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Bioethanol: Market and Production Processes 91

during batch fermentation. However, very high concentration of the
inhibitors will cause complete inactivation of the metabolism [18].
Several strategies may be considered for fermentation of hydrolyzate
to improve the in situ detoxification in batch fermentation and obtain
higher yield and productivity of ethanol. Having high initial cell den-
sity, increasing the tolerance of microorganisms against the inhibitors
by either adaptation of cells to the medium or genetic modification of the
microorganism, and choosing optimal reactor conditions to minimize
the effects of inhibitors are among these strategies.
Volumetric ethanol productivity is low in lignocellulosic hydrolyzates
when low cell-mass inocula are used due to poor cell growth. Usually,
high cell concentration, e.g., 10 g/L dry cells, have been used in order
to find a high yield and productivity of ethanol in different studies. In
addition, a high initial cell density helps the process for in situ detox-
ification by the microorganisms, and therefore, the demand for a detox-
ification unit decreases. In situ detoxification of the inhibitors may
even lead to increased ethanol yield and productivity, due to uncoupling
by the presence of weak acids, or due to decreased glycerol production
in the presence of furfural [21]. Adaptation of the cells to hydrolyzate
or genetic modification of the microorganism can significantly improve
the yield and productivity of ethanol. Optimization of reactor condi-
tions can be used to minimize the effects of inhibitors. Among the dif-
ferent parameters, cell growth is found to be strongly dependent on pH
[18, 21].

3.12.2 Fed-batch processes
In fed-batch processes (or semi-continuous processes), the substrate
and required nutrients are added continuously or intermittently to the
initial medium after the start of cultivation or from the point halfway
through the batch process. Fed-batch processes have been utilized to
avoid utilizing substrates that inhibit growth rate if present at high con-
centration, to overcome catabolic repression, to demand less initial bio-
mass, to overcome the problem of contamination, and to avoid mutation
and plasmid instability found in continuous culture. Furthermore, fed-
batch processes do not face the problem of washout, which can occur in
continuous fermentation. A major disadvantage of a fed-batch process
is the need for additional control instruments that require a substan-
tial amount of operator skill. In addition, for systems without feedback
control, where the feed is added on a predetermined fixed schedule,
there can be difficulty in dealing with any deviation (i.e., time courses
may not always follow the expected profiles) [70]. The fed-batch processes
without feedback control can be classified as intermittent fed-batch,
constant-rate fed-batch, exponential fed-batch, and optimized fed-batch.