Page 65 - Advances in bioenergy (2016)

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are produced by the enzymatic oxidation of cellulose. Sugar lactones have been shown to
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inhibit many different types of glycosyl hydrolases, for example, glucono-δ-lactone is a
potent inhibitor of β-glucosidases. 60

BIOCONVERSION PROCESSES

There are three distinguished (functional) stages of cellulose bioconversion into ethanol: the
pretreatment of the raw cellulosic material, the enzymatic saccharification, and the
fermentation. Depending on the material used, the fermentation stage may include hexoses
fermentation and/or pentoses fermentation. The stage of enzymes production needed for the

enzymatic saccharification completes the list of necessary biological stages for ethanol
production from lignocellulosic materials. Depending on the process design, these stages may
take place in different chronic combinations, being consecutive or simultaneous. The major
bioconversion processes are: The separate (or sequential) hydrolysis and fermentation (SHF),
the simultaneous saccharification and fermentation (SSF) and the consolidated bioconversion
process (CBP) in which, enzyme production, growth of microorganism, enzymatic hydrolysis
and fermentation are performed in the same reactor. When C5 sugars (pentoses) are involved, a
fourth process, the simultaneous saccharification and cofermentation (SScF) is often
considered.

SHF and SSF Processes

In SHF processes the enzymatic SSF stages are performed in different reactors. The main
advantage of this process is the fact that each step can be conducted at optimal conditions of
pH and temperature. However, the accumulation of glucose and cellobiose (occurring in most

experimental set ups during the hydrolysis step) has inhibitory effects on the activity of the
cellulases, decreasing significantly after some time the saccharification and accordingly the
overall yield of the process. In a SSF process, the enzymatic hydrolysis and fermentation are
not discernible stages, neither chronically nor topically. They are both performed in the same
reactor and thus, glucose released by the action of cellulases is converted directly to ethanol
by the fermenting microorganism. This continuous removal of glucose from the medium has a
beneficial effect on the overall yield due to the minimization of the end-product inhibition on
cellulolytic activities.

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Most reported experimental work on SSF, as thoroughly reviewed by Olofsson et al., have
focused on improving the process by increasing the substrate loading (i.e., the content of water
insoluble solids: WIS), decreasing enzyme and yeast concentration, and varying temperature
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and pH. In most cases SSF is compared favorably to SHF. According to Wingren et al., using
softwood as the raw material, the SSF process results in 10% lower ethanol production cost
( 4.81 SEK/L) than the SHF process ( 5.32 SEK/L), mainly due to the higher ethanol yield
that in most cases is achieved from SSF processes. Increase in the initial dry matter
concentration of the raw material, decrease in the microorganism dry weight used and the
recycling of the stillage stream would substantially reduce the cost of SSF process (by 20–