Biomedical Engineering Reference
In-Depth Information
7.2.2
Substrate Utilization
Clostridium acetobutylicum
is able to use polymeric substrates such as starch and
xylan, but not cellulose [
14
]. The degradation of starch is mediated by
'
-amylase,
and genetic data indicates that the presence of
'
-amylase is evident in at least two
'
-amylase genes. Larch wood xylan is hydrolyzed by the action of endoxylanases
and
-D-xylosidase.
Clostridium beijerinckii
also grows on starch and employs
the catalytic activities of glucoamylase and
“
-amylase. However, direct butanol
fermentation using lignocellulosic biomass as the raw material is a consolidated
bioprocess by a cellulolytic, solventogenic bacteria that remains a future task.
Only fairly active minicellulosomes were found in
C. acetobutylicum
ATCC 824,
and attempts to correct reading frame errors and improvement of promoters did
not lead to substantial cellulase production. Sabathé et al. [
15
] tried to modify a
C. acetobutylicum
strain to utilize cellulose directly. However, from the results, the
achievement of this objective also will need a long process.
At present, one of the major bottlenecks hampering economic viability is the
cost of substrates, accounting for up to 60 % of the total production costs. The high
cost of substrates, including molasses, whey permeate, corn, and starchy roots, is a
major factor affecting the economic viability of butanol production by fermentation.
Lignocellulose is the most abundant renewable resource on the planet and has
great potential as a substrate for fermentation [
16
]. For the use of lignocellulosic
substrates, several strategies have been studied, such as the use of hydrolysates,
coculture with true cellulolytic organisms, or the addition of cellulases to the
fermentation medium.
Many researchers have been carrying out lignocellulose degradation to sugars
for butanol fermentation since the 1980s. Yu et al. [
17
] reported that
C. aceto-
butylicum
was grown in acid hydrolysates of steam-exploded aspen wood chip
with final butanol yields of 9.0 g
'
L
1
(0.26 g of butanol/g sugar consumed).
However, it should be noted that pretreatment of agricultural residues, such as
corn fiber, with acid/alkali results in generation of inhibitors that inhibit fer-
mentation. Marchal et al. [
18
] described a one-step hydrolysis and fermentation
process involving the use of cellulase from
Trichoderma reesei
and conversion
of alkali-pretreated wheat straw into butanol and acetone by
C. acetobutylicum
.
The results obtained for solvent concentration (17.3 g
L
1
) and overall conversion
time (36 h) demonstrated an improved performance over the separate hydrolysis
and fermentation operation. Querishi et al. [
19
] studied butanol production from
wheat straw hydrolysate in batch cultures using
C. beijerinckii
P260. When wheat
straw hydrolysate was supplemented with 60 g
L
1
glucose, the resulting medium
L
1
sugars was successfully fermented (because of product
removal) to produce 47.6 g
containing 128.3 g
L
1
solvent, and the culture utilized all the sugars.
In China, research on substrate substitution for butanol fermentation also has
attracted increasing attention. Chen et al. [
20
] reported that 12.8 g
L
1
total solvent
concentration and 29.9 % yield were obtained in batch broth by
C. acetobutylicum
grown on rice straw enzymatic hydrolysate. Li and Chen [
21
,
22
] investigated
