251
Hydrolysis and Fermentation Technologies for Alcohols
Beta-glucosidase converts cellobiose to glucose by hydrolysis. In general,
glucosidase is any enzyme that catalyzes the hydrolysis of glucoside. Beta-glucosidase
catalyzes the hydrolysis of terminal, nonreducing beta-d-glucose residues with the
release of beta-d-glucose. Kadam and Demain [48] determined the substrate specificity of the beta-glucosidase and demonstrated that its addition to the cellulase complex
enhances the hydrolysis of Avicel, specifically by removing the accumulated cellobiose. They used C. thermocellum that is expressed in Escherichia coli to determine the
surface specificity of the enzyme. The hydrolysis of cellobiose to glucose is a liquidphase reaction. The action of beta-glucosidase on this reaction can be slowed or halted
by the inhibitive action of glucose accumulated in the solution. The accumulation may
also induce the entire hydrolysis to a halt as inhibition of the beta-glucosidase results
in the buildup of cellobiose, which in turn inhibits the action of exogluconases. Thus,
the hydrolysis of the cellulosic materials depends on the presence of all three enzymes
in proper amounts. If any of these enzymes is present in the amount less than the
required amount, the other enzymes will be inhibited or lack the necessary substrates
upon which to act.
While higher temperature increases the rate of hydrolysis, the high temperature
can inactivate or destroy the enzyme. To strike a balance between the increased
activity and the simultaneous deactivation rate, enzymatic hydrolysis is generally
operated at ~40°C–50°C. While enzymatic reactions are carried out at low temperatures, as mentioned earlier, dilute acid hydrolysis is generally carried out at high
temperatures (195°C–215°C).
One of the issues that need to be addressed in enzymatic hydrolysis is the loss of
enzyme that is left on the lignocellulose residues, on the cellulose substrate, or in
the solution. The enzyme adsorption capacity of the lignocellulose residue decreases
as the pretreatment temperature is increased, whereas the capacity of cellulose
increases with higher temperature. The reduction of enzyme on the residue is essential for the overall economics of the process.
An enzymatic hydrolysis process involving solid lignocellulosic materials can be
designed in many ways. Generally, substrate and enzymes are fed into the process,
and sugar solution along with the solid residue leaves the process at various points.
The enzyme adsorbed on the residue is lost and this hurts the economics of the
process. The recycling and reuse of the enzyme adsorbed on the residue is essential.
In essence, the enzymatic process should be designed in such a way that the loss of
enzymes is minimal.
9.4.3 FermenTATion
The hydrolysis and fermentation of cellulose can be carried out in sequence often
called as separate hydrolysis and fermentation (SHF) process or simultaneously
called as simultaneous saccharification and fermentation (SSF) process. Here we
briefly examine both of these processes.
9.4.3.1 separate hydrolysis and Fermentation
In the SHF process, hydrolysis and fermentation are carried out in two separate
vessels. The most expensive items in the overall process costs are the cost of
Hydrolysis and Fermentation Technologies for Alcohols
Beta-glucosidase converts cellobiose to glucose by hydrolysis. In general,
glucosidase is any enzyme that catalyzes the hydrolysis of glucoside. Beta-glucosidase
catalyzes the hydrolysis of terminal, nonreducing beta-d-glucose residues with the
release of beta-d-glucose. Kadam and Demain [48] determined the substrate specificity of the beta-glucosidase and demonstrated that its addition to the cellulase complex
enhances the hydrolysis of Avicel, specifically by removing the accumulated cellobiose. They used C. thermocellum that is expressed in Escherichia coli to determine the
surface specificity of the enzyme. The hydrolysis of cellobiose to glucose is a liquidphase reaction. The action of beta-glucosidase on this reaction can be slowed or halted
by the inhibitive action of glucose accumulated in the solution. The accumulation may
also induce the entire hydrolysis to a halt as inhibition of the beta-glucosidase results
in the buildup of cellobiose, which in turn inhibits the action of exogluconases. Thus,
the hydrolysis of the cellulosic materials depends on the presence of all three enzymes
in proper amounts. If any of these enzymes is present in the amount less than the
required amount, the other enzymes will be inhibited or lack the necessary substrates
upon which to act.
While higher temperature increases the rate of hydrolysis, the high temperature
can inactivate or destroy the enzyme. To strike a balance between the increased
activity and the simultaneous deactivation rate, enzymatic hydrolysis is generally
operated at ~40°C–50°C. While enzymatic reactions are carried out at low temperatures, as mentioned earlier, dilute acid hydrolysis is generally carried out at high
temperatures (195°C–215°C).
One of the issues that need to be addressed in enzymatic hydrolysis is the loss of
enzyme that is left on the lignocellulose residues, on the cellulose substrate, or in
the solution. The enzyme adsorption capacity of the lignocellulose residue decreases
as the pretreatment temperature is increased, whereas the capacity of cellulose
increases with higher temperature. The reduction of enzyme on the residue is essential for the overall economics of the process.
An enzymatic hydrolysis process involving solid lignocellulosic materials can be
designed in many ways. Generally, substrate and enzymes are fed into the process,
and sugar solution along with the solid residue leaves the process at various points.
The enzyme adsorbed on the residue is lost and this hurts the economics of the
process. The recycling and reuse of the enzyme adsorbed on the residue is essential.
In essence, the enzymatic process should be designed in such a way that the loss of
enzymes is minimal.
9.4.3 FermenTATion
The hydrolysis and fermentation of cellulose can be carried out in sequence often
called as separate hydrolysis and fermentation (SHF) process or simultaneously
called as simultaneous saccharification and fermentation (SSF) process. Here we
briefly examine both of these processes.
9.4.3.1 separate hydrolysis and Fermentation
In the SHF process, hydrolysis and fermentation are carried out in two separate
vessels. The most expensive items in the overall process costs are the cost of