Biological Fundamentals for the Biotechnology of Lignocellulose - Biotechnology of Lignocellulose - page 107

Using the method of batch feeding can reduce substrate concentration and
mass transfer resistance. It can relatively increase the enzyme dosage of substrate,
especially the amount of cellobiose. So, it can reduce the inhibition caused by the
accumulation of cellobiose generated and thus increase the hydrolysis yield. But,
this technology can only improve enzymatic efficiency to a certain extent.
In addition, it is also possible to adopt continuous enzymatic technology, by
which material is fed in and fed out at a certain speed simultaneously for a long time.
The dilution D is defined as the ratio of the flow rate F and liquid volume V , that is,
D

F / V . Compared with enzymolysis in batches, continuous enzymatic technology
could obtain relatively high productivity. However, this technology needs to add not
only the substrate but also the cellulase, which increases the cost of the enzyme [ 80 ].
The major drawback of direct enzymolysis is that it cannot completely eliminate
the inhibitory effect of glucose on the reaction, and the substrate residues and
enzyme cannot be reused.

D

3.4.2.2 Simultaneous Cellulase Production and Enzymatic Hydrolysis

To reduce the cost of cellulase, a simultaneous process of cellulase production
and enzymatic hydrolysis was developed. This process is the coupling of cellulase
production and cellulose enzymatic saccharification, which are carried out in the
same reactor. Previous studies have shown that the immobilized Trichoderma reesei
mycelia growing in the specific limiting conditions can be batch cultured repeatedly,
which allows cellulase production coupled with enzymatic saccharification in one
reactor. In the intermittent feed conditions, the amount of cornstalks reaches
120 g L 1 , and saccharification reaches 89.20 % after 7 days. The cost of this
process is low, but there are strict sterilization demands for equipment. It also cannot
completely eliminate the inhibition of glucose to the reaction.

3.4.2.3 Simultaneous Saccharification and Fermentation

The so-called simultaneous saccharification and fermentation process is that the
reaction of saccharification and ethanol fermentation process happen in the same
tank simultaneously. The enzymatic hydrolysis of cellulose takes place simultane-
ously and continuously with the saccharification process in the same apparatus.
The hydrolyzate, glucose, is used by the continuous fermentation of bacterial
cells, which can eliminate the feedback inhibition to cellulase. The advantages of
this process include reducing product inhibition, promoting enzymatic hydrolysis
of cellulose, decreasing the amount of enzyme, simplifying the equipment, and
enhancing production efficiency. But, there are also many problems, such as the
differences between temperatures of hydrolysis and fermentation. In the saccharifi-
cation process, the optimum temperature of cellulase is about 50 ı C; the optimum
fermentation temperature of yeast is 31-38 ı C, which causes a problem in that the
enzymatic hydrolysis cannot be carried out at the optimum temperature.