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Bioethanol: Market and Production Processes 89

xylose, and wood hydrolyzate [60]. M. indicus has several industrial
advantages compared to baker’s yeast for ethanol production, such as
(a) capability of utilizing xylose, (b) having a valuable biomass, e.g., for
production of chitosan, and (c) high optimum temperature of 37 C [61].
Skory et al. [62] examined 19 Aspergilli and 10 Rhizopus strains for their
ability to ferment simple sugars (glucose, xylose, and arabinose) as well
as complex substrates. An appreciable level of ethanol has been produced
by Aspergillus oryzae, R. oryzae, and R. javanicus.
The dimorphic organism M. circinelloides is also used for production
of ethanol from pentose and hexose sugars. Large amounts of ethanol have
been produced during aerobic growth on glucose under nonoxygen-
limiting conditions by this mold. However, ethanol production on galac-
tose or xylose has been less significant [63]. Yields as high as 0.48 g/g
ethanol from glucose by M. indicus, under anaerobic conditions, have
been reported [64]. However, the yield and productivity of ethanol from
xylose is lower than that of P. stipitis [65].
Although filamentous fungi have been industrially used for a long time
for several purposes, a number of process-engineering problems are
associated with these organisms due to their filamentous growth.
Problems can appear in mixing, mass transfer, and heat transfer.
Furthermore, attachment and growth on bioreactor walls, agitators,
probes, and baffles cause heterogeneity within the bioreactor and prob-
lems in measurement of controlling parameters and cleaning of the
bioreactor [66, 67]. Such potential problems might hinder industrial
application of M. indicus for ethanol production. However, this fungus
is dimorphic, and its morphology can be controlled to be yeast-like or
pellet-like through fermentation [65].

3.12 Fermentation Process
In this section, we will discuss different fermentation processes appli-
cable for ethanol production. Fermentation processes, as well as other
biological processes, can be classified into batch, fed-batch, and contin-
uous operation. All these methods are applicable in industrial fermen-
tation of sugar substances and starch materials. These processes are
well established, the fed-batch and continuous modes of operation being
dominant in the ethanol market. When configuring the fermentation
process, several parameters must be considered, including (a) high
ethanol yield and productivity, (b) high conversion of sugars, and (c) low
equipment cost. The need for detoxification and choice of the microor-
ganism must be evaluated in relation to the fermentation configuration.
Presentation of a variety of inhibitors and their interaction effects,
e.g., in lignocellulosic hydrolyzates, makes the fermentation process
more complex than with other substrates for ethanol production [17, 21].