b i o s y s t em s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 4 7 0e4 7 5

Avai lab le at www.sc iencedi rect .com

journa l homepage : www.e lsev i er . com/ locate / i ssn /15375110

Research Paper

Effect of SC-CO2 pretreatment in increasing rice strawbiomass conversion

Miao Gao a,b, Feng Xu a, Shurong Li b, Xiaoci Ji a, Sanfeng Chen a,**, Dequan Zhang b,*aState key laboratory of Agrobiotechnology and College of Biological Sciences, China Agriculture University, Beijing 100193, PR Chinab Institute of Agro-food Science and Technology Chinese Academy of Agricultural Sciences (IAFST), Beijing 100193, China

a r t i c l e i n f o

Article history:

Received 26 November 2009

Received in revised form

20 May 2010

Accepted 20 May 2010

Published online 2 July 2010

* Corresponding author. Tel.: þ86 010 628187** Corresponding author. Tel.: þ86 010 627315

E-mail addresses: chensf@cau.edu.cn (S.1537-5110/$ e see front matter ª 2010 IAgrEdoi:10.1016/j.biosystemseng.2010.05.011

Rice straw, pretreated with supercritical carbon dioxide (SC-CO2) at 10e30 MPa and

40e110 �C for 15e45 min at a 1:1 g/g liquid: solid ratio, was analysed for glucose yield from

enzymatic hydrolysis, main chemical composition, and supermolecular structure. Rice

straw that was pretreated with SC-CO2 at 30 MPa and 110 �C for 30 min had a final glucose

yield of 32.4� 0.5%, compared with 27.7� 0.5% for unpretreated straw, after enzymatic

hydrolysis with a mixture of cellulose and b-glucosidase. Unpretreated and pretreated rice

straw differed in chemical composition, with SC-CO2 pretreatment removing some non-

lignocellulose material. Scanning electron microscopy showed that pretreated rice straw

had extensive anomalous porosity and lamellar structures. Also, SC-CO2 pretreatment

rendered fibres relatively fluffy and soft, which enhanced cellulase enzymatic hydrolysis.

ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved.

1. Introduction digestibility at the enzymatic hydrolysis stage, which is key to

As an agricultural residue, rice straw is a low-cost and abun-

dant (700 Mt yr�1 in China) rawmaterial for the bioconversion

of lignocellulose biomass to biofuel (ethanol and biodiesel).

The main components of rice straw are cellulose (32e47%),

hemicellulose (19e27%), and lignin (5e24%) (Gil et al., 2002;

Keikhosro, Shauker, & Mohammad, 2006). Rice straw is diffi-

cult to digest, and the final sugar yield from enzymatic

hydrolysis without pretreatment is generally less than 20% of

the theoretical maximum (Kyoung & Juan, 2001). The diges-

tion of rice straw is limited by several factors, including

cellulose crystallinity, degree of polymerisation, moisture

content, available surface area, and lignin content (Galbe &

Zacchi, 2007; Hendriks & Zeeman, 2009; Zhao et al., 2009).

Pretreatment increases the accessible surface area of

lignocellulose, degrades straw content, and improves biomass

40.51.Chen), dqzhang0118@126. Published by Elsevier Lt

biomass conversion. Pretreatment methods are generally

categorised into physical, chemical, physicochemical, and

biological processes and include steam explosion (Chen & Liu,

2007; Li & Chen, 2008), ozonolysis (Teresa, Gerardo, Irune,

Monica, & Silvia, 2009), alkali treatment (Wang, Liu, Li, Wu,

& Ye, 2007), dilute-acid hydrolysis (Keikhosro et al., 2006),

liquid hot water treatment (Perez et al., 2009), electron-beam

irradiation (Jin et al., 2009; Yang, Shen, Yu, &Wang, 2008), wet

oxidation (Carlos, Mette, Henrik, & Anne, 2008), and super-

critical fluid pre-treatment (Hendriks & Zeeman, 2009; Kyoung

& Juan, 2001; Zheng, Lin, & George, 1998; Zheng et al., 1995).

Supercritical fluid pretreatment agents include supercrit-

ical H2O, ammonia, and carbon dioxide (SC-CO2). Recent

attention has focused on SC-CO2 because of several potential

benefits. CO2 has a low critical temperature (31.1 �C) and

pressure (7.36 MPa), in which state it is characterised by “gas-

.com (D. Zhang).d. All rights reserved.

b i o s y s t em s e ng i n e e r i n g 1 0 6 ( 2 0 1 0 ) 4 7 0e4 7 5 471

like” mass transfer and “liquid-like” solvating power

(Muzafera, Mateja, Maja, Zoran, & Zeljko, 2007; Zhang & Hu,

2006). In the supercritical state, CO2 also has low viscosity

(3e7� 10�5 N sm�2) and zero surface tension, so it can quickly

penetrate complex structures and materials (Zhang et al.,

2006). Moreover, CO2 is non-flammable, is nontoxic, leaves

no residue, and is inexpensive and readily available. Also, SC-

CO2 could replace commonly used nonpolar organic solvents

(Muzafera et al., 2007).

In SC-CO2 extraction of pine wood, researchers reported no

significant change in the microstructure morphology of pre-

treated pine wood, indicating that SC-CO2 was of no value in

pretreating lignocellulose (Ritter & Campbel, 1986, 1991).

However, SC-CO2 explosive technology is a potentially effec-

tive pretreatment method, enhancing the rate of Avicel

(microcrystalline cellulose) hydrolysis as well as increasing

cellulosic material permeability (Musrizal, Akio, Masafumi,

Tsuyoshi, & Kunio, 2003) and glucose yield by as much as

50% (Zheng et al., 1995). The disruption of cellulosic structure

by the explosive release of CO2 pressure should increase the

accessible surface area of the substrate and reduce the degree

of cellulosic crystallinity, thus improving sugar production

after enzymatic hydrolysis (Zheng et al., 1995, 1998). Moreover,

explosive pretreatment with high-pressure CO2 acidifies the

liquid, resulting in acid hydrolysis, especially of hemicellulose

(Puri & Mamers, 1983; Hendriks & Zeeman, 2009). Kyoung

found that sugar yields from enzymatic hydrolysis of aspen

and southern yellow pine pretreated with SC-CO2 at 21.4 MPa

and 165 �C for 30 min were 84.7� 2.6% and 27.3� 3.8% of the

theoretical maximum, respectively (Kyoung & Juan, 2001).

Past studies have shown that high glucose yields can be

obtained from the enzymatic hydrolysis of some cellulose

materials pretreated with SC-CO2 at lower temperatures. For

example, Avicel registered a glucose yield of 72.6% after SC-

CO2 pretreatment at 20.7 MPa and 35 �C (Zheng et al., 1998).

However, some lignocellulose materials do not exhibit high

glucose yields, even at higher temperatures: southern yellow

pine had a sugar yield of only 27.3� 3.8% after SC-CO2

pretreatment at 21.4 MPa and 165 �C (Kyoung & Juan, 2001).

Thus, glucose yields from the enzymatic hydrolysis of various

materials pretreated with SC-CO2 vary widely.

Yields from the enzymatic hydrolysis of rice straw pre-

treated with SC-CO2 have yet to be reported. Here, the efficacy

of SC-CO2 pretreatment of rice straw at various temperatures

(40e110 �C), pressures, and lengths of time was investigated

and changes in the main chemical composition and super-

molecular structure analysed.

2. Materials and methods

2.1. Materials

Rice straw obtained in Beijing, China, was milled to a particle

size <2 mm using a laboratory hammer mill (Micro-Motor

Manufacturing, Tianjin, China). The milled straw was dried at

60 �C to constant weight and stored in a plastic bag main-

tained at 4 �C for further use.

Cellulase (EC 3.2.1.4, fromTrichoderma viride) was purchased

from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). b-

glucosidase was extracted from Aspergillus fumigatus. Carbox-

ymethyl cellulase and b-glucosidase activity were determined

to be 40 IUml�1 and 15 IUml�1 respectively.

CO2 was purchased from Beijing Analytical Apparatus Co.

(Beijing, China) and was purified to 99.99% purity using an

activated carbon column.

2.2. SC-CO2 pretreatment of rice straw

Pretreatment with supercritical carbon dioxide extractor

(ISCO Inc., USA) was performed in a 10-ml stainless steel

container at various pressures (10, 20, and 30 MPa), tempera-

tures (40, 80, and 110 �C), and lengths of time (15, 30, and

45 min). When the SC-CO2 treatment system reached the

desired temperature, the container of rice straw was loaded

into the high-pressure vessel, which was then closed tightly.

The connecting valves were then carefully opened, and liquid

CO2 from the siphon cylinder, which was maintained at

temperature with a water bath, was pumped into the

pretreatment vessel, fully pressurising it within 2 min. The

connecting valves were left open throughout the pretreat-

ment to maintain the desired pressure. After pretreatment for

the desired time, the treatment vessel was depressurised by

tightly closing the connecting valves and slowly opening the

decompression valve. Depressurisation took approximately

3 min. The pretreated rice straw was removed from the

stainless steel container, dried at 60 �C to constant weight,

and stored in a plastic bag for further use. All SC-CO2

pretreatment conditions were performed in triplicate, and the

system was cleaned with 70% (v/v) aqueous ethanol solution

between each treatment run.

2.3. Enzymatic hydrolysis of pretreated rice straw

Enzymatic hydrolysis tests were performed on samples under

various pretreatment conditions to determine improvements

to the system. Enzymatic digestibility tests were conducted as

follows: reaction conditions were 50 �C, pH 5.0 (0.05 M citrate

buffer solution), with incubation in a shake flask at 150 rpm

for 48 h. The enzyme mixture doses were cellulase at

30 FPU g�1 dry matter and b-glucosidase at 15 CBU g�1 dry

matter. The initial solid concentration was 2% (w/v) in 15 ml

total liquid. Sodium azide was added to the buffer solutions at

a final concentration of 50 ppm to prevent microbial growth

during incubation. After the enzymatic hydrolysis reaction,

the mixture was centrifuged immediately at 12,000 rpm for

10 min, and the supernatant was immediately collected for

analysis. Each sample test was performed in triplicate.

2.4. Determination of reducing glucose with the3,5-dinitrosalicylic acid method

The 3,5-dinitrosalicylic acid method was used to determine

reducing glucose. Each centrifuged hydrolysate was diluted

with deionisedwater and reactedwith 3,5-dinitrosalicylic acid

reagents, after which 1 ml 3,5-dinitrosalicylic acid was added

to each 1-ml aliquot hydrolysate to stop the reaction. All

samples were instantly placed into a hot water bath for 5 min

and then in a cold water bath for 10 min. The glucose

b i o s y s t em s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 4 7 0e4 7 5472

concentration was measured using a UVeVis spectropho-

tometer (UNIC WFJ7200, Shanghai) set at 540 nm.

2.5. Straw composition analysis

Cellulose, hemicellulose, and lignin contents were measured

using themethod of Van Soest (Van et al., 1991). Cellulose was

calculated as acid detergent fibre (ADF) e acid detergent lignin

(ADL), hemicellulose as neutral detergent fibre (NDF) e ADF,

and lignin as ADL e ADF-insoluble ash. NDF, ADF, and ADL

contentswere determined using an ANKOM220 Fibre Analyser

(ANKOMTechnology Corp., Fairport, NY, USA), and sequential

analysis of the rice straw and ADF-insoluble ash was done

using a muffle furnace. Each sample was analysed in

triplicate.

Fig. 1 e Effect of pretreatment pressure on the enzymatic

hydrolysis of rice straw. SC-CO2 pretreatment was

performed at a liquidesolid ratio of 1 g gL1 at 110 �C for

30 min. Enzymatic hydrolysis was conducted at 50 �C and

150 rpm in 0.05 M citric acid buffer (pH 5.0) with enzyme

loading of cellulase at 30 FPU gL1 and b-glucosidase at

15 CBU gL1 dry matter.

2.6. Scanning electron microscope (SEM) observation ofstructural fibre morphology

A field emission scanning electron microscope (JSM-6700F,

JEOL Ltd., Tokyo, Japan) was used to image the dried fibre

fraction of rice straw pretreated with SC-CO2 at 30 MPa and

110 �C for 30 min. The samples were sputtered with a thick

layer of gold spread uniformly from all sides at two different

angles.

3. Results and discussion

3.1. Effect of pretreatment pressure on enzymatichydrolysis

Differences in the enzymatic hydrolysis of rice straw pre-

treated with SC-CO2 at various pressures were investigated.

Fig. 1 shows the relationship between pretreatment pressure

and the glucose yield of enzymatic hydrolysis.

Glucose yield increased with the duration of enzymatic

hydrolysis, reaching a maximum of 30.2, 33.6, 33.8, and 35.4%

at 48 h at pretreatment pressures of 0, 10, 20, and 30 MPa,

respectively. The glucose yield increased with increasing

pretreatment pressure at 0e10 MPa, showed little change at

10e20 MPa, and continued to rise at >20 MPa, thus demon-

strating that pretreatment pressure plays an active role in

improving the glucose yield of enzymatic hydrolysis. Zheng

et al. (1998) reported that higher pressure carbon dioxide

resulted in higher glucose yields from enzymatic hydrolysis of

cellulosic products, such as Avicel, recycled paper mix, and

sugarcane bagasse. However, Kyoung and Juan (2001) reported

that increasing CO2 pressure from 21.4 to 27.6 MPa did not

increase the enzymatic digestibility of aspen or southern

yellow pine but in fact significantly decreased the sugar yield

of aspen (Kyoung & Juan, 2001). However, a negative effect of

increased CO2 pressure was not observed, possibly because of

the structural differences of the materials. Aspen and

southern yellow pine are denser and have a higher lignin

content, in contrast to the looser structure and lower lignin

content of Avicel, recycled paper, and the rice strawused here.

Thus, CO2 pressure may play different roles in different

environments.

3.2. Effect of pretreatment time on enzymatic hydrolysis

To explore the effect of pretreatment duration, the reaction

times of SC-CO2 pretreatment were varied. All of the experi-

ments were performed at 30 MPa pressure, 110 �C, and

a 1 g g�1 liquidesolid ratio. Fig. 2 compares the glucose yields

of enzymatic hydrolysis for various pretreatment times.

Glucose yields were 31.2, 32.4, and 32.5% at pretreatment

times of 15, 30, and 45 min, respectively, whereas the glucose

yield of untreated rice straw was 27.7%. Thus, pretreatment

duration failed to significantly affect glucose yields, but the

difference in glucose yield between pretreated and untreated

rice straw was significant. Hence, unlike temperature,

pretreatment duration does not appear to be a key factor in

improving glucose yield.

3.3. Effect of pretreatment temperature on enzymatichydrolysis

Rice straw was subjected to SC-CO2 pretreatment tempera-

tures of 40, 80, and 110 �C. Fig. 3 shows the relationship

between pretreatment temperature and the glucose yield of

enzymatic hydrolysis, with the yield increasing slowly as the

pretreatment temperature rose from 40 to 110 �C. After 48 h

hydrolysis, the glucose yield of SC-CO2-pretreated rice straw

reached 29, 30.5, and 32.4% at pretreatment temperatures of

40, 80, and 110 �C, respectively, whereas the glucose yield of

untreated rice straw was 27.6%. Thus, temperature is an

important factor in increasing the conversion efficiency of rice

straw. However, the glucose yield of pretreated rice straw was

far lower than that of Avicel (Zheng et al., 1998), which reached

72.6% after SC-CO2 pretreatment at 20.7 MPa and 35 �C and

subsequent enzymatic hydrolysis for 24 h. This suggests

Fig. 2 e Effect of pretreatment time on the enzymatic

hydrolysis of rice straw. SC-CO2 pretreatment was

conducted at a liquidesolid ratio of 1 g gL1 at 30 MPa and

110 �C. Enzymatic hydrolysis was as described in Fig. 1.

b i o s y s t em s e ng i n e e r i n g 1 0 6 ( 2 0 1 0 ) 4 7 0e4 7 5 473

that the structure and composition of rice straw are more

complex than those of Avicel, making rice strawmore difficult

to degrade.

The conversion efficiency of rice straw pretreated with SC-

CO2 is similar to that of g radiation pretreatment, which

yielded 8.63e10.24% glucose yield at a dose of 0e500 kGy

(Yang et al., 2008), but is significantly lower than that of steam

explosion, liquid hot water, and wet oxidation, which had

conversion efficiencies of up to 93.6% at 1.2 MPa and 195 �C for

10 min (Carlos et al., 2008). Hemicellulose connects lignin and

cellulose, conferring more rigidity on the cellulo-

seehemicelluloseelignin network (Farzaneh, Lizbeth, Hasan,

& Bruce, 2005; Hendriks & Zeeman, 2009), and hemicellulose

and lignin begin to solubilise at about 180 �C under neutral

conditions (Hendriks & Zeeman, 2009). Our tests might have

achieved a higher conversion rate if SC-CO2 the pretreatment

had been conducted at a temperature greater than 180 �C.

Fig. 3 e Effect of pretreatment temperature on the

enzymatic hydrolysis of rice straw. SC-CO2 pretreatment

was conducted at a liquidesolid ratio of 1 g gL1 at 30 MPa

for 30 min. Enzymatic hydrolysis was as described in

Fig. 1.

3.4. Structural constituents of rice straw

To increase the glucose yield of enzymatic hydrolysis,

pretreatment choices usually focus on removing composi-

tional impediments and modifying physico-chemical struc-

tural barriers (Sun & Chen, 2008). Accordingly, differences in

the chemical composition of pretreated and unpretreated rice

straw were investigated.

Fig. 4 shows the differences in the main chemical compo-

sition of unpretreated rice straw and rice straw pretreated at

30 MPa and 110 �C for 30 min. Unpretreated and pretreated

rice straw differ in crude cellulose, cellulose, hemicellulose,

and lignin contents. Hemicellulose and lignin contents of

untreated and pretreated rice straw were 5.7% and 22.6% and

21.8% and 5.8%, respectively (Fig. 4), suggesting that SC-CO2

pretreatment at 30 MPa and 110 �C for 30 min partly removes

hemicellulose but does not affect lignin. The crude cellulose

and cellulose contents of pretreated rice strawwere 68.7% and

41.2%, respectively, compared to 68.7% and 43.5%, respec-

tively, for untreated rice straw. SC-CO2 pretreatment led to

increasing the remaining cellulose compared to unpretreated

rice straw. This result was not consistent with the reported

reduction of cellulose by SC-CO2 pretreatment (Sun & Chen,

2008; Teresa et al., 2009). This phenomenon may be

explained by a feature of SC-CO2 that is effective in extracting

fat-soluble components of rice straw (Muzafera et al., 2007).

3.5. Scanning electron microscope observations of ricestraw structure

Scanning electron microscope (SEM) was used to compare

morphological changes in rice straw after pretreatment

and enzymatic hydrolysis. Fig. 5 shows SEM images of rice

straw before and after pretreatment at 30 MPa and 110 �C for

30 min at a liquidesolid ratio of 1 g g�1. Transverse sections of

rice straw showed obvious histological changes before and

after pretreatment (Fig. 5a, b). Before SC-CO2 pretreatment,

the straw surface was smooth, tight, and contiguous, whereas

after pretreatment it showed extensive anomalous porosity

and lamellar structures, and fibres became relatively fluffy.

Microfibrils were separated from the initial connected

Fig. 4 e Chemical composition of unpretreated and SC-

CO2-pretreated rice straw. The rice straw was pretreated at

30 MPa and 110 �C for 30 min, with 100% moisture content.

Fig. 5 e SEM micrographs of rice straw before and after

SC-CO2 pretreatment. (a) Transverse section of rice straw

stem before pretreatment. (b) Transverse section of rice

straw stem after SC-CO2 pretreatment.

b i o s y s t em s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 4 7 0e4 7 5474

structure and fully exposed, helping to expose the fasciculi

to enzyme access and attack and resulting in a high enzymatic

hydrolysis yield (Sun & Chen, 2008). Thus SC-CO2 ruptures the

physical structural barriers of rice straw and exposes fine

fibrils with more surface area and roughness, helping to

achieve a high enzymatic hydrolysis yield. Moreover, pre-

treated rice straw was much softer to the touch than was

untreated straw, as Zhang et al. (2006) also discovered.

4. Conclusions

Our results suggest that SC-CO2 pretreatment of rice straw at

low temperatures somewhat enhances the glucose yields of

enzymatichydrolysis, changes themainchemical composition,

andbreaksdown thestructureof rice straw.Theglucoseyieldof

enzymatic hydrolysis increasedwith increases in pretreatment

pressure, temperature, and time, reaching a maximum of

33.42� 2.1% at 30 MPa and 110 �C for 30min. The pretreatment

pressure plays an active role in SC-CO2 pretreatment of rice

straw, in contrast to SC-CO2 pretreatment of aspen and

southern yellow pine. Moreover, pretreatment temperature is

a key factor in further enhancing the glucose yield of enzymatic

hydrolysis. The main chemical composition of unpretreated

and pretreated rice straw differs, with SC-CO2 pretreatment

resulting in increased lignocellulose content and the removal of

some non-lignocellulose material. Finally, SEM observation

showed extensive anomalous porosity and lamellar structures

in pretreated rice straw, with fibres becoming relatively fluffy

and soft, thus exposing the fasciculi to enzyme access and

attack and resulting in a high enzymatic hydrolysis yield.

This study is a first step toward total utilisation of rice

straw by SC-CO2 pretreatment. To accelerate the biomass

conversion of rice straw, higher pretreatment temperatures

should be investigated.

Acknowledgements

The authors thank Fu Qin and Tao Haiteng for the technical

assistance of SC-CO2 pretreatment. Yan Jianhe is appreciated

for their equipment assistance of the ANKOM220 Fibre

Analyser. The authors are also grateful to Wu Lixian, Wang

Jianping, Li Huamin and Yuanyuan for helpful discussions.

r e f e r e n c e s

Carlos, M., Mette, H. T., Henrik, H. N., & Anne, B. T. (2008). Wetoxidation pretreatment, enzymatic hydrolysis andsimultaneous saccharification and fermentation ofclovereryegrass mixtures. Bioresource Technology, 99,8777e8782.

Chen, H. Z., & Liu, L. (2007). Unpolluted fractionation of wheatstraw by steam explosion and ethanol extraction. BioresourceTechnology, 98, 666e676.

Farzaneh, T., Lizbeth, L. P., Hasan, A., & Bruce, E. D. (2005).Optimization of the ammonia fiber explosion (AFEX)treatment parameters for enzymatic hydrolysis of corn stover.Bioresource Technology, 96, 2014e2018.

Galbe, M., & Zacchi, G. (2007). Pretreatment of lignocellulosicmaterials for efficient bioethanol production. Advances inBiochemical Engineering Biotechnology, 108, 41e65.

Gil, G., Herminia, D., & Juan, C. P. (2002). Autohydrolysis ofcorncob: study of non-isothermal operation forxylooligosaccharide production. Journal of Food Engineering, 52,11e18.

Hendriks, A. T. W. M., & Zeeman, G. (2009). Pretreatments toenhance the digestibility of lignocellulose biomass. BioresourceTechnology, 100, 10e18.

Jin, S. B., Ja, K. K., Young, H. H., Byung, C. L., In-Geol, C., &Kyoung, H. K. (2009). Improved enzymatic hydrolysis yield ofrice straw using electron beam irradiation pretreatment.Bioresource Technology, 100, 1285e1290.

Keikhosro, K., Shauker, K., & Mohammad, J. T. (2006). Conversionof rice straw to sugars by dilute-acid hydrolysis. BiomassBioenergy, 30, 247e253.

Kyoung, H. K., & Juan, H. (2001). Supercritical CO2 pretreatment oflignocellulose enhanced enzymatic cellulose hydrolysis.Bioresource Technology, 77(2), 139e144.

Li, H. Q., & Chen, H. Z. (2008). Detoxification of steam-explodedcorn straw produced by an industrial-scale reactor. ProcessBiochemistry, 43, 1447e1451.

Musrizal, M., Akio, A., Masafumi, I., Tsuyoshi, Y., & Kunio, T.(2003). Feasibility of supercritical carbon dioxide as a carriersolvent for preservative treatment of wood-based composites.Journal of Wood Science, 49, 65e72.

Muzafera, P., Mateja, P., Maja, H., Zoran, N., & Zeljko, K. (2007).Hydrolysis of carbonxymethyl cellulose catalyzed by celluloseimmobilized on silica gels at low and high pressures. Journal ofSupercritical Fluids, 43, 74e80.

Perez, J. A., Ballesteros, I., Ballesteros, M., Saez, F., Negro, F. S., &Manzanares, P. (2009). Optimizing liquid hot waterpretreatment conditions to enhance sugar recovery fromwheat straw for fueleethanol production. Fuel, 87, 3640e3647.

Puri, V. P., & Mamers, H. (1983). Explosive pretreatment oflignocellulosic residues with high-pressure carbon dioxide forthe production of fermentation substrates. Biotechnology andBioengineering, 25(12), 3149e3161.

Ritter, D. C., & Campbel, A. G. (1986). The effect of supercriticalcarbon dioxide extraction on pine wood structure. EighthSymposium on Biotechnology for Fuels and Chemicals 179e182.

Ritter, D. C., & Campbel, A. G. (1991). Supercritical carbon dioxideextraction of southern pine and ponderosa pine. Wood FiberScience, 23. 98el13.

Sun, F. B., & Chen, H. Z. (2008). Enhanced enzymatic hydrolysis ofwheat straw by aqueous glycerol pretreatment. BioresourceTechnology, 99, 6156e6161.

b i o s y s t em s e ng i n e e r i n g 1 0 6 ( 2 0 1 0 ) 4 7 0e4 7 5 475

Teresa, G. C., Gerardo, G. B., Irune, I., Monica, C., & Silvia, B.(2009). Effect of ozonolysis pretreatment on enzymaticdigestibility of wheat and rye straw. Bioresource Technology,100, 1608e1613.

Van Soest, P. J., Robertson, J. B., & Lewis, B. A. (1991). Methods forDietary Fiber, Neutral Detergent Fiber, and NonstarchPolysaccharides in Relation to Animal Nutrition. Journal ofDairy Science, 74, 3583e3597.

Wang, J. K., Liu, J. X., Li, J. Y., Wu, Y. M., & Ye, J. A. (2007).Histological and rumen degradation changes of rice strawstem epidermis as influenced by chemical pretreatment.Animal Feed Science Technology, 136, 51e62.

Yang, C. P., Shen, Z. Q., Yu, G. C., & Wang, J. L. (2008). Effect andaftereffect of g radiation pretreatment on enzymatichydrolysis of wheat straw. Bioresource Technology, 99,6240e6245.

Zhang, D. Q., & Hu, X. D. (2006). Food processing technology ofsupercritical CO2 fluid. Chemical Industry Press.

Zhang, J., Thomas, A. D., Michael, A. M., Michael, J. D., Martin, L. B.,& Yuehuei, H. A. (2006). Sterilization using high-pressurecarbon dioxide. Journal of Supercritical Fluids, 28, 354e372.

Zhao, H., Jones, C. L., Gary, A. B., Xia, S. Q., Olarongbe, O., &Vernecia, N. P. (2009). Regenerating cellulose from ionicliquids for an accelerated enzymatic hydrolysis. Journal ofBiotechnology, 139, 47e51.

Zheng, Y. Z., Lin, H. M., & George, T. T. (1998). Pretreatment forcellulose hydrolysis by carbon dioxide explosion. Progress inBiotechnology, 14, 890e896.

Zheng, Y. Z., Lin, H. M., Wen, J. Q., Cao, N. J., Yu, X. Z., & George, T.T. (1995). Supercritical carbon dioxide explosion asa pretreatment for cellulose hydrolysis. Biotechnology Letters, 17(8). 854e50.