Sunflower - Bioenergy Crop Plants - page 727

30.6.3 S imultanEouS S accharification and f ErmEntation
Cellulose hydrolysis to glucose is a significant component of the total production cost of ethanol
from wood (Nguyen and Saddler 1991). An overall economic process must include achieving a
high glucose yield (>85% theoretical) at high substrate loading (>10% w/v) over short residence
times (<4 days). It has been shown that simultaneous saccharification of cellulose to glucose and
fermentation of glucose to ethanol [simultaneous saccharification and Fermentation (SSF)] improves
the kinetics and economics of biomass conversion. This SSF reduces the accumulation of hydrolysis
products that are inhibitory to cellulase and β-glucosidase, reduces contamination risk because of
ethanol, and reduces capital equipment requirements (Sun and Cheng 2002; Karimi et al. 2006;
Olofsson et al. 2008) . An important drawback of yeast-based SSF is that the reaction has to operate
at a compromised temperature of approximately 30°C instead of an optimal enzyme temperature in
the range 45-50°C (Taherzadeh and Karimi 2007). Inhibition of cellulose by the produced ethanol
is a problem of SSF; Wyman (1996) reported a 5% reduction in cellulose activity by 30 g/L ethanol.
There are no reports on SSF of sunflower wastes except for one by Ruiz et al. (2006), who carried
out SSF of steam-pretreated (220°C, 5 min) sunflower stalks and obtained 21 g/L of ethanol while
using 10% (w/v) substrate concentration. Elsewhere, through selection of improved cellulase and
yeasts better suited to the SSF process, 90-95% yields with 4-5% ethanol concentrations could be
achieved in only 3-7 days for various feedstocks (Spindler et al. 1991). Doran et al. (1994) performed
simultaneous saccharification and fermentation of pretreated sugarcane bagasse using recombinant
ethanol producing Klebsiella oxytoca strain P2 and Genencor Spezyme CE. Srinivas et al. (1995)
used single-stage bioconversion of cellulosic material to ethanol using intergeneric fusants of
T. reesei QM9414 and Saccharomyces cerevisiae NCIM 3288. Under optimal conditions, the fusant
produced 0.17 g ethanol in 30 h. Barron et al. (1995) used a thermotolerant strain Kluveromyces
marxianus in simultaneous saccharification and ethanol formation from cellulose in which an
ethanol yield of 10 g/L at 45°C was obtained, representing 39% of the theoretical yield. Moritz and
Duff (1996) described a simultaneous saccharification and extractive fermentation (SSER) process
for ethanol production from cellulosic substrates. In batch SSEF reactors with 2-5% aqueous
phase, 50% conversion of 25% Solka Floc was achieved in 48 h using 2 FPU cellulase/g. Torget
et al. (1996) reported that pretreated poplar saw dust can be converted to ethanol at a yield 91% of
theoretical, with an ethanol concentration of up to 4.0% (w/v) in 55 h via a SSF process. Szakacs and
Tengerdy (1997) selected Gliocladium sp. TUB F-498, a wild strain of a lignocellulolytic fungus,
as a potential in situ enzyme source for the bioprocessing of pretreated poplar wood to ethanol in
an SSF process.
Krishna et al. (1998) optimized ethanol production by SSF of pretreated sugarcane leaves using
a cellulolytic enzyme complex from T. reesei QM 9414 and S. cerevisiae NRRL-Y-132. The optimal
temperature and substrate concentration chosen for SSF were 40°C and 10%, respectively. Meunier
Goddik et al. (1999) performed SSF of dilute-acid-pretreated and untreated poplar wood for 5 days
using T. reesei cellulases and S. cerevisiae fermentation. Cellulose conversion varied from 8% for
untreated poplar to 78% for the 180°C pretreated poplar.
30.7 ethanol FermentatIon
Efforts directed at ethanol production from biomass at industrial levels have failed because
of economic constraints. The main problems encountered in the efficient conversion of the
lignocellulosic hydrolysates to ethanol are twofold. Firstly, after pretreatment, the hydrolysate
contains not only fermentable sugars but also a broad range of compounds having inhibitory
effects on the microorganisms used for fermentation (Endo et al. 2008). The composition of these
compounds depends on the type of lignocellulosic material used and the chemistry and nature of the
pretreatment process. Secondly, the hemicellulose hydrolysates contain not only hexoses but also
pentoses. Hexoses can easily be fermented by S. cerevisiae with well-known processes, but this is