Page 119 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 93

may be converted by the fermenting microorganisms; for this reason an
SSF process could tolerate the inhibitor formation from the pretreatment
to a greater extent [215]. Therefore the rate of hydrolysis is increased, and
a lower enzyme loading is required, resulting in higher ethanol yields and
in a reduced risk of contamination caused by enzymes. Techno-
economical assessments suggest that a 50% decrease in the enzyme loading
is advantageous if the yield is less than 6% 7% and required residence
time is not raised by more than 30% [216].
The risk of contamination in SSF is less than that determined in the
SHF, due to the presence of ethanol in the broth that makes the reaction
mixture less susceptible to the action of undesired microorganisms.
Although too little attention has been paid to inhibit cellulase by pro-
duced ethanol; however, the alcohol inhibition may be a limiting factor
in producing high ethanol concentration in SSF, it was indeed reported
that 30 g/L ethanol reduces the enzyme activity by 25% [217]. Anyway,
the SSF offers an easier operation and requires a decreased number of ves-
sels in comparison to SHF, since no hydrolysis reactors are necessary,
therefore resulting in a lower capital cost of the process [62]. The reduc-
tion of capital investment has been valued to be greater than 20% [211].
The main drawback of SSF is the different optimum temperatures of
the hydrolysis and fermentation processes which means a problematic
control and optimization of process parameters [218]. As discussed earlier,
the optimum temperature for saccharification by cellulolytic enzymes is
usually between 45°C and 50°C, whereas the fermentation step by fer-
menting microorganism is best done between 28°C and 37°C. For
instance, the optimum temperature of S. cerevisiae is between 30°C and
35°C; this yeast is inactive at temperature exceeding 40°C [219].
Therefore in SHF process, the enzymatic hydrolysis temperature can be
optimized regardless of the fermentation temperature, whereas a compro-
mise is needed in SSF process [220]. Tang performed the SSF process, by
using S. cerevisiae in the form of dry yeast, at a temperature of about 40°
C, achieving a realistic compromise between the optimal temperatures for
hydrolysis and fermentation [221].
Currently, hydrolysis remains the rate-limiting step in SSF.
Nevertheless, given the difficulty to reduce the optimal temperature of
cellulases via protein engineering, several thermotolerant bacteria and
yeasts were selected for their capacity to ferment ethanol. The yeasts as
Candida acidothermophilum and K. marxianus can be used in SSF with the
purpose of increasing the temperature, approaching that optimal of