Page 113 - Biofuels Refining and Performance

96 Chapter Three

low as 6 g/L. Although glucose also decreases the cellulase activity, the
inhibitory effect of glucose is lower than that of cellobiose. On the other
hand, glucose is a strong inhibitor for -glucosidase. At a level of 3 g/L
of glucose, the activity of -glucosidase reduces by 75% [27, 80]. Another
possible problem in SHF is contamination. Hydrolysis is a lengthy
process (one or possibly several days), and a dilute solution of sugar
always has a risk of contamination, even at rather high temperatures
such as 45–50 C.

3.12.7 Simultaneous saccharification
and fermentation (SSF)
SSF combines enzymatic hydrolysis of cellulose and fermentation in
one step. As cellulose converts to glucose, a fermenting microorganism
is presented in the medium and it immediately consumes the glucose
produced. As mentioned, cellobiose and glucose significantly decrease the
activity of cellulase. SSF gives higher reported ethanol yields and
requires lower amounts of enzyme, because end-product inhibition from
cellobiose and glucose formed during enzymatic hydrolysis is relieved
by the yeast fermentation. SSF has the following advantages compared
to SHF:

Fewer vessels are required for SSF, in comparison to SHF, resulting
in capital cost savings.
Less contamination during enzymatic hydrolysis, since the presence
of ethanol reduces the possibility of contamination.
Higher yield of ethanol.
Lower enzyme-loading requirement.
On the other hand, SSF has the following drawbacks compared to SHF:

SSF requires that enzyme and culture conditions be compatible with
respect to pH and temperature. In particular, the difference between
optimum temperatures of the hydrolyzing enzymes and fermenting
microorganisms is usually problematic. Trichoderma reesei cellulases,
which constitute the most active preparations, have optimal activity
between 45 C and 50 C, whereas S. cerevisiae has an optimum tem-
perature between 30 C and 35 C. The optimal temperature for SSF
is around 38 C, which is a compromise between the optimal temper-
atures for hydrolysis and fermentation. Hydrolysis is usually the
rate-limiting process in SSF [27]. Several thermotolerant yeasts (e.g.,
C. acidothermophilum and K. marxianus) and bacteria have been
used in SSF to raise the temperature close to the optimal hydrolysis
temperature.