Biomedical Engineering Reference
In-Depth Information
Natural lignocellulose contains hemicellulose and lignin. Lignin and hemicel-
lulose form a solid bond layer to surround the cellulose. It can be inferred that
hemicellulose mainly surrounds the amorphous region of cellulose, and lignin
surrounds the crystalline region of cellulose. Hemicellulose is easier to remove than
lignin. Thus, it is generally believed that the existence of lignin affects the enzymatic
hydrolysis yields of cellulose. Therefore, lignin should be removed to improve the
enzymatic hydrolysis yields of cellulose. But, there still are some issues under
debate, such as whether lignin has inhibitory effects on the enzymatic hydrolysis
process, whether there is a machinery space obstacle effect of lignin, whether the
absorbed cellulase still has activity, and so on [
5
]. In addition, the residual rate
of lignin has an obvious impact on the pretreatment cost of natural lignocellulose.
There is a great difference in cost between the 100 % removal rate of lignin and
20-65 %. Increasing research has proved that it is not necessary to remove all lignin
[
6
]. According to different sources of natural lignocellulose, removing 20-65 % of
the lignin can significantly improve the enzymatic hydrolysis yields [
2
].
In summary, pretreatment of natural lignocellulose has significant effects not
only on enhancing the enzymatic hydrolysis rate but also on decreasing the amount
of cellulase (saturated, invalid adsorption).
4.2.2
The Proposal on the Component Separation Concept
of Natural Lignocellulose
In general research reports or monographs, the purpose of pretreatment for lignocel-
lulose materials was to improve the enzymatic conversion of cellulose by removing
the lignin or destroying the crystal structure between cellulose macromolecules. So,
the utilization of natural lignocellulose focused only on the cellulose. Various means
were used to remove other components in lignocellulose and took advantage of the
isolated cellulose, which is basically consistent with the pulp and papermaking
industry process. Delignification treatment of natural lignocellulose follows the
method of strong acid or strong alkali dissolving the lignin at a high temperature in
the pulping process. It consumes plenty of energy and chemical raw materials and
causes resource waste and environmental pollution. From the point of ecological and
economic benefits, these methods are difficult to use in microbial transformation of
natural lignocelluloses.
At present, the microbial transformation of natural lignocelluloses worldwide
has achieved great progress but has not fully realized industrialization because of
the high cost of pretreatment and cellulase, both directly related to the pretreating
process. Taking the cost of cellulase as an example, to improve the enzymatic
hydrolysis rate of cellulose, the enzymes and the availability of the substrate should
both be considered. Considering cost reduction, the ideal pretreating process not
only can improve substrate availability but also can reduce the invalid adsorption of
cellulase and the inhibition of other components on cellulase, consequently reducing
cellulase dosage. Therefore, pretreatment of natural lignocellulose materials is a key
factor for the microbial transformation to fully access industrialization.
