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Enhanced enzymatic saccharification of rice straw by microwave pretreatment.

Enhanced enzymatic sacchari?cation of rice straw by microwave pretreatment

Huan Ma a ,Wei-Wei Liu b ,Xing Chen c ,Yue-Jin Wu a,*,Zeng-Liang Yu a

Key Laboratory of Ion Beam Bioengineering,Institute of Plasma Physics,Chinese Academy of Sciences,Hefei 230031,China b

School of Engineering,Anhui Agricultural University,Hefei 230036,China c

Hefei Institute of Intelligent Machines,Chinese Academy of Sciences,Hefei 230031,China

a r t i c l e i n f o Article history:

Received 3June 2008

Received in revised form 26August 2008Accepted 26August 2008

Available online 17October 2008Keywords:Microwave Pretreatment Rice straw

Enzymatic sacchari?cation

Response surface methodology (RSM)

a b s t r a c t

In this study,Box–Behnken design and response surface methodology were employed to plan experi-ments and optimize the microwave pretreatment of rice straw.Experimental results show that micro-wave intensity (MI),irradiation time (IT)and substrate concentration (SC)were main factors governing the enzymatic sacchari?cation of rice straw.The maximal ef?ciencies of cellulose,hemicellulose and total sacchari?cation were respectively increased by 30.6%,43.3%and 30.3%under the optimal conditions of MI 680W,IT 24min and SC 75g/L.The chemical composition analysis of straw further con?rmed that microwave pretreatment could disrupt the silici?ed waxy surface,break down the lignin–hemicellulose complex and partially remove silicon and lignin.

1.Introduction

Rice straw is one of the most abundant agriculture residues in China with a production of 180million tons every year (Cai et al.,2007).The conventional utilization of rice straw,e.g.,?eld burning,is being phased out in China,because of its low heat of combustion.However,the available sugars,which are locked in the rice straw,could be subsequently converted to different valuable products such as ethanol and thus attract increasing interests.A key issue for utilization of lignocellulosic biomass is the disruption of com-plex matrix of polymers to liberate the monosaccharides.Thus,development of pretreatment methods that increase the material digestibility for the subsequent enzymatic hydrolysis becomes a focus in this research ?eld (J?rgensen et al.,2007).

Over the years,a number of different methods,including uncat-alyzed steam explosion,liquid hot water,dilute acid,?ow-through acid pretreatment,lime,wet oxidation and ammonia ?bre/freeze explosion,have been developed for the pretreatment of lignocellu-losic biomass (Liu and Wyman,2005).The general idea of these methods is to remove or alter the hemicellulose or lignin,decrease the crystallinity of cellulose and increase the surface area (Mosier et al.,2005).Actually,rice plant,one of the typical silicon-accumu-lating organisms,accumulates up to 10%silicon in the shoot,and so is the case of rice straw (Van Soest,2006).Therefore,silicon is another limiting factor in rice straw pretreatment.Additionally,most of these pretreatment methods require high-temperature

or high-pressure reactions and the application of dose of chemi-cals which may be toxic to the enzymes or the fermentative microorganisms.The removal of these toxicants is always costly and complicated.

Microwave irradiation,different from the conventional heating methods,has been successfully applied in many ?elds because of either non-thermal or thermal effects,which arise from the heating rate,‘‘hot spots”,acceleration of ions and collision with other mol-ecules,and rapid rotation of dipoles such as water with an alter-nating (2450million times/s)electric ?eld (Banik et al.,2003).Since Azuma et al.(1984)and Ooshima et al.(1984)demonstrated that microwave irradiation played a positive role in biomass diges-tion,it has become desirable to investigate the key operating parameters affecting the pretreatment so as to optimize the condi-tions for a further ef?cient hydrolysis of biomass.However,so far most of the previous studies (Keshwani et al.,2007;Zhu et al.,2006)have been conducted merely by one-variable-at-a-time experiments,which are time-consuming,and may also result in wrong conclusions for optimizing a multivariable system without consideration of the interactive effects among the variables.More-over,it is dif?cult to fairly evaluate the in?uence of microwave irradiation on recalcitrant structures of straw,since there are few reports concerning the silicon removal and the physicochemical surface characteristics of rice straw without adding other chemi-cals,such as alkali,acid or H 2O 2in the microwave pretreatment process.

Response surface methodology (RSM)is a collection of statisti-cal techniques for designing experiments,building models,evalu-ating the effects of factors (Yue et al.,2008),which extracts the

*Corresponding author.Tel.:+865515593172;fax:+865515591310.E-mail address:yjwu@https://www.wendangku.net/doc/9c17243693.html, (Y.-J.Wu).Bioresource Technology 100(2009)

1279–1284

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j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /b i o r t e c

maximal information with the minimal number of runs.In order to systemically study the effect of microwave on digestibility of rice straw without dose of other chemicals,a Box–Behnken design (BBD)was applied in this study to optimize the pretreatment con-ditions and investigate the effects of microwave intensity(MI), irradiation time(IT),and substrate concentration(SC)on the cellu-lose sacchari?cation(CS)as well as hemicellulose sacchari?cation (HS)of rice straw.Changes of main chemical components for pre-treated straw were also determined to evaluate the in?uence of microwave irradiation on recalcitrant structures.

2.Methods

2.1.Raw materials and microwave pretreatment

Rice straw used in the experiments was harvested at maturity in October2006from a local farm in Hefei,China.The air-dried rice straw was milled to powder.The particles that passed through a 20-mesh sieve(850l m)but were retained by a40-mesh (450l m)sieve were stored in a sealed plastic bag at room temper-atures.The chemical compositions of rice straw were determined in?ve replicates.

Microwave pretreatments were carried out in a Galanz/WP800T microwave at a frequency of2.45GHz.The microwave oven had a maximal power of800W with six discrete settings.The mixtures were placed in a sealed glass vessel and treated by the microwave according to the experimental design.After pretreatment,the slur-ry was?ltered through a Whatman?lter paper to separate resi-dues and liquid.The?ltered cakes were dried at35°C and stored for enzymatic hydrolysis,while the liquid fraction was collected to determine the glucose and xylose yields obtained in the process of microwave pretreatment.

2.2.Enzymatic hydrolysis

The cellulase enzyme used in this study was a commercial Trichoderma reesei cellulase kindly supplied by Guangzhou Global Green Tech.Ltd.,China.The activity of carboxymethyl-cellulase,?lter paper enzyme,b-glucosidase and hemicellulase were5.82?104unit/ml,826FPU/ml,9.1unit/ml and285unit/ml,respectively. The hydrolysis experiments were conducted in100ml Erlenmeyer ?asks with the hydrolysis mixture,which consisted of1.0g of rice straw,600l l of liquid cellulase and20ml of acetate buffer solu-tion at pH 4.8.The buffer solution was supplemented with 40mg/L of antibiotics tetracycline and30mg/L of cycloheximide to prevent microbial contamination.The hydrolysis mixture was incubated at40°C in an orbital shaker at110rpm for100h.After incubation,samples were collected and centrifuged for sugar anal-ysis.The enzymatic hydrolysis experiments were performed in?ve replicates.

2.3.Experimental design and data analysis

In this study,BBD was used to evaluate the main and interac-tion effects of the factors:MI(X1),IT(X2),and SC(X3)on CS(Y1) and HS(Y2)obtained from both the pretreatment and enzymatic hydrolysis experiments.The range and levels of the variables investigated are given in Table1,whereas the experimental de-signs with the observed responses and predicted values for CS and HS are presented in Table2.A polynomial quadratic equation was?tted to evaluate the effect of each independent variable to the response:

Y?b0t

i?1b i x it

i?1

b ii x2

i?1

j?it1

b ij x i x je1T

where x i,x j are the input variables,which in?uence the response

variable Y,b0the offset term,b i the i th linear coef?cient,b ii the qua-

dratic coef?cient and b ij is the ij th interaction coef?cient.The re-

sponse surfaces of the variables inside the experimental domain

were analyzed using Design Expert software(Version7.1,Stat-Ease

Inc.,Silicon Valley,CA,USA).Subsequently,?ve additional con?r-

mation experiments were conducted to verify the validity of the

statistical experimental strategies.

2.4.Analytical methods

The glucose concentration of rice straw was analyzed using an

SBA-40C Biosensor(Biology Institute of Shandong Academy of Sci-

ences,Shandong,China)with25l l of centrifuged supernatant col-

lected from the hydrolysis products.The xylose content was

determined with the phloroglucinol method(Ashwell,1966).

Moisture,total solids and volatile solids were analyzed accord-

ing to NREL Laboratory Analytical Procedures(LAP)(Sluiter et al.,

2008),while neutral detergent?bre,acid detergent?bre,cellulose

and hemicellulose were determined with the sequential analysis

method developed by Van Soest and Robertson(1985).Acid-insol-

uble lignin,acid-soluble lignin,acid-insoluble ash and acid-soluble

ash were determined using the procedures recommended by NREL

LAP003and014(Hyman et al.,2007).

Filter-paper activity of cellulase was measured following the

standard procedures recommended by the Commission Biotech-

nology,IUPAC(Ghose,1987).b-Glucosidase activity was assayed

for30min at50°C using an SBA-40C Biosensor to determine the

glucose concentration released.Carboxymethyl-cellulase activity

was determined according to Mandels et al.(1976).Xylanase activ-

ity was analyzed according to the method of Mohana et al.(2008).

2.5.Calculation methods

In order to clearly re?ect the effect of microwave pretreatment

on the recalcitrant structures and major available components of

Table1

Independent variables used in the BBD and actual factor levels corresponding to

coded factor levels

Independent

variable

Code Uncode Actual factor level

at coded factor levels

à10+1

MI(W)X1x1320500680

IT(min)X2x2202530

SC(gTS/L)X3x3607590

Table2

BBD with the observed and predicted values for CS and HS in straw

Run Coded variable level CS,Y1(%)HS,Y2(%)

X1X2X3Observed Predicted Observed Predicted

1à1à1031.130.815.715.0

2à11031.232.415.215.1

3à10à130.630.115.415.6

4à10131.130.814.615.2

50à1à132.633.516.216.7

601à132.531.917.117.0

70à1132.533.216.816.8

801134.433.518.017.5

900036.236.619.819.0

1000036.936.618.518.9

1100036.736.618.618.9

121à1039.137.918.318.5

1311034.735.018.619.3

1410à134.735.119.418.8

1510135.035.619.919.7

1280H.Ma et al./Bioresource Technology100(2009)1279–1284

rice straw,the CS and HS were calculated respectively based on the yields of glucose and xylose obtained from both pretreatment and enzymatic hydrolysis.

CS was expressed as gram of cellulose digested per1g of cellu-lose of biomass(untreated or microwave pretreated rice straw) used in both pretreatment and hydrolysis systems.The water mol-ecule added upon hydrolysis of the cellulose polymer was corrected by multiplying the glucose reading by0.9,according to NREL laboratory analytical procedures(LAP)009(Brown and Torget,1996).CS was calculated from the following equation:

%CS?

grams glucose formed?0:9

grams cellulose in rice straw added

?100e2T

The content of xylose released from rice straw was used to evaluate HS,as xylose is the most important sugar of hemicellulose and of noticeable amount in rice straw(Karimi et al.,2006).The calcula-tion formula is as follows:

%HS?

grams xylose released

grams hemicellulose in rice straw added

?100e3T

3.Results and discussion

3.1.Preliminary results

Preliminary experiments(one factor at a time approach)were performed to determine the main factors and the appropriate ranges in which the optima lie.The effects of four factors(MI,IT, SC and soak time in water before microwave treatment)on straw sacchari?cation were tested.Among the variables screened,MI, IT,and SC were identi?ed as the most signi?cant variables(data not shown)with a range of320–680W,20–30min,60–90gTS/L, respectively.Although the samples with a low-solid content(and accordingly a high water content)receive more energy in micro-wave pretreatment,which eventually facilitates the disruption of the lignocellulosic complex(Hu et al.,2008),60–90gTS/L was cho-sen as the appropriate range of SC after both energy ef?ciency and minimized water usage were taken into account.The enzymatic digestibility did not substantially change in the soak time range of0–24h in water before microwave irradiation.

3.2.RSM results

Further optimization of the microwave effect on straw sacchar-i?cation was achieved by employing BBD of experiments.Data were analyzed using Design Expert software to yield analysis of variance(ANOVA),regression coef?cients and regression equation. The polynomial equation,describing the CS as a simultaneous function of the MI(X1),IT(X2)and SC(X3),is shown in Eq.(4)

Y1?36:605t2:438X1à0:318X2t0:322X3à1:137X1X2

à0:062X1X3t0:506X2X3à1:369X2

1à1:230X2

à2:360X2

e4T

Table3depicts the ANOVA for the?tted model.The model F value of7.38implies that the model was signi?cant,and the coef?cient of variation(R2=0.93)indicates a high correlation between the ob-served and predicted values.At the same time,the lack-of-?t statis-tics,which is used to test the adequacy of the model,indicates that the P-value of0.0553was not signi?cant.No abnormality was ob-served from the diagnoses of residuals.Thus,it can be concluded that the model was statistically sound.The P-value denoting the signi?cance of the coef?cients was also important in understanding the pattern of the mutual interactions between the variables.The independent variables MI and the quadratic term of SC had signi?-cant effects on the CS,since the P-values less than0.05indicates the signi?cant model terms.

The regression model for the HS is presented in Eq.(5).ANOVA given in Table4indicates that the predicted model matched the observed values.Additionally,the P-values suggest that the inde-pendent variables X1and quadratic term of X2had signi?cant ef-fects on the HS

Y2?18:972t1:910X1t0:218X2t0:147X3t0:199X1X2t0:321X1X3t0:087X2X3à0:844X2

à1:166X2

à0:803X2

e5TThe3D response surfaces and the2D contour plots of the responses using Eq.(4)for the CS and Eq.(5)for the HS are respectively shown in Figs.1and2.To depict the interactive effects of independent variables on responses,one variable was kept constant while the other two variables varied in certain ranges.The shapes of response surfaces and contour plots indicate the nature and extent of the interaction between different factors(Prakash et al.,2008).Less prominent or negligible interactions were shown by the circular nature of the contour plots(Figs.1b and2c),while comparatively prominent interactions were otherwise shown by the elliptical nat-ure of the contour plots(Figs.1a,c,2a and b).

Similarly,corresponding to Figs.1a and2a,the interactive effect between MI and IT signi?cantly in?uenced the straw digestibility, Table3

ANOVA of the quadratic model for the CS

Source of

variations

Degree of

freedom

Sum of

squares

Mean

squares

F-value P a>F R2b

Model0.0202

984.73009.41457.3813*0.9300

0.0017

X1147.553447.553337.3057*

X210.81040.81040.63580.4614

X310.83380.83380.65410.4554

X1X21 5.1726 5.1726 4.05790.1001

X1X310.01560.01560.01220.9163

X2X31 1.0243 1.02430.80360.4111

X211 6.9174 6.9174 5.42670.0672

X221 5.5884 5.5884 4.38410.0905

0.0102

120.566520.566516.1344*

Residual5 6.3954 1.2791

Lack-of-?t3 6.1576 2.052517.25780.0553

a Probability values(P-values).

b Coef?cient of determination,CV-3.33.

Table4

ANOVA of the quadratic model for the HS

Source of

variations

Degree of

freedom

Sum of

squares

Mean

squares

F-value P a>F R2b

Model0.0337

939.0628 4.3403 5.7953*0.9250

0.0015

X1129.193229.193239.0088*

X210.38170.38170.51010.5070

X310.17370.17370.23200.6504

X1X210.15500.15500.20720.6681

X1X310.41250.41250.55110.4912

X2X310.03070.03070.04110.8474

1 2.6275 2.6275 3.51090.1198

0.0488

X221 5.0217 5.0217 6.7101*

X231 2.3788 2.3788 3.17860.1347 Residual5 3.74470.7489

Lack-of-?t3 2.60530.8684 1.52430.4197

a Probability values(P-values).

b Coef?cient of determination,CV-4.35.

H.Ma et al./Bioresource Technology100(2009)1279–12841281

no matter the CS or HS.It supports previous studies that increasing severity(reaction time and temperature)of biomass-pretreatment could enhance hemicellulose removal and cellulose digestibility as well(Kabel et al.,2007).However,an extended IT with a higher MI could also lead to a decrease in straw digestibility,because in-creases in IT and MI lead to high temperature which could trigger decomposition of released sugar in pretreatment process(Liu and Wyman,2005;Kabel et al.,2007).For example,at high tempera-ture dissolved xylose can be decomposed into furfural,resulting in a much lower total xylose yield.Besides,furfural is an undesir-

1282H.Ma et al./Bioresource Technology100(2009)1279–1284

able end-product that is known to inhibit fermentation processes (Eggeman and Elander,2005).

Furthermore,both CS and HS were found to increase with a simultaneous increase in SC at the middle levels,while a further in-crease in the level results in a gradual decrease in sacchari?cations. It might be caused by the different‘‘energy effect”with different SC.In other words,the samples with a high SC(and thus relatively low water loading)will receive less energy absorbed by water due to oscillation of water molecules(Yang and Wyman,2004),which is not bene?cial to the structure disruption of rice straw.As a re-sult,a higher straw digestibility was obtained for samples at a rel-atively low SC.

3.3.Optimization and con?rmation experiments

The optimal conditions for microwave pretreatment were ex-tracted by Design Expert software though a graphical optimization. Taking both costs and ef?ciency into consideration,the optimum operating parameters were found to be:x1=680W,x2=24min, x3=75g/L.Under these conditions,con?rmation experiments were conducted in?ve replicates.The observed mean CS and HS were found to be largely consistent with the predicted values.

Additionally,the CS,HS and total sacchari?cation(TS)of rice straw were respectively increased by30.6%,43.3%and30.3%after microwave irradiation(Table5),demonstrating the bene?cial ef-fects of microwave pretreatment.At the same time,it is further con?rmed the rationality and practicability of optimal conditions for microwave pretreatment in this work.

3.4.Changes of main chemical components in straw with microwave pretreatment under optimum conditions

All compositions(expect moisture)were calculated based on the dry weight of samples.As shown in Table6,the acid-soluble lignin and acid-soluble ash of the rice straw pretreated by micro-wave decreased from2.1%to1.9%and5.3%to3.2%,respectively. The increases in cellulose and hemicellulose content were predom-inantly attributed to the decreases in lignin and ash.These results indicate that microwave pretreatment could partially disrupt the lignin structure and expose more accessible surface area of cellu-lose to cellulase.Furthermore,lignin removal could also reduce unproductive binding of cellulase to lignin(Lu et al.,2002).Conse-quently,it improved enzymatic biocatalysis and increased the yields of desired products and recycled more cellulase.Thus,the costs associated with enzymatic sacchari?cation of biomass could be remarkably reduced.On the other hand,it should be noted that microwave pretreatment removed7.5%of acid-insoluble ash(silica content)in rice straw.It also improved straw digestibility,as Si deposition in cell walls acts as another physical barrier,protecting the plant from enzymatic hydrolysis(Rˇezanka and Sigler,2008).

4.Conclusions

The conditions for microwave pretreatment of rice straw were optimized by using RSM and BBD.The optimal conditions were found as follows:MI at680W,IT at24min and SC at75g/L.Under these optimal conditions,the CS,HS and TS reached to37.8%,20.2% and31.8%with increased rates of30.6%,43.3%and30.3%as com-pared with the raw rice straw,respectively.These results show that microwave irradiation is an ef?cient pretreatment method to enhance rice straw digestibility.The chemical composition anal-ysis further con?rms that microwave pretreatment could disrupt the silici?ed waxy surface,break down lignin–hemicellulose com-plex,partially remove silicon and lignin,and expose more accessi-ble surface area of cellulose to cellulase.The structural properties of pretreated rice straw were also characterized using Fourier transform infrared spectroscopy,X-ray diffraction and atomic force microscopy.We will report these results in another paper.More-over,we will apply system designs and optimization of batch or continuous microwave pretreatment processes to increase straw digestibility,improve energy ef?ciency and reduce the https://www.wendangku.net/doc/9c17243693.html,-bination of microwave pretreatment and biological treatment is also of our concern.

Acknowledgements

We acknowledge the?nancial support from the Natural Sci-ence Foundation of China(No.3037863)and the Key Technolo-gies Research and Development Program of Anhui Province (0701*******).

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Table5

Predicted and experimental CS and HS under optimum conditions a

Control b(%)Predicted yield(%)Measured yield c(%)Increasing rate(%)

CS28.9±0.137.937.8±0.130.6±0.4

HS14.1±0.219.920.2±0.343.3±0.8

TS24.4±0.131.831.8±0.230.3±0.6

a The optimum conditions:MI680W,IT24min,SC75g/L.

b Rice straw used in control was untreated.

c Mean±standar

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Chemical composition(percent by dry weight)of rice straw

Composition(%)Untreated straw Microwave pretreated straw

Moisture 5.4±0.2 6.1±0.3

Total solids94.6±0.293.9±0.3

Volatile solids83.8±0.286.7±0.5

Ash16.2±0.213.3±0.5

Neutral detergent?bre70.1±1.384.0±0.7

Acid detergent?bre53.9±1.260.5±1.0

Hemicellulose16.2±0.223.6±1.3

Cellulose33.4±0.941.8±0.3

Acid-soluble lignin 2.1±0.0 1.9±0.0

Acid-insoluble lignin 6.8±0.8 6.9±0.4

Acid-soluble ash 5.3±0.9 3.2±1.1

Acid-insoluble ash10.9±0.110.1±0.5

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