such as extractive removal from solid phase and solubilization of acidsoluble lignin) take place, leading to liquors with a complex composition. Xylose-containing oligosaccharides show potential as ingredients for functional foods. In this field, xylooligo* To whom correspondence should be addressed. Tel.: +34988387033. Fax: +34988387001. E-mail: [email protected].

saccharides selectively enhance the growth of bifidobacteria, causing beneficial health effects.7-11 When oligosaccharides are to be employed for food purposes, the reaction media must be refined to remove extractive- and lignin-derived compounds. Purification of xylooligosaccharides is a complex problem requiring multistep processing. In this field, chromatography and ion-exchange have been proposed for purification of xylooligosaccharides obtained by hydrolytic processing of wood.6 Interestingly, the nonsaccharide components of liquors from aqueous processing of biomass show antioxidant and antimicrobial properties,12 a fact fostering their utilization for replacing commercial antioxidants manufactured via chemical synthesis. This alternative is being encouraged by factors such as contradictory data on the safety of synthetic compounds, and growing demand and increased consumer preferences for natural food additives. In this field, ethyl acetate extraction allows the removal of a variety of potentially valuable compounds coming from extractives and acid-soluble lignin. In literature studies,13-15 lignin-derived products have been identified in reaction media coming from hydrolytic processing of biomass. The nonsaccharide compounds isolated in the processing of autohydrolysis liquors lack commercial value, and the development of practical applications for this fraction would be of scientific and economic interest. On the basis of the chemical nature of compounds soluble in ethyl acetate, the antioxidant activity of this fraction offers the possibility of commercial developments in both food and cosmetic fields. For this purpose, the identification of representative nonsaccharide compounds present in hydrolyzates is of major importance. This work deals with the kinetics of barely husks hemicellulose degradation by autohydrolysis, with the production of oligosaccharides by this method and with the chemical characterization of reaction byproducts

10.1021/ie0342762 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/02/2004

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1609

(from extractives and acid-soluble lignin) defining their practical applications. Experimental Section Raw Material and Chemical Characterization. Barley husks were collected in a local factory, air-dried, milled, screened to pass a 1-mm sieve, homogenized in a single lot, and stored in a dry dark place until use. Aliquots from the above homogenized lot were subjected to moisture determination and to quantitative acid hydrolysis with 72% sulfuric acid following standard methods. The solid residue after hydrolysis was considered as Klason lignin. Hydrolyzates were analyzed for monosaccharides (glucose coming from cellulose; xylose and arabinose coming from hemicelluloses) and acetic acid (coming from acetyl groups) by HPLC as reported elsewhere.16 Uronic acids were determined spectrophotometrically17 using galacturonic acid as a standard for quantification. Proteins were determined from the nitrogen content of samples (measured by the Kjeldahl method, protein ) 6.25‚N), whereas ashes were determined by calcination (TAPPI standard T-244-om-93). Hydrothermal Processing of Barley Husks and Analytical Determinations. Barley husks were contacted with water in a batch stainless steel reactor (Parr Instruments Company, Moline, IL) with temperature control using a liquid/solid ratio of 8:1 kg/kg and heated to the desired temperature following the standard temperature profile of the reactor. Solid residues from treatments were recovered by filtration, washed with water, air-dried, subjected to moisture determination to measure the amount of dissolved substrate, and subjected to the same analytical processing as the raw material. The experimental data allowed the calculation of solid recovery and its contents of hemicellulose components to be used in mathematical modeling. Samples of liquors from hydrothermal treatments were filtered through 0.45-µm membranes and used for direct HPLC determination of glucose, xylose, arabinose, furfural, and acetic acid. A second sample of liquors was subjected to quantitative posthydrolysis (with 4% sulfuric acid at 121 °C for 20 min) before duplicate HPLC analysis and uronic acid determination. The increase in the concentrations of monosaccharides and acetic acid caused by posthydrolysis provided a measure of the oligomer concentration and their degree of substitution with acetyl groups.16 Characterization of Reaction Byproducts Derived from Extractives and Acid-Soluble Lignin. Aliquots from the liquid phase obtained in aqueous treatments were separated by filtration and extracted with ethyl acetate at a liquor/ethyl acetate volume ratio of 1:3 (v/v) in a single extraction stage. The organic phase was vacuum-evaporated to remove both solvent and volatile-dissolved compounds, and the remaining solid phase was assayed for phenolic compounds and derivatized for GC-MS analysis. Total phenols in the extracts obtained after ethyl acetate evaporation were determined by absorbance readings at 745 nm of the complex formed by extracts with the Folin-Denis reagent. A standard curve made with chlorogenic acid was used to express the concentrations of Folin-Denis phenolics as chlorogenic acid equivalents. To obtain volatile derivatives for GC-MS analysis, dried extracts (0.05 g) were silylated by adding 0.200 mL of pyridine, 1 mL of bis(trimethylsilyl)trifluoracetamide (BSTFA),

and 0.05 mL of trimethylchlorosilane (TMCS) for 30 min at 60 °C.18 Silylated products were subjected to GCMS analysis using a Hewlett-Packard 5989 GC fitted with a MS detector (Hewlett-Packard 5972). Chromatographic separation was performed with a 60 m × 0.25 mm HP-5MS column operating with 1 mL/min helium as carrier gas. Temperature was kept at 90 °C for 10 min followed by heating at 5 °C/min up to 205 °C and further heating at 8 °C/min up to 250 °C, with a final isothermal stage at 250 °C to the complete desired analysis time (60 min). Nonsaccharide compounds were identified by both comparison of spectral data with reported results and direct comparison with derivatized standard compounds. Only compounds identified at a confidence level >95% are reported. Fitting of Data. Empirical equations (selected from built-in functions of the TableCurve software, Jandel Scientific, San Rafael, CA) were used to fit the temperature profiles (which were measured for each experiment and averaged before fitting). The empirical equations obtained were used in the numerical solving of the differential equations involved in the mathemathical models describing hemicellulose decomposition. The set of differential equations was solved by a fourth order Runge-Kutta method. A commercial optimization routine dealing with the Newton’s method was used to calculate the preexponential factors and the activation energies by minimizing the sum of the squares of deviations between experimental and calculated data. Results and Discussion Chemical Composition of the Raw Material. A detailed knowledge of the feedstock composition is necessary for calculating the theoretical yields in oligosaccharides as well as for assessing the substrate reactivity: increased contents of acidic substituents (such as acetyl and/or uronic groups) result in increased concentrations of hydronium ions (responsible for hydrolytic activity) in the autohydrolysis medium, enabling the hydrolytic degradation of hemicellulose polysaccharides under comparatively mild operational conditions. Among the structural components of barley husks, cellulose is quantified in terms of the glucose contained in the liquors from quantitative saccharification. Cellulose is expected to suffer little alteration in hydrothermal treatments under the experimental conditions used in this work, even if the total glucan can include small proportions of easily hydrolyzable glucose polymers which can be degraded to glucose or glucosecontaining oligomers. Klason lignin, which has been measured as the acid-insoluble residue after quantitative acid hydrolysis, is of minor importance for the objectives of this study. Hemicelluloses have been measured in terms of xylan, araban, acetyl groups, and uronic substituents. Xylan is the main hemicellulosic polymer, whereas arabinose units linked to the xylan backbone (referred to as “araban” in this work) are highly susceptible to hydrothermal degradation and can give furfural as undesired reaction byproduct. Acidic hemicellulose components (acetyl groups and uronic acid substituents) promote the hydrolytic degradation of polysaccharide chains. Finally, other feedstock components (proteins and ash) also have been determined for the summative analysis of barley husks. The rest of the components (such as extractives or acid-soluble lignin) are of minor importance for this study, and are reported

1610 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 1. Composition of Barley Husks (Oven-Dry Basis) fraction

weight percent

cellulose xylan araban acetyl groups uronic acids Klason lignin protein ash others

21.4 26.8 5.74 1.69 2.39 19.2 5.90 15.5 1.39

as “others”. Table 1 shows the analytical data determined for the various fractions present in the raw material. Hydrothermal Processing. Aqueous processing (also called autohydrolysis or hydrothermal processing) of lignocellulosic substrates uses water and lignocellulosic feedstocks as the only reagents. When these treatments are carried out under mild conditions, hemicelluloses are depolymerized leading to sugar oligomers potentially useful as ingredients for prebiotic foods. The suitability of autohydrolysis reaction products for a given purpose is conditioned by a variety of factors, including the type, number, and distribution of substituents, the biological activities of the oligosaccharides produced, and the presence of undesired compounds in the crude autohydrolysis liquors.19 These factors depend on both the raw material employed and on the autohydrolysis conditions.20,21 Kinetic models have been reported for the hydrolytic degradation of the hemicellulosic fraction of various xylan-containing raw materials, including agricultural byproducts.22,23 The hydrolytic degradation of hemicelluloses is based on the hydronium-catalyzed breakdown of glycosidic bonds. This reaction may follow a two-stage kinetics,24 which has been ascribed to the presence of two substrate fractions with different susceptibility.25 The two-fraction hypothesis can be justified by reasons such as the existence of mass and heat transfer limitations in partially converted substrates, accessibility problems, variation in particle size and available surface area in the hydrolytic process, and differences in reactivity caused by a different composition or a different substitution pattern of the unreacted substrate. The fraction of hemicelluloses susceptible to hydrolysis (denoted “susceptible fraction”, R) depends on both the lignocellulose feedstock and the reaction conditions under which the reaction was performed.24 In the mathematical interpretation of the oligomer generation during the autohydrolysis of lignocellulose, it must be considered that the breakdown of a given bond can lead to different reaction products (monosaccharides or oligosaccharides) depending on the position of the cleaved bond in the polymer chain. When hemicelluloses have been consumed to a significant extent, oligomers are the most abundant reactive species in the medium. The first reaction stages lead mainly to highmolecular-weight oligomers, which are split first in smaller fragments and then in monomeric sugars. Under harsh conditions, pentose sugars can be dehydrated to furfural, and furfural can be decomposed. It can be noted that when the raw material has significant contents of both xylan and araban, furfural can be produced from both xylose and arabinose. To achieve a complete understanding of the hydrolytic breakdown of xylan and araban as well as the cleavage of acetyl groups in the hydrothermal processing of

Figure 1. Reaction mechanism proposed for hemicellulose decomposition in hydrothermal treatments of barley husks: XnS ) xylan fraction susceptible to hydrothermal degradation; XOH ) high-molecular-weight xylose oligomers; XOL ) low-molecularweight xylose oligomers; X ) xylose; F ) furfural; DP ) furfuraldegradation products; Arn ) arabinan; ArO ) arabinooligosaccharides; AcO- ) acetyl groups; AcOH ) acetic acid; kSX...kC ) kinetic coefficients. Table 2. Data Obtained in the Fitting of Experimental Data to the Proposed Mathematical Model R

(a) Susceptible Xylan Fraction, g/g 0.966

(b) Preexponential Factors (s-1) Corresponding to the Kinetic Coefficients shown in Figure 1 kSX0 1.08‚1013 k1X0 2.17‚1014 k2X0 1.97‚1013 k3X0 1.34‚107 kF0 1.81‚107 kA0 1.06‚1022 k2A0 5.41‚109 k3A0 1.35‚104 kC0 1.01 (b) Activation Energies [J/(mol‚K)] Corresponding to the Kinetic Coefficients shown in Figure 1 EaSX0 2.16‚105 Ea1X0 1.50‚105 Ea2X0 1.42‚105 Ea3X0 8.30‚104 EaF0 8.28‚104 EaA0 2.16‚105 Ea2A0 1.10‚105 Ea3A0 5.77‚104 EaC0 6.90‚104 (c) Regression Coefficients (R2) Obtained in Fitting for the Various Model Variables XnS 0.994 XO 0.98 X 0.94 F 0.77 Arn 0.991 ArO 0.976 Ar 0.97 Ac 0.992

barley husks, a variety of different approaches have been considered, including the existence of one or two reactive fractions in both araban and xylan, the generation of one or two types of oligomers (with different polymerization degree) from xylan or araban, the hydrolysis of oligomers into monosaccharides, the dehydration of pentoses into furfural, the direct generation of furfural from low-sugar oligomers, the decomposition of furfural, and the cleavage or acetyl groups from both the hemicellulose remaining in solid phase and acetylated oligomers. The simplest reaction mechanism allowing a satisfactory interpretation of experimental (shown in Figure 1) is based on the following assumptions: (i) xylan is made up of two fractions, but only one of them (denoted susceptible xylan, XnS) reacts under the assayed conditions; (ii) the susceptible xylan fraction R (defined as the ratio between initial reacting

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polymer XnS0 and initial amount of polymer Xn0) is independent from temperature under the reaction conditions tested; (iii) the degraded xylan yields highmolecular-weight oligomers, which are converted into low molecular oligomers and then into xylose; (iv) araban is totally reactive and yields a single type of oligomers, which decompose into arabinose; (v) in a subsequent reaction, pentoses (arabinose or xylose) yield furfural, which is converted into degradation products; (vi) acetyl groups (linked to polysaccharides or oligomers) yield acetic acid; (vii) all the involved reactions show a pseudohomogeneous, irreversible, first-order kinetics; (viii) all the kinetic coefficients present an Arrhenius-type dependence on temperature; and (ix) for calculation purposes, the araban degradation was considered to proceed independently from xylan hydrolysis. The set of differential equations and stoichiometric conditions corresponding to the above mechanism are:

[ ]

dXnS -EaSX ) -k0SX‚exp ‚X dt R‚T(t) nS

[ ]

(1)

dXOH -EaSX ) k0SX‚exp ‚X - k01X‚ dt R‚T(t) nS exp

[ ]

[ ] -Ea1X

R‚T(t)

‚XOH (2)

dXOL -Ea1X ) k01X‚exp ‚XOH - k02X‚ dt R‚T(t) -Ea2X ‚XOL (3) exp R‚T(t)

[ ] [ ]

[ ]

-Ea2X -Ea3X dX ‚XOL - k03X‚exp ‚X (4) ) k02X‚exp dt R‚T(t) R‚T(t) R)

XnS0 Xn0

(5)

XO ) XOH + XOL

(6)

Xn ) XnS + (1 - R)‚Xn0

(7)

dArn -EaA ) -k0A‚exp ‚Arn dt R‚T(t)

(8)

[ ]

[ ]

[ ]

[ ]

-Ea2A -Ea3A dAr ) k02A‚exp ‚ArO - k03A‚exp ‚Ar dt R‚T(t) R‚T(t) (10)

[ ]

The set of equations was solved by fitting the experimental temperature/time series of data obtained in nonisothermal hydrolysis experiments by means of empirical equations and expressing the concentrations of the several xylan- or araban-derived products as pentosan equivalents (g equivalent pentosan/100 g ovendry barley husks). In the same way, both acetyl groups and acetic acid were expressed as acetyl group equivalents (g equivalent acetyl groups/100 g oven-dry barley husks). The initial conditions for solving the above set of differential equations were as follows:

for t ) 0, Xn ) Xn0; XnS ) XnS0;

-EaA -Ea2A dArO ‚Arn - k02A‚exp ‚ArO ) k0A‚exp dt R‚T(t) R‚T(t) (9)

[ ]

Figure 2. Experimental and calculated concentration profiles of xylan (Xn), xylooligosaccharides (XO), and xylose (X).

[ ] [ ]

-Ea3X -Ea3A dF ) k03X‚exp ‚X + k03A‚exp ‚ dt R‚T(t) R‚T(t) -EaF ‚F (11) Ar - k0F‚exp R‚T(t) DP ) (Xn0 + Arn0) (Xn + XO + X + Arn + ArO + Ar + F) (12) -EaC dAcO ) -k0C‚exp ‚AcO dt R‚T(t)

[ ]

(13)

AcOH ) AcO0 - AcO

(14)

Arn ) Arn0; Ac- ) AcO0; XOH ) XOL ) X ) ArO ) Ar ) DP ) AcOH ) 0 where Xn0, Arn0, and AcO0 are the compositional data of the raw material given in Table 1. Table 2 shows the results determined for the various regression parameters as well as for the statistical parameter R2, whereas Figures 2-5 show both experimental data and model predictions. Figure 2 shows that the amount of residual xylan in solid phase decreases exponentially with time to reach a minimum different from zero, owing to the presence of nonsusceptible xylan (a finding introduced in the kinetic model by means of the susceptible fraction R). Xylooligosaccharides behaved as typical reaction intermediates, with a well defined maximum of about 18 g equivalent xylan/100 g raw material. Considering the compositional data shown in Table 1, it can be inferred that about 67% of the initial xylan can be directly converted into xylooligosaccharides under the optimal conditions. Interestingly, under the same conditions, the xylose concentration is comparatively low (about 3 g equivalent xylan/100 g raw material), a fact favoring

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Figure 3. Experimental and calculated concentration profiles of araban (Arn), arabinooligosaccharides (ArO), and arabinose (Ar).

the further purification of xylooligosaccharides from autohydrolysis liquors. Figure 3 shows that the residual araban in solid phase decreases steadily up to total removal, confirming that all the araban is susceptible to autohydrolysis in the operational range. The concentration profiles of arabinooligosacchares are not well defined (owing to the experimental error caused by its low proportion in the raw material), but its comparative analysis with the concentration profiles of xylooligomers confirms the higher susceptibility of araban to the hydrolytic reaction, leading to the production of significant proportions of arabinooligosaccharides at lower temperatures (190195 °C) than in the former case. The concentration profiles of arabinose showed a maximum (1.5 g equivalent araban/1000 g raw material) at about 200 °C, with a correspondent arabinooligosaccharide concentration of just 2.2 g equivalent araban/1000 g raw material. The decreased proportion oligosaccharides/monosaccharides in comparison with that observed for xylan-decomposition products is caused by the higher lability of arabinooligosaccharides. Figure 4 shows the fair agreement between experimental and calculated data for acetyl groups linked to polysaccharides or oligosaccharides and acetic acid, the corresponding hydrolysis product. Under the conditions leading to a maximal oligosaccharide concentration, 42% of the initial acetyl groups were cleaved. Characterization of Reaction Products and Byproducts. Once the autohydrolysis conditions leading to maximal oligosaccharide yield were identified (see Figure 2), a new experiment was carried out under the selected conditions, and the fraction of solids dissolved in treatments was determined to enable the formulation of material balances. The dissolved fraction of barley

Figure 4. Experimental and calculated concentration profiles of acetyl groups linked to polysaccharides or oligosaccharides (AcO) and acetic acid (AcH).

husks accounted for 42.4% of its dry weight. Autohydrolysis treatments preserve a part of the functional groups present in the raw material, yielding multiple reaction products with variable degrees of polymerization and complex substitution pattern.20,21 Most of the dissolved fraction corresponded to oligosaccharides (including arabino- and xylo-oligosaccharides with acetyl and uronic substituents), which accounted for 25.8 wt % of the raw material. As monosaccharides (glucose, arabinose and xylose), acetic acid, and furfural accounted for 5, 2.8, and 1.1 wt % of the dissolved fraction, it can be inferred that about 18% of the dissolved solid corresponds to nonsaccharide materials and to furfural degradation products. The nonsaccharide components are byproducts of the hydrothermal reaction (including lignin-, extractive-, protein-, and ash-derived products), which have to be removed in additional processing stages to obtain food-grade oligosaccharides. To obtain further information on the phenolic content of nonsaccharide byproducts, spectrophotometric determination of Folin-Denis-reactive compounds was carried out. The chlorogenic acid equivalent of Folin-Denis phenols accounted for 5% of the solid dissolved in treatments. As both low-molecular-weight phenols and extractivederived products are soluble in ethyl acetate, and considering that antioxidant and antimicrobial properties have been reported for ethyl acetate soluble compounds present in liquors from hydrolytic treatments of biomass,12 autohydrolysis liquors were extracted with ethyl acetate and subjected to GC-MS analysis. The ion-chromatograms of derivatized extracts showed fatty acids derived from the extractive fraction of barley husks as main nonsaccharide products. Hexadecanoic acid (26% of total area) and octadecanoic acid (24.4% of total area) were the main fatty acids present in auto-

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hydrolysis media, with minor amounts of oleic acid, 9-12 octadecanedienoic acid, and tetradecanoic acid (which accounted for 6, 3, and 1% of the total area). The extracts also showed a resin acid (dehydroabietic acid), which accounted for 2.9% of the total area. All these compunds are typical components of lipophilic biomass extractives. Acids belonging to this group have been proposed for the manufacture of resins, as raw materials for the synthesis of other useful compunds such as industrial rubber, for applications in cosmetic industries, and for uses as surfactants and components of soaps.26 Phenolic compunds accounted for 11% of the total area, with vanillin (3% of total area) as the major component. Other lignin-derived compounds identified were 3,4-dihydroxybenzaldehyde, 3-methoxy-4-hydroxybenzoic acid, and cinnamic acid, which accounted for 2, 1.7, and 1.5% of the total area. Low-molecular-weight, lignin-derived phenols are suitable as food antioxidants.12-15 Conclusions Nonisothermal autohydrolysis has been studied as a method for obtaining substituted oligosaccharides. In treatments at temperatures up to 220 °C, the concentration profiles of araban, xylan, xylo- and arabinooligosaccharides, monosaccharides, furfural, furfuraldegradation products, acetyl groups, and acetic acid were determined, and kinetic models providing a good interpretation of experimental data have been developed. On the basis of the model predictions, operational conditions leading to maximal concentrations of oligosaccharides have been identified. In an autohydrolysis experiment carried out under the optimal conditions, both the concentrations of hemicellulose-degradation products and the percent of solid recovery were measured, allowing a preliminary estimate of the concentrations of nonsaccharide products in the reaction media, which should be removed to achieve food-grade oligosaccharides. Concerning the nonsaccharide compounds, Folin-Denis reactive phenols have been quantified, and other reaction byproducts (monomeric phenols, fatty acids, and dehydroabietic acid) have been identified in the reaction media. Acknowledgment We are grateful to the Spanish Ministry of Science of Technology for the financial support of this work (in the framework of the Research Project “Food Ingredients from Agroindustrial Wastes”, reference PPQ2002-00184, which had partial financial support from the FEDER funds of the European Union). Nomenclature AcO ) concentration of acetyl groups bonded to polysaccharides or oligomers, g/100 g oven-dry raw material AcOH ) acetic acid concentration, g equivalent acetyl groups/100 g oven-dry raw material Ar ) concentration of arabinose, g equivalent araban/100 g oven-dry raw material Arn ) concentration of araban, g/100 g oven-dry raw material ArO ) concentration of arabinooligomers, g equivalent araban/100 g oven-dry raw material DP ) concentration of furfural degradation products, g equivalent pentosan/100 g oven-dry raw material

EaSX...EaC ) activation energies corresponding to the kinetic coefficients involved in the mathematical model (see Figure 1), J/(mol‚K) F ) concentration of furfural, g equivalent pentosan/100 g oven-dry raw material k0X...k0C ) preeexponential factors corresponding to the kinetic coefficients involved in the mathematical model (see Figure 1), s-1 T ) temperature, K X ) xylose concentration, g equivalent xylan/100 g ovendry raw material Xn ) concentration of xylan, g/100 g oven-dry raw material XnS ) concentration of susceptible xylan, g /100 g ovendry raw material XO ) concentration of total xylooligomers, g equivalent xylan/100 g oven-dry raw material XOH ) concentration of high-molecular-weight xylooligomers, g equivalent xylan/100 g oven-dry raw material XOL ) concentation of low-molecular-weight xylooligomers, g equivalent xylan/100 g oven-dry raw material R ) susceptible xylan fraction, g susceptible xylan/g xylan contained in the raw material

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Received for review November 28, 2003 Revised manuscript received January 28, 2004 Accepted February 4, 2004 IE0342762