Production of butanol (a biofuel) from agricultural residues: Part II – Use of corn stover and switchgrass hydrolysates☆

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 5 6 6 – 5 7 1

Avai lab le at www.sc iencedi rect .com

ht tp : / /www. e lsev ier . com/ loca te / b i ombi oe

Production of butanol (a biofuel) from agricultural residues:Part II – Use of corn stover and switchgrass hydrolysates5

Nasib Qureshi a,*, Badal C. Saha a, Ronald E. Hector a, Bruce Dien a, Stephen Hughes b,Siqing Liu b, Loren Iten a, Michael J. Bowman a, Gautam Sarath c, Michael A. Cotta a

a United States Department of Agriculture (USDA), Agricultural Research Service (ARS), National Center for Agricultural

Utilization Research (NCAUR), Bioenergy Research, 1815 N. University Street, Peoria, IL 61604, USAb USDA-ARS-NCAUR, Renewable Product Technology, 1815 N. University Street, Peoria, IL 61604, USAc USDA-ARS, Grain, Forage, and Bioenergy Research Unit, University of Nebraska, 314 Biochemistry Hall, East Campus,

Lincoln, NE 68583, USA

a r t i c l e i n f o

Article history:

Received 30 July 2009

Received in revised form

21 December 2009

Accepted 28 December 2009

Available online 20 January 2010

Keywords:

Butanol

Clostridium beijerinckii P260

Agricultural residue hydrolysates

Corn (Zea mays) stover

Energy crop – switchgrass

(Panicum virgatum)

Fermentation

Productivity

Yield

Overliming

5 Mention of trade names or commercial pnot imply recommendation or endorsement

* Corresponding author. Tel.: þ1 309 681 631E-mail address: [email protected]

0961-9534/$ – see front matter Published bydoi:10.1016/j.biombioe.2009.12.023

a b s t r a c t

Acetone butanol ethanol (ABE) was produced from hydrolysed corn stover and switchgrass

using Clostridium beijerinckii P260. A control experiment using glucose resulted in the

production of 21.06 g L�1 total ABE. In this experiment an ABE yield and productivity of 0.41

and 0.31 g L�1 h�1 was achieved, respectively. Fermentation of untreated corn stover

hydrolysate (CSH) exhibited no growth and no ABE production; however, upon dilution

with water (two fold) and wheat straw hydrolysate (WSH, ratio 1:1), 16.00 and 18.04 g L�1

ABE was produced, respectively. These experiments resulted in ABE productivity of

0.17–0.21 g L�1 h�1. Inhibitors present in CSH were removed by treating the hydrolysate

with Ca(OH)2 (overliming). The culture was able to produce 26.27 g L�1 ABE after inhibitor

removal. Untreated switchgrass hydrolysate (SGH) was poorly fermented and the culture

did not produce more than 1.48 g L�1 ABE which was improved to 14.61 g L�1. It is suggested

that biomass pretreatment methods that do not generate inhibitors be investigated.

Alternately, cultures resistant to inhibitors and able to produce butanol at high concen-

trations may be another approach to improve the current process.

Published by Elsevier Ltd.

1. Introduction such as corn in the United States and sugarcane in Brazil. It

Recent increases in fuel price have challenged all nations

across the world to develop their own biofuels from renewable

resources such as lignocellulosic crops. However, availability

of renewable agricultural biomass is geographically specific

roducts in this article is sby the U.S. Department

8; fax: þ1 309 681 6427.ov (N. Qureshi).Elsevier Ltd.

should be noted that use of corn in the United States appears

not to be cost effective, as there are challenges like food and

feed Vs fuel. As corn demand for converting to fuel ethanol

increased during the last year (2008), corn prices rose to high

levels (256.28 $ tonne�1) [1] thus making it difficult or cost

olely for the purpose of providing scientific information and doesof Agriculture.

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 5 6 6 – 5 7 1 567

ineffective to use this substrate for fuel ethanol production. As

a result of increase in corn prices, various laboratories across

the nation have initiated biofuel production program from

wheat straw, barley straw, corn stover, and energy crops such

as switchgrass, reed canary grass, and alfalfa. Wheat straw

(26.46 $ tonne�1), barley straw (28.66 $ tonne�1), pea straw

(48.50 $ tonne�1), corn stover (55.12 $ tonne�1), grass hay

(55.12 $ tonne�1), and switchgrass (66.14 $ tonne�1) can be

purchased at much lower prices than corn [1]. In our recent

work we have demonstrated that wheat straw is a good

substrate for butanol production [2,3]. This feedstock and

biofuel (butanol) has numerous attractive properties [4,5].

It is indicated that one single crop residue/biomass would

not be able to meet the biofuel demand. Hence, focus should

be placed on all types of biomass listed in the first paragraph

of this section. With this view, we attempted to expand the

use of different agricultural substrates for butanol production.

In the preceding paper, use of barley straw was investigated

[6]. The objective of the present investigation was to use corn

stover to produce butanol, as this lignocellulosic material is

abundant in the mid-western region of the United States.

Another substrate that we investigated was switchgrass,

a biomass feedstock promoted as a perennial energy crop. It

has been found that both of these substrates require addi-

tional treatments to promote fermentation. The hydrolysates

of corn stover and switchgrass were prepared employing

dilute sulfuric acid pretreatments followed by enzymatic

hydrolysis.

2. Materials and methods

2.1. Strain and inoculum development

Clostridium beijerinckii P260 was a generous gift from Professor

David Jones, University of Otago (Dunedin, New Zealand).

Details of culture propagation have been given in the

preceding article [6].

2.2. Corn stover hydrolysate (CSH)

Corn stover (Pioneer 32B83 variety: leaves, stems, and cobs)

was obtained from a local farmer (Forest: Geo-coordinates

N40.48, W88.93, Elevation 310 m; Illinois, USA) and stored dry

at room temperature until needed. The corn was harvested in

September/October 2007. Corn stover consists of leaves, cob,

husk, and stalks of the maize plants left in field after corn

(grains) harvest. 86 g of milled (1.27 mm sieve screen) corn

stover (stalks, cobs, and leaves) was suspended in 1% (v/v)

sulfuric acid (H2SO4) and mixed well. Then the mixture was

transferred to a 316 stainless steel reactor and heated to 160 �C

in a fluidized sand bath for 20 min. After cooling to 25 �C, the

pH of the mixture was adjusted to 5.0 with 400 g L�1 NaOH

solution. Enzymatic hydrolysis was carried out as described

for barley straw in the preceding paper [6]. It should be noted

that pretreatment conditions were different (higher temper-

ature) for corn stover (than wheat straw & barley straw)

because release of sugars from this substrate is difficult and

requires a higher temperature.

2.3. Switchgrass hydrolysate (SGH)

Switchgrass (Cave-in-Rock variety) was obtained from an

established stand located at Mead (Geo-coordinates N41.140,

W-96.483, and Elevation 348.6 m), Nebraska (USA) [7]. All field

plots were fertilized for high productivity under local soil

conditions. Plant materials were harvested (August 2007) at

10 cm stubble height. After harvest the biomass was air dried

on a greenhouse bench. The dried switchgrass was milled

using a hammer mill (1.27 mm sieve screen) and stored at

room temperature until needed. The milled switchgrass was

pretreated with dilute [1% (v/v)] H2SO4 and hydrolysed as

described for barley straw hydrolysis and WSH [2,6]. To

remove inhibitors, CSH and SGH were treated with lime

[Ca(OH)2; known as overliming] as reported for BSH in the

previous article [6].

2.4. Fermentation

Fermentation studies were performed as described for BSH in

the preceding paper [6]. Prior to fermentation, pH of various

media contained in bottles was adjusted to 6.5 using 400 g L�1

NaOH or diluted (10�) H2SO4. Following pH adjustment, the

bottles were kept in an anaerobic jar for 48–72 h for anaero-

biosis. After this period, the bottles were inoculated with

highly motile culture (7 mL cell culture in 100 mL medium)

followed by transferring the bottles to anaerobic environment.

Where applicable, the hydrolysate fermentation medium was

supplemented with filter sterilized glucose solution (from

400 g L�1) to raise total sugar level to 60 g L�1. This was done to

keep the total sugar level approximately 60 g L�1 in all the

experiments.

2.5. Analyses

Samples were analyzed for cell mass, sugars, and solvents as

described in previous reports [2,6]. The results presented here

are an average of duplicate experiments and have an error

range of �5–8%.

3. Results and discussion

In order to compare results obtained in this investigation,

a control batch fermentation was operated to produce butanol

using glucose as a substrate and C. beijerinckii P260. These

results are given in the preceding paper [6]. The control

experiment resulted in the production of 21.06 g L�1 total ABE.

During the fermentation, an ABE yield and productivity of 0.41

and 0.31 g L�1 h�1 were obtained, respectively. The fermen-

tation was initiated with 58.3 g L�1 glucose in the medium.

CSH was prepared and subjected to fermentation. In the

undiluted/untreated CSH medium, the culture did not show

any growth and/or fermentation. To promote fermentation of

CSH, similar treatment approaches were applied as described

for BSH reported in the previous article [6]. The first approach

was to dilute the hydrolysate with distilled water followed by

supplementing with sterile glucose solution to raise the total

sugar level to approximately 60 g L�1. The culture exhibited

a long lag phase of approximately 40 h before it initiated

0

5

10

15

20

25

0 20 40 60 80 100 120

Pro

duct

s [g

L]

Fermentation Time [h]

AcetoneButanolEthanolHAcHBuABE

0

0.5

1

1.5

2

2.5

3

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120

Cel

l Con

c. [

gL]

Suga

rs [g

L]

Fermentation Time [h]

Glu

XylArabMan

GalTotal SugCells

A

B

Fig. 2 – Production of ABE from a mixture of CSH and WSH

(1:1 ratio) using C. beijerinckii P260. For HAc and HBu see

Fig. 1. A. Products; B. Sugars and cell concentration.

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 5 6 6 – 5 7 1568

accumulation of a significant amount of ABE (Fig. 1A). After

96 h of fermentation (at that time fermentation stopped)

16.00 g L�1 ABE (acetone 4.7, butanol 10.4, and ethanol

0.9 g L�1) was produced (Fig. 1A). This resulted in a produc-

tivity of 0.17 g L�1 h�1. In the beginning of the fermentation,

6.55 g L�1 acetic acid was present which was reduced to

4.01 g L�1 during the course of fermentation suggesting that

the culture was able to utilize some acetic acid. At the end of

fermentation, butyric acid concentration was 0.16 g L�1. The

culture grew relatively fast and in 24 h a cell concentration of

1.89 g L�1 was achieved (Fig. 1B). At 0 time, 60.1 g L�1 total

sugars were present of which glucose, xylose, arabinose, and

galactose were 49.9, 7.4, 1.1, and 1.7 g L�1, respectively (Fig. 1B).

The culture used 37.3 g L�1 sugar, thus leaving 22.8 g L�1

(glucose 21.1, xylose 1.3, and galactose 0.4 g L�1) as residual

sugars. This experiment resulted in a yield of 0.43.

In the next experiment, CSH and WSH were mixed in

a ratio of 1:1 and fermented. The culture fermented this

mixture at a greater rate than CSH and water and produced

18.04 g L�1 ABE (Fig. 2A) in 84 h, resulting in a productivity of

0.21 g L�1 h�1. The individual levels of solvents were acetone

5.10, butanol 12.30, and ethanol 0.64 g L�1. Acid levels were

4.36 (acetic) and 0.35 (butyric) g L�1. Initial sugar levels were

glucose 38.7, xylose 15.0, arabinose 3.9, and galactose 1.7 g L�1

(Fig. 2B). The residual sugar levels were glucose 14.7 and

xylose 3.1 g L�1. The culture used 41.5 g L�1 sugars to produce

18.04 g L�1 ABE, thus resulting in a yield of 0.43. In this

experiment, a maximum cell concentration of 0.80 g L�1 was

0

5

10

15

20

25

0 20 40 60 80 100 120

Pro

duct

s [g

L-1

]

Fermentation Time [h]

AcetoneButanolEthanolHAcHBuABE

0

0.5

1

1.5

2

2.5

3

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120

Cel

l Con

c. [

gL-1

]

Suga

rs [g

L-1

]

Fermentation Time [h]

GluXylArabManGalTotal SugCells

B

A

Fig. 1 – Production of ABE from diluted (1:1, with water) CSH

using C. beijerinckii P260. A. Products, HAc – acetic acid, HBu –

butyric acid; B. Sugars and cell concentration.

obtained (Fig. 2B). This cell concentration is much lower than

obtained in the above experiment (1.89 g L�1) where CSH was

diluted 2 fold with water.

Overliming of CSH was applied to reduce inhibitor toxicity.

Fermentation stopped at 85 h and during this time, acetone

8.00, butanol 14.50, and ethanol 3.77 g L�1 were produced to

give a total of 26.27 g L�1 ABE (Fig. 3A). It should be noted that

untreated CSH was not fermented at all. This experiment

resulted in a solvent productivity of 0.31 g L�1 h�1. It was

observed that the culture started gassing within 3 h of inoc-

ulation. In this system, a maximum cell concentration of

0.77 g L�1 was measured (Fig. 3B). In the medium, an initial

sugar level of 60.3 g L�1 (glucose 32.6, xylose 22.8, arabinose

3.6, and galactose 1.3 g L�1) was present. During the fermen-

tation, 59.8 g L�1 sugars were used, leaving behind 0.5 g L�1

unused xylose. This resulted in an ABE yield of 0.44. This yield

is higher than the control fermentation of glucose and the

possible reasons behind this are use of less sugars for cell

growth and use of acetic acid that was present in the

fermentation medium. At 0 time, 11.1 g L�1 acetic acid was

present which was reduced to 5.69 g L�1. It should be noted

that the fermentation could be possible due to the removal of

inhibitors as a result of overliming. In the four experiments

[control, CSH diluted with water, CSH mixed with WSH, and

lime treated (LT) CSH], ABE concentrations of 21.06, 16.00,

18.04, and 26.27 g L�1, respectively, were obtained. Levels of

ABE in the treated CSH were higher than the control.

Measuring cell concentration in the fermentation broth is

another way of evaluating the system. In the control reactor,

0

5

10

15

20

25

30

0 20 40 60 80 100

Pro

duct

s [g

L-1

]

Fermentation Time [h]

Acetone

Butanol

Ethanol

HAc

HBu

ABE

0

0.5

1

1.5

2

2.5

3

0

10

20

30

40

50

60

70

0 20 40 60 80 100

Cel

l Con

c. [g

L-1

]

Suga

rs [g

L-1

]

Fermentation Time [h]

Glu

Xyl

Ara

Man

Gal

Total Sug

Cells

A

B

Fig. 3 – Production of ABE from lime treated CSH using

C. beijerinckii P260. A Products; B. Sugars and cell

concentration.

0

5

10

15

20

25

0 20 40 60 80 100

Pro

duct

s [g

L]

Fermentation Time [h]

Acetone

Butanol

Ethanol

HAc

HBu

ABE

0

0.5

1

1.5

2

2.5

3

0

10

20

30

40

50

60

70

0 20 40 60 80 100

Cel

l Con

c. [

gL]

Suga

rs [g

L]

Fermentation Time [h]

Glu

Xyl

Arab

Man

Gal

Total Sug

Cells

A

B

Fig. 4 – Production of ABE from diluted (1:1, with water)

SGH using C. beijerinckii P260. A. Products; B. Sugars and

cell concentration.

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 5 6 6 – 5 7 1 569

a cell concentration of 2.66 g L�1 was obtained. In the three

systems where CSH was fermented after treatments, reduced

cell growth was obtained, suggesting that the culture experi-

enced growth inhibition. Corn stover hydrolysate plus water,

CSH plus WSH, and lime treated CSH resulted in maximum

cell concentrations of 1.89, 0.80, and 0.77 g L�1, respectively. As

described in our preceding paper, specific productivities for

the four systems are compared. The control experiment

resulted in a specific productivity of 0.12 h�1, while

CSH þWSH resulted in a specific productivity (g ABE/g cell h)

of 0.27 h�1. Lime treated CSH resulted in a specific productivity

of 0.40 h�1 which is much higher than the control experiment.

In a report, Parekh et al. [8] used CSH to produce ABE using

Clostridium acetobutylicum P262. The corn stover was pre-

treated with SO2. In their report, they achieved an ABE yield of

0.33 (calculated value) which is lower than that achieved in

the present studies.

For fermentation of SGH, this substrate was initially fer-

mented without dilution or mixing with WSH. Fermentation

was weak and in 72 h fermentation 1.48 g L�1 ABE (acetone

0.41, butanol 0.97, and ethanol 0.10 g L�1) was produced. Total

initial sugar concentration was 60.0 g L�1 (glucose 33.2, xylose

20.4, arabinose 3.2, and galactose 3.2 g L�1) and residual sugar

level following fermentation was 42.1 g L�1 (glucose 24.1,

xylose 16.2, and arabinose 1.8 g L�1). In subsequent fermen-

tations, SGH was diluted two fold by adding water and sugar

level was raised to 58.9 g L�1 (glucose 45.5, xylose 10.2, arabi-

nose 1.6, and galactose 1.6 g L�1) by supplementing with

glucose solution. The culture produced 14.61 g L�1 ABE

(Fig. 4A) in 84 h resulting in a productivity of 0.17 g L�1 h�1. The

levels of acetone, butanol, and ethanol were 4.35, 9.55, and

0.71 g L�1, respectively. The individual levels of residual sugars

were glucose 19.6, xylose 0.4, and galactose 1.4 g L�1 (Fig. 4B).

In this system, a maximum cell concentration of 1.6 g L�1 was

obtained (Fig. 4B). This fermentation resulted in a solvent yield

of 0.39.

For the next fermentation, a 1:1 mixture of SGH and WSH

was used. The culture did not produce more than 8.91 g L�1

ABE (Table 1) in 96 h of fermentation resulting in a produc-

tivity of 0.09 g L�1 h�1. The reason for low ABE and low

productivity of these SGH plus WSH cultures is not apparent.

Further, when SGH was treated with lime, and attempts were

made to produce ABE, a cell concentration of only 0.20 g L�1

was measured suggesting that inhibitors were still present in

the medium. We are attempting to treat lime treated SGH with

XAD (trade name) resin to remove any residual inhibitors

followed by fermentation [9].

In the present studies, we have demonstrated that as

a result of inhibition, the culture (strain P260) was not able to

grow and produce ABE in undiluted/untreated CSH (and SGH;

showed poor growth and fermentation). By diluting these

hydrolysates we were able to reduce inhibitory effects and

produce solvents. Lime treated CSH resulted in the production

of 26.27 g L�1 ABE which is superior than the control experi-

ment where glucose was used as a substrate. Results from BSH

[6] and CSH fermentations suggest that overliming proved to

be a good technique to detoxify the hydrolysates. However,

Table 1 – Production of ABE from switchgrass hydrolysate(SGH) in batch reactor using C. beijerinckii P260.

Products and fermentationparameters

SGH mixedwith WSHa

Acetone [g L�1] 2.45

Butanol [g L�1] 5.79

Ethanol [g L�1] 0.67

Total ABE [g L�1] 8.91

Sugar used [g L�1] 24.1

Productivity [g L�1 h�1] 0.09

Yield [–] 0.37

a Supplemented with glucose solution to raise initial sugar level to

60 g L�1.

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 5 6 6 – 5 7 1570

fermentation of SGH was poor after overliming. It should be

noted that lime treatment of hydrolysates resulted in the

reduction of sugar levels by 1–4% due to dilution [6].

In our previous paper, we were able to identify three

chemicals that were present in WSH, BSH, CSH, and SGH [6].

These chemicals were acetic acid, furfural, and hydrox-

ymethyl furfural (HMF). The concentration range of acetic acid

was 6.43–10.10 g L�1, while levels of furfural (0.04–0.64 g L�1)

and HMF (0.12–0.52 g L�1) were much lower (than acetic acid).

One of the objectives behind using these agricultural resi-

dues (wheat straw, barley straw, corn stover and switchgrass)

has been to use low cost substrates for this fermentation and

bring it closer to commercialization. This fermentation

appears to have a potential to be an economically viable

process as it was 3–4 decades ago [10]. In order to make it

a viable process, various laboratories around the world have

made significant technological progress [11–16]. In addition to

the use of agricultural residues, cutting edge technologies

such as application of high productivity reactors, and cost-

efficient product recovery technologies are also being inves-

tigated in our laboratory. In the present studies, we have been

able to overcome fermentation inhibitor [9,17,18] problems

associated with CSH and SGH (partially) fermentation.

Furthermore, it is our aim to identify the problems associated

with SGH fermentation as it was difficult to ferment this

substrate even after overliming. To reduce the cost of butanol

production we intend to integrate fermentation with high

productivity reactors and energy efficient product recovery

technologies.

4. Conclusions

A control experiment resulted in the production of 21.06 g L�1

total ABE from glucose using C. beijerinckii P260. In this experi-

ment, an ABE yield and productivity of 0.41 and 0.31 g L�1 h�1

was achieved. We were able to produce solvents from over-

limed CSH and the culture accumulated 26.27 g L�1 (ABE

productivity 0.31 g L�1 h�1, and yield 0.44) ABE. Overlimed SGH

did not show significant cell growth, resulting in poor

fermentation. The maximum ABE concentration that was

produced from SGH was 14.61 g L�1 (productivity 0.17 g L�1 h�1)

when diluted two fold with water. It is concluded that other

pretreatment approaches may be used to pretreat cellulosic

biomass that do not generate fermentation inhibitors. Alter-

nately cultures that can metabolize or tolerate inhibitors and

still produce ABE in high concentrations should be developed.

Acknowledgments

N. Qureshi would like to thank Professor David Jones (retired

from Otago University, New Zealand) for his generous gift of

Clostridium beijerinckii P260. N. Qureshi would also like to thank

Adam Wallenfang, John Michael Henderson and Greg Ken-

nedy (USDA, NCAUR, Bioenergy Research) for helping us with

some of these studies.

r e f e r e n c e s

[1] Qureshi N, Saha BC, Iten L, Sarath G, Dien BS, Cotta MA.Agricultural residues and energy crops as novel substratesfor butanol fermentation. In: Tenth international workshopand conference on the regulation of metabolism, genetics,and development of the solvent and acid forming Clostridia.The Netherlands: Wageningen University; Sept. 28 – Oct. 1,2008.

[2] Qureshi N, Saha BC, Cotta MA. Butanol production fromwheat straw hydrolysate using Clostridium beijerinckii.Bioprocess and Biosystems Engineering 2007;30:419–27.

[3] Qureshi N, Saha BC, Hector RE, Hughes SR, Cotta MA. Butanolproduction from wheat straw by simultaneoussaccharification and fermentation using Clostridiumbeijerinckii: part I – batch fermentation. Biomass andBioenergy 2008;32(2):168–75.

[4] D’Aquino R. Cellulosic ethanol – tomorrow’s sustainableenergy source (update). Chemical Engineering Progress 2007;3:8–10.

[5] Anon. The next big thing: biobutanol. Fuels & LubesInternational 2006;Quarter 3:19–22.

[6] Qureshi N, Saha BC, Dien BS, Hector RE, Cotta MA.Production of butanol (a biofuel) from agricultural residues:part I – use of barley straw hydrolysate. Biomass andBioenergy, in this issue, doi:10.1016/jbiombioe.2009.12.024.

[7] Dien BS, Jung H-JG, Vogel KP, Casler MD, Lamb JFS, Iten L,et al. Chemical composition and response to dilute-acidpretreatment and enzymatic saccharification of alfalfa, reedcanarygrass, and switchgrass. Biomass and Bioenergy 2006;30:880–91.

[8] Parekh SR, Parekh RS, Wayman M. Ethanol and butanolproduction by fermentation of enzymatically saccharifiedSO2 prehydrolysed lignocellulosics. Enzyme and MicrobialTechnology 1988;10:660–8.

[9] Qureshi N, Ebener J, Ezeji TC, Dien B, Cotta MA, Blaschek HP.Butanol production by Clostridium beijerinckii BA101. Part I:use of acid and enzyme hydrolysed corn fiber. BioresourceTechnology 2008;99:5915–22.

[10] Zverlov VV, Berezina O, Velikodvorskaya GA, Schwarz WH.Bacterial acetone and butanol production by industrialfermentation in the Soviet Union: use of hydrolyzedagricultural waste for biorefinery. Applied Microbiology andBiotechnology 2006;71:587–97.

[11] Ezeji T, Blaschek HP. Fermentation of dried distillers’ grainsand soluble (DDGS) hydrolysates to solvents and value-added products by solventogenic clostridia. BioresourceTechnology 2008;99:5232–42.

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 5 6 6 – 5 7 1 571

[12] Maddox IS. The acetone-butanol-ethanol fermentation:recent progress in technology. Biotechnology and GeneticEngineering Reviews 1989;7:190–220.

[13] Shaheen R, Shirley M, Jones DT. Comparative fermentationstudies of industrial strains belonging to four species ofsolvent-producing Clostridia. Journal Molecular Microbiologyand Biotechnology 2000;2(1):115–24.

[14] Qureshi N. Solvent production. In: Schaechter M, editor.Chapter in encyclopedia of microbiology. Elsevier Ltd; 2009.p. 512–28.

[15] Durre P. New insights and novel developments in clostridialacetone/butanol/isopropanol fermentation. AppliedMicrobiology and Biotechnology 1998;49:639–48.

[16] Harris LM, Blank L, Desai RP, Welker NE, Papoutsakis ET.Fermentation characterization and flux analysis ofrecombinant strains of Clostridium acetobutylicum with aninactivated solR gene. Journal of Industrial Microbiology andBiotechnology 2001;27:322–8.

[17] Marchal R, Ropars M, Pourquie J, Fayolle F, Vandecasteele JP.Large-scale enzymatic hydrolysis of agriculturallignocellulosic biomass. Part 2: conversion into acetone-butanol. Bioresource Technology 1992;42:205–17.

[18] Ezeji TC, Qureshi N, Blaschek HP. Butanol production fromagricultural residues: impact of degradation products onClostridium beijerinckii growth and butanol fermentation.Biotechnology and Bioengineering 2007;97(6):1460–9.