Biochemical Engineering Journal Search for optimum

Biochemical Engineering Journal 46 (2009) 199–204

Contents lists available at ScienceDirect

Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej

Search for optimum conditions of sugarcane bagasse hemicellulose extraction M. Brienzo, A.F. Siqueira, A.M.F. Milagres ∗ Department of Biotechnology, School of Engineering of Lorena, University of São Paulo – USP Estrada Municipal do Campinho, s/no – CP 116, 12602-810 Lorena SP, Brazil

a r t i c l e

i n f o

Article history: Received 16 March 2009 Received in revised form 13 May 2009 Accepted 16 May 2009

Keywords: Hemicellulose Biomass Lignocellulose Sugarcane bagasse Extraction Biopolymers

a b s t r a c t A process has been elaborated for one-step low lignin content sugarcane bagasse hemicellulose extraction using alkaline solution of hydrogen peroxide. To maximize the hemicellulose yields several extraction conditions were examined applying the 24 factorial design: H2 O2 concentration from 2 to 6% (w/v), reaction time from 4 to 16 h, temperature from 20 to 60 ◦ C, and magnesium sulfate absence or presence (0.5%, w/v). This approach allowed selection of conditions for the extraction of low and high lignin content hemicellulose. At midpoint the yield of hemicellulose was 94.5% with more than 88% of lignin removed. Lignin removal is suppressed at low extraction temperatures and in the absence of magnesium sulfate. Hemicellulose in 86% yield with low lignin content (5.9%) was obtained with 6% H2 O2 treatment for 4 h and 20 ◦ C. This hemicellulose is much lighter in color than samples obtained at the midpoint condition and was found suitable for subsequent enzymatic hydrolysis. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Sugar mills generate approximately 135 kg of bagasse (dry weight) per metric ton of sugarcane. Bagasse is a rich source of not only cellulose, but also hemicellulose, represented by l-arabino-(4-O-methyl-d-glucurono)-d-xylan. The two polysaccharides represent about 70% of bagasse. Entwined around the two sugar polymers is lignin, covalently linked to hemicellulose [1]. Nowadays about 50% of generated sugarcane bagasse is used to generate heat and power to run the sugar mills and ethanol plants. The remaining portion is usually stockpiled [2]. However, because the heating value of carbohydrates is approximately half of that of lignin [1], it would be beneficial to develop a more economical use of carbohydrates. One such possibility would be to extract the hemicelluloses before cellulose to convert them to higher value-added products such as prebiotic xylooligosaccharides or polymers and composites for chemical and pharmaceutical applications. Hemicelluloses are known as valuable in pulp additives [3], natural barrier for packaging films [4] and as components of skin substitutes in case of damage of superficial epidermal layers [5]. As hemicelluloses are relatively tightly bound in the plant cell wall network to lignin and cellulose, it is difficult to separate them without significant modification of their structure [6,7]. Different treatments have been applied to hemicellulose extraction, and heat treatment is often combined with addition of chemicals such as

alkali, acid or hydrogen peroxide [7–10]. Alkaline peroxide is an effective agent for both delignification and solubilization of hemicelluloses [7]. In these conditions carbohydrates are less damaged and delignification is more efficient [7,11–14]. However, the use of hydrogen peroxide treatment of bagasse requires prior removal heavy metals with chelating agents. The metals catalyze decomposition of the peroxide anion in the alkaline medium leading to the formation of hydroxyl radicals [15] which cause hemicellulose depolymerization and diminish its recovery [16]. However, chelation not only removes heavy metals, but also alkali earth metals which act as natural stabilizers of the peroxide during the treatment [14]. This is the reason why magnesium ions are added later in a surplus to the chelating agents to prevent hemicellulose degradation. We report here optimization of conditions for hemicellulose solubilization from sugarcane bagasses in a single step using alkaline solution of hydrogen peroxide. The hemicellulose yield and its lignin content were determined under variety of parameters that received little attention in the past. Specifically, we investigated the effect of temperature and hydrogen peroxide concentration in the presence and absence of magnesium sulfate as a stabilizer. Conditions for the extraction of low and high lignin content hemicellulose have been elaborated. 2. Materials and methods 2.1. Dewaxed sugarcane bagasse

∗ Corresponding author. Tel.: +55 1231595019; fax: +55 1231533165. E-mail address: [email protected] (A.M.F. Milagres). 1369-703X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2009.05.012

Sugarcane bagasse, milled to particles smaller than 25 meshes, was washed with ethanol to remove waxy compounds. The

200

M. Brienzo et al. / Biochemical Engineering Journal 46 (2009) 199–204

dewaxed bagasse contained 42.4% of cellulose, 25.2% of hemicellulose, 19.6% of lignin and 1.6% of ash on a dry weight basis. 2.2. Extraction of hemicellulose The extraction of hemicellulose was done according to the reported method with some modifications [10]. Prior the extraction the milled bagasse was washed with 0.2% (w/v) ethylenediamine tetraacetic acid (EDTA) for 1 h at 90 ◦ C to remove metal cations, such as ions of iron and manganese. These ions promote decomposition of hydrogen peroxide reducing its delignification performance. The material was submitted to an oxidative delignification using alkaline solution of hydrogen peroxide. A sample of 10 g of dewaxed bagasse was treated with alkaline peroxide (2–6%) and magnesium sulfate (0–0.5%) with the pH adjusted to 11.6 with NaOH, in a reaction volume of 200 mL and incubated in thermostat-controlled water bath (20–60 ◦ C). After 4–16 h, the insoluble residue was collected by filtration, washed with distilled water until the pH was neutral, and then dried at 45 ◦ C. The supernatant fluid was adjusted to pH 6 with 6 M HCl and then concentrated to about one-third of its volume under air circulation at 45 ◦ C. The concentrated sample was poured into three volumes of 95% ethanol to precipitate hemicellulose that was then washed four times with 70% ethanol. The decanted hemicellulose was dried by air circulation at 45 ◦ C. The reported yield of hemicellulose (in %) refers to the hemicellulose content in dewaxed bagasse [17]. A two-level, four variables, temperature (x1 ), hydrogen peroxide concentration (x2 ), magnesium sulfate (x3 ) and time (x4 ) were applied to determine the best combination of extraction variables for the hemicellulose extraction. The variables’ range and experimental design are shown in Table 1. Three assays at the midpoint of the design were carried out to estimate the random error needed for the analysis of variance. The hemicellulose and lignin content were taken as dependent variables or response of the experimental design. STATISTICA (Version 6.0) was used for regression and graphical analyses of the data obtained. The fitness of the polynomial model equation was expressed by the coefficient of determination R2 , and its statistical significance was checked by F-test at a probability (p) of 0.001 and 0.05. The significances of the regression coefficient were also tested by F-test. 2.3. Chemical composition Isolated hemicelluloses were soaked first in 72% sulfuric acid at 45 ◦ C for 7 min (300 mg in 1.5 mL of sulfuric acid). Then the acid was diluted to 3% concentration by addition of 45 mL of water, and the mixture was heated at 121 ◦ C/L atm for 30 min. The resulting material was cooled and filtered through porous glass filter number 3. The solids were dried to constant weight at 105 ◦ C and determined as insoluble lignin (Klason) [18]. The soluble lignin concentration in the filtrate was determined by measuring the absorbance at 205 nm [19]. The concentrations of monomeric sugars in the soluble fraction were determined by HPLC using a BIORAD HPX87H column, eluted at the 0.6 mL/min with 5 mM sulfuric acid. Sugars were detected at 45 ◦ C using a refractometric detector. Uronic acids were determined with the carbazole procedure [20]. Around 0.4 mL of the acid hydrolysate was mixed with 2.4 mL of concentrated sulfuric acid. When the mixture reached room temperature, 0.1 mL of solution of carbazole (0.1%, w/v) in ethanol was added. The tube was sealed and placed in a boiling water bath for 20 min, followed by cooling in an ice-water bath until room temperature was reached. The absorbance of the solution measured at 525 nm which was converted to concentration of uronic acids using a calibration curve constructed with galacturonic acid (17–139 nmol/mL).

2.4. Analysis of hemicellulose by FT-IR spectroscopy The 100 ␮m sieved sample was dried at 60 ◦ C overnight and stored under P2 O5 . One to two milligrams of sample was homogenized with 225 mg of KBr for 1 min and recorded between 400 and 4000 cm−1 in a Perkin Elmer FT-2000 FT-IR spectrometer using 64 scans. The baseline was corrected to the regions near 4000, 2400 and 800 cm−1 and the spectrum normalized to the band nearest to 900 cm−1 .

3. Results and discussion 3.1. Influence of the extraction parameter on hemicellulose yield The yields of extracted hemicellulose as a function of the extraction time, temperature, concentration of H2 O2 and magnesium sulfate are shown in Table 1. The yields varied within a range 38.3–94.5%. The lowest yield was obtained using the variables at low levels, except for magnesium sulfate (run 9) and the highest yield was obtained at midpoint conditions. There was an increasing trend in the yield of hemicellulose from 2 to 6% H2 O2 . Time was another factor that influenced the extraction efficiency of the hemicellulose. The increase in hemicellulose yields with the time indicates that longer extraction time might be important for high hemicellulose yields. However, extraction times longer than 10 h may induce changes in molecular structure of polysaccharides [21]. Statistical analysis showed that concentration of H2 O2 , magnesium sulfate and duration of the treatment had a significant effect (p < 0.05) with a positive signal, indicating that hemicellulose yield was high when these variables were increased. The temperature was the unique variable that did not present significant effect on hemicellulose yield (Table 2). The greatest influence on the response was produced by hydrogen peroxide (p < 0.01). Its interaction with the temperature was positive, while the interaction with the time was negative. Note that similar yields of hemicelluloses were obtained by increasing the time and keeping H2 O2 in low concentration, or vice versa, differently from other studies in which improvements of the hemicellulose extraction was reached by increasing both H2 O2 concentration and reaction time [11,21,22]. All the interactions were significant at least at a 95% confidence level. This analysis was performed without estimation of the curvature with a determination coefficient (R2 ) of 0.97, suggesting a first-order equation to explain the hemicellulose yield variations as a function of the evaluated variables in the studied region. Although a second-order equation could be more suitable to represent hemicellulose yield, the high hemicellulose yield, 94.5% achieved in the experimental design, indicates that additional assays would not result in a significant increase in the yield values. The concentration of magnesium sulfate and its interaction with hydrogen peroxide had negative influence on hemicellulose extraction. Meanwhile the addition of magnesium sulfate in low concentrations can be beneficial for the hemicellulose extraction, when compared to experiments with its absence, its addition in excess (0.5%) limits the hemicellulose extraction as a consequence of alkali reduction in reactions that lead the production of Mg(OH)2 [11]. In the case of the small charge of magnesium sulfate (0.25%), the peroxide seems to decompose in alkaline medium and only very strongly bounded residual lignin are not effectively removed as a consequence of excess of residual peroxide [11,23]. Magnesium may act as a natural stabilizer of the peroxide during the sugarcane bagasse hemicellulose extraction. We expected that, with magnesium in greater concentrations than heavy metal content in sugarcane bagasse, it could prevent the decomposition of the peroxide anion in the alkaline medium leading to the formation of


201

Table 1 The yields of hemicellulose (% total dry weight of hemicelluloses in sugarcane bagasse, w/w) and lignin (% dry hemicellulose) solubilized in H2 O2 treatment of sugarcane bagasse at pH 11.6 according to a 24 full factorial design. Run

Temperature (◦ C)

Hydrogen peroxide (%, w/v)

Reaction time (h)

Magnesium sulfate (%, w/w)

Lignina content (%)

Hemicelluloseb yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

20 60 20 60 20 60 20 60 20 60 20 60 20 60 20 60 40 40 40

2 2 6 6 2 2 6 6 2 2 6 6 2 2 6 6 4 4 4

4 4 4 4 16 16 16 16 4 4 4 4 16 16 16 16 10 10 10

0 0 0 0 0 0 0 0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.25 0.25 0.25

10.0 10.2 5.9 12.2 14.1 13.7 7.4 9.6 9.3 9.1 5.0 7.2 12.9 9.2 4.6 6.1 11.0 10.4 9.0

52.0 49.1 86.0 74.2 67.0 49.5 78.0 60.8 38.3 46.0 52.5 79.8 72.6 66.6 60.9 64.9 90.5 93.5 94.5

a b

Represent the content of associated lignin in isolated hemicellulose fractions. Obtained by the difference between isolated hemicellulose fractions and lignin.

hydroxyl radicals. This in turn, results in the decrease in hemicellulose polymerization degree and its solubilization. However, adding increasing concentrations of magnesium sulfate to the bagasse did not lead to the higher hemicellulose extraction. The improper dosage of magnesium sulfate caused the decrease in hemicellulose recovery. A negative influence of the magnesium excess was also reported for pulp bleaching [24]. By employing multiple regression analysis on the experimental data, the predicted response y1 for the yield of hemicellulose can be obtained by the following equation: y1 = 59.38 + 6.86x2 + 2.49x3 − 3.37x1 x3 + 4.88x1 x4 − 5.80x2 x3 − 2.83x2 x4 + 3.22x3 x4

tion, all the mutual interactions among the tested variables are significant. Through the three-dimensional plot it is very easy and convenient to understand the interactions between two variables and to locate their optimum ranges. By analyzing the plot (Fig. 1A), the optimal values of the tested variables to obtain hemicellulose of approximately 94.5% lie around the midpoint. From the above analysis, the best value of hemicellulose yield occurs at a condition of low lignin removal (Table 1) that for some applications is not appropriated [3–5]. The hemicellulose extracted at 6% (w/v) hydrogen peroxide, for 4 h at 20 ◦ C has a great potential for use, since valuable product and high yields of it (about 86%) can be recovered.

(1) 3.2. Content of associated lignin in hemicellulose

where x1 , x2 , x3 and x4 were the coded values of the test variables, extracting temperature (◦ C), hydrogen peroxide concentration (%), extracting time (h) and magnesium sulfate concentration (%), respectively. The 3D response surface is the graphical representation of regression equation. In the present study, two independent response surface plot was generated using STATISTICA (Version 6.0) (as shown in Fig. 1A). It is clear that the yield of hemicellulose is sensitive to minor alterations of H2 O2 and time. The other variables, temperature and MgSO4 , were kept at low levels. In addi-

The amount of lignin content, based on dry weight of the hemicelluloses, varied from 4.6 to 14.1% depending on the extraction conditions (Table 1). The experiments performed at high H2 O2 concentration caused the major lignin removal and practically did not interfere with the hemicellulose fraction. Only the hydrogen peroxide and magnesium sulfate had highly significant effects on lignin removal, with a negative signal indicating that lignin removal was high when the hydrogen peroxide and magnesium sulfate were increased (Table 2).

Table 2 Effect estimates (EE) and level of significance (p) for hemicellulose extraction and lignin removal according to a 24 full factorial design. Variables and interactions

x1 x2 x3 x4 x2 x3 x1 x4 x1 x3 x3 x4 x2 x4 x1 x2 Model Lack of fit

Hemicellulose yield

Lignin content of hemicellulose

Standard error

Probability (p > F)

Standard error

Probability (p > F)

−2.41 18.15 9.38 8.95 −20.77 14.33 −12.46 9.95 −9.76 6.73

0.53 0.00a 0.01b 0.01b 0.02 0.00a 0.00a 0.00a 0.01 0.01 0.02 0.06

1.95 −7.36 2.11 −4.75

0.30 0.00a 0.26 0.02b

x1 , temperature; x2 , H2 O2 concentration; x3 , reaction time; x4 ; MgSO4 . a Significance at 0.001 level. b Significance at 0.05 level.

0.00 0.23

202


The other two studied variables, time and temperature, did not present significant effects for lignin removal. The interactions of variables were not significant at 95% confidence. The analysis of variance was performed for a linear model and high value of R2 (>0.96) was achieved, showing a close agreement between experimental results and the theoretical values predicted by the first-order polynomials. Therefore, a multiple regression analysis was performed to fit the first-order polynomial equation to the experimental data points. Residual lignin (y2 ) was correlated as a function of hydrogen peroxide concentration (x2 ) and magnesium sulfate (x4 ) (coded values) resulting in Eq. (2): y2 (%) = 9.16 − 1.90x2 − 1.23x4 ,

R2 = 0.96

(2)

Three-dimensional response surface described by the abovementioned first-order polynomials were fitted to the experimental data points concerning the lignin removal (Fig. 1B). This figure shows that lignin removal is well-fitted to a flat-surface and the lowest lignin content in hemicellulose can be attained performing sugarcane bagasse extraction with 6% hydrogen peroxide and 0.5% magnesium sulfate. It could be evidenced that the lignin contents are directly associated with the hemicellulose darkening, however as additional lignin removal from the hemicellulose was very difficult to be reached with only one step of peroxide reaction, more efforts were not given to obtain a more purified hemicellulose. 3.3. Characterization of the hemicelluloses The monosaccharide compositions of hemicelluloses extracted from bagasse under variety of conditions were determined after acid hydrolysis. All analyzed samples showed a similar pen-

Fig. 1. Response surface plots showing the effect of hydrogen peroxide (x2 ) and reaction time (x3 ) on the yield of hemicellulose (A) and hydrogen peroxide (x2 ) and magnesium sulfate (x4 ) corresponding to lignin residual in hemicellulose (B).

Table 3 The contents of neutral sugars (relative % dry weight of hemicelluloses, w/w) and uronic acids of hemicellulose obtained from sugar cane bagasse. Runs

Xylose

Arabinose

Glucose

Uronic acids

Sum

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

80.4 79.3 80.9 77.0 79.0 79.2 77.9 74.9 76.9 80.1 75.5 73.1 74.9 73.5 76.7 79.1 78.6 78.1 82.6

4.4 5.7 3.8 6.0 5.1 6.5 4.8 6.1 5.7 6.5 5.3 7.2 5.3 6.9 7.0 5.8 6.8 5.8 6.2

3.9 6.0 4.2 5.0 4.6 6.1 3.8 6.4 5.4 7.9 4.7 7.2 5.8 6.6 5.4 5.2 6.5 5.4 5.6

4.4 5.5 3.9 6.9 5.5 6.5 4.3 5.2 7.0 4.4 5.2 5.8 5.3 6.5 6.6 3.6 6.5 6.5 5.2

93.1 96.5 92.8 94.8 94.1 98.3 90.7 92.6 95.0 98.9 91.0 93.4 91.3 93.5 95.6 93.8 98.4 95.9 99.7

tosan content, but differences were found with other components (Table 3). There was a clear increase in arabinose, glucose and uronic acid content in samples obtained at higher temperatures (Table 3) indicating slight release of carbohydrates not associated with arabinoglucuronoxylan [25]. Glucose content observed (3.8–7.9%) was probably overestimated by small amounts of mannose and galactose present as contaminant in hemicellulose preparation [10]. The FT-IR spectra of some samples of hemicelluloses, with contrasting lignin and hemicellulose content, such as 1, 3, 12, 16 and 17 (Table 1) are showed in Fig. 2. The FT-IR spectra showed predominant resonance at 1050 cm−1 corresponding to vibrations of the C–O, C–C and C–OH linkage. The peak at 980 cm−1 indicated the presence of arabinan [26], and signal at 1175 cm−1 can be attributed the presence of arabinosyl side chains. A sharp band at 900 cm−1 indicated the presence of ␤-glucosidic linkage (C–O–C) between the sugar units in the hemicellulose [7,26]. The peaks at 1240 cm−1 indicated the presence of the C–H, OH or CH2 groups, and the vibration at 1465 indicates CH2 . The occurrence of a small band at 1510 cm−1 confirmed that lignin was present mainly in the samples 1 and 17, which present higher lignin content. In general, the hemicellulose preparations showed typical IR spectra without significant

Fig. 2. FT-IR spectra of hemicelluloses. Numbers correspond the condition of variables’ range and experimental design as shown in Table 1, (A) experiment 1; (B) experiment 3; (C) experiment 12; (D) experiment 16; (E) experiment 17.


203

sugar in all isolated hemicelluloses, comprising 73.1–82.6% of the total hemicellulose, while arabinose, glucose and glucuronic acid were present in small amounts. The alkaline hydrogen peroxide oxidation afforded hemicelluloses containing 4.6–14.1% of associated lignin. In terms of high yield, light color and minimal degradation of hemicellulose, an optimal treatment was found to be at 6% (w/v) hydrogen peroxide, for 4 h and 20 ◦ C. According to these results the alkaline hydrogen peroxide treatment of sugarcane bagasse has a great potential for isolation of sugarcane bagasse hemicellulose in high yields reaching 90%. Acknowledgement Fig. 3. Scores plot of FT-IR spectra of different hemicellulose samples.

The authors are grateful to Fundação de Amparo a Pesquisa do Estado de São Paulo–FAPESP for financial support (Grants 06/03564-9 and 08/03204-8). References

Fig. 4. Loads plot for a date matrix containing FT-IR spectra of different hemicellulose samples.

differences ascribed to varied peroxide treatments. However, the intensity of the band at 1650 cm−1 varied among samples presenting a close correlation with the color of the material. This band has been assigned to C O groups and increased in intensity from the brighter samples (assays 1 and 3) to darkest one (assay 17). Besides the lignin content, the color of the material has relation, probably, with oxidative processes that produce hexenuronic acids and chromophores from lignin. According to Dalimova [27], the strength in this region depends on the degree of oxidation. The absence of signal at 1750 cm−1 indicates that the alkaline peroxide treatment saponified ester bonds, presumable the acetates. Monitoring structural or compositional changes in hemicelluloses by analysis of isolated bands in the FT-IR spectra is difficult; hence the spectral information is overlapped. On the other hand, performing principal component analysis (PCA) on the spectral information helps discriminate among samples [28]. The PCA score plot showed that samples obtained with different extraction conditions were clearly separated on the PC1 axis (Fig. 3). Sample 16, a sub-group containing samples 1, 3 and 12, and sample 13 were discriminated along PC1 that contained most of the spectral information (85%). Samples 1, 3 and 12 were discriminated only along PC2. The loadings plot for PC1 (Fig. 4), showed that the bands near to 1040 and 1650 cm−1 were responsible for most of the spectral differences observable in the samples. 4. Conclusions Sugarcane bagasse hemicelluloses are predominantly removed during alkaline hydrogen peroxide extractions. In general, the conclusion must be drawn that, at high magnesium sulfate, the hydrogen peroxide has pronounced effect on the extraction of more purified hemicellulose with a high yield. It can also be concluded that hemicellulose yield cannot be enhanced by using more drastic treatments. Isolated hemicelluloses showed only negligible deviations in their carbohydrate compositions with increasing hydrogen peroxide concentration. Xylose was the predominant component

[1] P. McKendry, Energy production from biomass (part 1): overview of biomass, Bioresour. Technol. 83 (2002) 37–46. [2] F. Saavedra, S. Karacsonyi, J. Monzon, D. Cordero, J. Gonzales, The hemicelluloses of sugar cane, in: Proceedings of XX Congress, São Paulo, 1989, pp. 269–275. [3] D.U. Lima, R.C. Oliveira, M.S. Buckeridge, Seed storage hemicelluloses as wetend additives in papermaking, Carbohydr. Polym. 52 (2003) 367–373. [4] J. Hartman, A.N. Albertsson, M.S. Lindblad, J. Sjöberg, Oxygen barrier materials from renewable sources: material properties of softwood hemicellulose-based films, J. Appl. Polym. Sci. 100 (2006) 2985–2991. [5] D. Melandri, A. Angelis, R. Orioli, G. Ponzielli, P. Lualdi, N. Giarratana, V. Reiner, Use of a new hemicellulose dressing (Veloderm1) for the treatment of splitthickness skin graft donor sites A within-patient controlled study, Burns 32 (2006) 964–972. [6] J.M. Brillouet, J.P. Joseleau, J.P. Utille, D. Lelievre, Isolation, purification and characterization of a complex heteroxylan from industrial wheat bran, J. Agric. Food Chem. 30 (1982) 488–494. [7] J.X. Sun, X.F. Sun, R.C. Sun, Y.Q. Su, Fractional extraction and structural characterization of sugarcane bagasse hemicelluloses, Carbohydr. Polym. 56 (2004) 195–204. [8] Y.J. Lee, C.H. Chung, F. Donal, Sugarcane bagasse oxidation using a combination of hypochlorite and peroxide, Bioresour. Technol. 100 (2008) 275–279. [9] C. Martín, H.B. Klinke, A.B. Thomsen, Wet oxidation as a pretreatment method for enhancing the enzymatic convertibility of sugarcane bagasse, Enzyme Microb. Technol. 40 (2007) 426–432. [10] F. Xu, J.X. Sun, F.C. Liu, R.C. Sun, Comparative study of alkali and acid organic solvent-soluble hemicellulosic polysaccharides from sugarcane bagasse, Carbohydr. Res. 341 (2006) 253–261. [11] J.M. Fang, R.C. Sun, D. Salisbury, P. Fowler, J. Tomkinson, Comparative study of hemicelluloses from wheat straw by alkali and hydrogen peroxide extractions, Polym. Degrad. Stabil. 66 (1999) 423–432. [12] L. Ban, X. Chai, J. Guo, W. Ban, L.A. Lucia, Chemical response of hardwood oligosaccharides as a statistical function of isolation protocol, J. Agric. Food Chem. 56 (2008) 2953–2959. [13] C. Maes, J.A. Delcour, Alkaline hydrogen peroxide extraction of wheat bran nonstarch polysaccharides, J. Cereal Sci. 34 (2001) 29–35. [14] A. Thakore, J. Oei, B. Ringrose, A. Gibson, M. Wajer, The use magnesium hydroxide as a cost effective cellulose protector in the pressurized alkaline peroxide (Eop) bleaching stage, Pulp Paper (Canada) 106 (2005) 46–49. [15] G.X. Pan, J.L. Bolton, G.J. Leary, Determination of ferulic and p-coumaric acids in wheat straw and the amounts released by mild acid and alkaline peroxide treatment, J. Agric. Food Chem. 46 (1998) 5283–5288. [16] J.F. Kadla, H. Chang, The reaction of peroxides with lignin and lignin model compounds, in: D.S. Argyropoulos (Ed.), Oxidative Delignification Chemistry: Fundamentals and Catalysis, ACS Symposium series, vol. 785, McGill University, American Chemical Society, Washington, 2001, pp. 108–129. [17] Y. Wang, J. Zhang, A novel hybrid process, enhanced by ultrasonication, for xylan extraction from corncobs and hydrolysis of xylan to xylose by xylanase, J. Food Eng. 77 (2006) 140–145. [18] B.L. Browning, Methods of Wood Chemistry, John Willey & Sons, New York, 1967. [19] C.W. Dence, The determination of lignin, in: Y.L. Lin, C.W. Dence (Eds.), Methods in Lignin Chemistry, Springer, Berlin, 1992, pp. 33–62. [20] J. Li, K. Kisara, S. Danielsson, M.E. Lindström, G. Gellerstedt, An improved methodology for the quantification of uronic acid units in xylans and other polysaccharides, Carbohydr. Res. 342 (2007) 1442–1449. [21] W. Cai, X. Gu, J. Tang, Extraction, purification, and characterization of the polysaccharides from Opuntia milpa alta, Carbohydr. Polym. 71 (2008) 403–410. [22] R.C. Sun, J. Tomkinson, Y.X. Wang, B. Xiao, Physico-chemical and structural characterization of hemicelluloses from wheat straw by alkaline peroxide extraction, Polymer 41 (2000) 2647–2656.

204


[23] L. Lapierre, R. Berry, J.R. Bouchard, The effect of magnesium ions and chelants on peroxide bleaching, Hollzforschung 57 (2003) 627–633. [24] A. Wójciak, Influence of magnesium addition on hydrogen peroxide bleaching of the acid treated pinewood kraft pulp, Electro. J. Polish Agric. Univ. 9 (2006) 1–14. [25] B.P. Lavarack, G.J. Griffin, D. Rodman, The acid hydrolysis of sugarcane bagasse hemicellulose to produce xylose, arabinose, glucose and other products, Biomass Bioenergy 23 (2002) 367–380.

[26] R.C. Sun, J. Tomkinson, Characterization of hemicelluloses obtained by classical and ultrasonically assisted extractions from what straw, Carbohydr. Polym. 50 (2002) 263–271. [27] G.N. Dalimova, Oxidation of hydrolyzed lignin from cotton-seed husks by hydrogen peroxide, Chem. Nat. Compd. 41 (2005) 85–87. [28] P. Fardim, N. Duran, Effects of kraft pulping on the interfacial properties of Eucalyptus pulp fibres, J. Braz. Chem. Soc. 16 (2005) 915–921.