Biomedical Engineering Reference
In-Depth Information
6.2.2.3
Simultaneous Saccharification and Cofermentation
SSCF carries out enzymatic hydrolysis and xylose and glucose fermentation
in a single bioreactor using mixed bacteria or bacteria with engineered xylose
metabolism. It has several advantages, such as lower cost, shorter process time,
lower contamination risk, and fewer inhibitory effects during enzymatic hydrolysis.
Thus, it attracts increasing research.
Koskinen et al. obtained two thermophilic bacteria (strains AK15 and AK17)
and used them to ferment lignocellulose. They could use both glucose and xylose
to produce ethanol and hydrogen. The strain AK17 was tolerated exogenously and
added ethanol up to 4 % (v/v) [
3
,
29
]. Ryabova et al. obtained a mutant strain of
Hansenula polymorpha
that was vitamin B
2
deficient from the yeast
Pichia stipitis
.
This strain could ferment both glucose and xylose up to 45
ı
C[
3
,
30
]. Kim et al.
used barley hull pretreated using aqueous ammonia to produce ethanol. The addition
of xylanase along with cellulase resulted in a synergetic effect on ethanol production
in SSCF using treated barley hull and recombinant
E. coli
(KO11). With 3 % (w/v)
glucan loading and 4 mL xylanase enzyme loading, the SSCF of the treated barley
hull resulted in a 24.1 g
L
1
ethanol concentration at 15 FPU cellulase/gram glucan
loading, which corresponds to 89.4 % of the maximum theoretical yield based on
glucan and xylan [
23
,
31
]. Zhang et al. developed a kinetic model to predict batch
SSCF by the xylose-utilizing yeast
Saccharomyces cerevisiae
RWB222 and the
commercial cellulase preparation Spezyme CP. The model accounted for cellulose
and xylan enzymatic hydrolysis and competitive uptake of glucose and xylose
[
23
,
32
].
6.2.2.4
Direct Microbial Conversion
DMC combines cellulase production, cellulose enzymatic saccharification, and fer-
mentation of hexose and pentose into a single step, thus reducing the reaction vessel
and saving costs. But, in this process, the resistant ethanol concentration of the
strain is low, and a variety of by-products is generated. Thus, the final ethanol yield
is low. In DMC,
Clostridium thermocellum
,
Clostridium thermohydrosulphaircum,
and
Thermoanaerobacter ethanolicus
are the focus of studies [
33
]. Christakopoulos
et al. found that the cellulase hyperproducing strain F3 of
Fusarium oxysporum
fermented glucose, xylose, cellobiose, and cellulose directly to ethanol in 1989.
Conversion of cellulose to ethanol was markedly affected by the pH of both aerated
preculture and anaerobic fermentation. Optimum values of cellulose conversion to
ethanol were obtained when aerated and anaerobic processes were carried out at
pH 5.5 and 6, respectively. Maximum ethanol concentrations of 9.6 and 14.5 g
L
1
corresponded to 89.2 and 53.2 % of the theoretical yield [
23
,
34
]. Balusu et al. used
Clostridium cellulolyticum
SS19 to produce ethanol from cellulose in anaerobic
submerged fermentation. The concentrations of filter paper, corn steep liquor,
cysteine hydrochloride, and ferrous sulfate in the medium that were optimal for
