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Water for Energy and Fuel Production
3. For 1 and 3 h reaction times, the overall xylose yields were 86% and 93%,
respectively.
4. Recycle leachate, dilute acid, and prehydrolysis acid solutions were stable
during the storage for several days.
5. Vacuum drying was adequate in the acid concentration step.
6. Cellulose hydrolysis by cooking stover containing 66%–78% acid for 6 h at
100°C resulted in 75%–99% cellulose conversion to glucose.
7. Fiberglass-reinforced plastics of vinyl ester resin were used for the construction of process vessels and piping.
More detailed description of the process is described by Lee and Shah [2].
9.4.2.2 enzymatic hydrolysis
Cellulose differs from other carbohydrates that are generally used as a substrate for
fermentation in that cellulose is insoluble and polymerized as beta-1,4 glycosidic
linkages. Each cellulose molecule is an unbranched polymer of 15–10,000 d-glucose
units. Hydrolysis of crystalline cellulose is the rate-limiting step in the overall conversion of biomass to ethanol because aqueous enzyme solutions have difficulty acting on insoluble, impermeable highly structured cellulose. Cellulose needs to be
efficiently solubilized such that an entry can be made into cellular metabolic pathways. Solubilization is brought about by enzymatic hydrolysis catalyzed by the cellulose system of certain bacteria and fungi. Cellulase is a class of enzyme produced
primarily by fungi, bacteria, and protozoans that catalyze the hydrolysis of cellulose
commonly known as cellulolysis.
The discussion on enzymatic hydrolysis is broken into three parts: enzyme system, enzyme production and inhibition, and mechanism of cellulose hydrolysis that
considers cellulase enzyme adsorption on the substrate.
9.4.2.2.1 Enzyme System
There are mechanistically and structurally different types of cellulases. Each cellulolytic microbial group has an enzyme system unique to it. The capabilities of
enzyme can vary from hydrolysis of soluble derivatives of cellulose to disrupting
the cellulose complex. Cellulase is actually composed of a number of distinctive
enzymes based on the specific types of reactions catalyzed. In fact, cellulase can be
characterized into five general groups:
1. Endocellulase cleaves the internal bonds to disrupt the crystalline structure
of cellulose and expose individual polysaccharide chains.
2. Exocellulase detaches two or four saccharide units from the ends of
exposed chains produced by endocellulase, resulting in the production
of disaccharides or tetrasaccharides, such as cellobiose. There are two
principal types of exocellulases or cellobiohydrolases (CBHs): (1) CBH-I
that works processively from the reducing end and (2) CBH-II that works
processively from the nonreducing end of cellulose. Here the processivity implies the ability of enzyme to continue repetitively its catalytic
Water for Energy and Fuel Production
3. For 1 and 3 h reaction times, the overall xylose yields were 86% and 93%,
respectively.
4. Recycle leachate, dilute acid, and prehydrolysis acid solutions were stable
during the storage for several days.
5. Vacuum drying was adequate in the acid concentration step.
6. Cellulose hydrolysis by cooking stover containing 66%–78% acid for 6 h at
100°C resulted in 75%–99% cellulose conversion to glucose.
7. Fiberglass-reinforced plastics of vinyl ester resin were used for the construction of process vessels and piping.
More detailed description of the process is described by Lee and Shah [2].
9.4.2.2 enzymatic hydrolysis
Cellulose differs from other carbohydrates that are generally used as a substrate for
fermentation in that cellulose is insoluble and polymerized as beta-1,4 glycosidic
linkages. Each cellulose molecule is an unbranched polymer of 15–10,000 d-glucose
units. Hydrolysis of crystalline cellulose is the rate-limiting step in the overall conversion of biomass to ethanol because aqueous enzyme solutions have difficulty acting on insoluble, impermeable highly structured cellulose. Cellulose needs to be
efficiently solubilized such that an entry can be made into cellular metabolic pathways. Solubilization is brought about by enzymatic hydrolysis catalyzed by the cellulose system of certain bacteria and fungi. Cellulase is a class of enzyme produced
primarily by fungi, bacteria, and protozoans that catalyze the hydrolysis of cellulose
commonly known as cellulolysis.
The discussion on enzymatic hydrolysis is broken into three parts: enzyme system, enzyme production and inhibition, and mechanism of cellulose hydrolysis that
considers cellulase enzyme adsorption on the substrate.
9.4.2.2.1 Enzyme System
There are mechanistically and structurally different types of cellulases. Each cellulolytic microbial group has an enzyme system unique to it. The capabilities of
enzyme can vary from hydrolysis of soluble derivatives of cellulose to disrupting
the cellulose complex. Cellulase is actually composed of a number of distinctive
enzymes based on the specific types of reactions catalyzed. In fact, cellulase can be
characterized into five general groups:
1. Endocellulase cleaves the internal bonds to disrupt the crystalline structure
of cellulose and expose individual polysaccharide chains.
2. Exocellulase detaches two or four saccharide units from the ends of
exposed chains produced by endocellulase, resulting in the production
of disaccharides or tetrasaccharides, such as cellobiose. There are two
principal types of exocellulases or cellobiohydrolases (CBHs): (1) CBH-I
that works processively from the reducing end and (2) CBH-II that works
processively from the nonreducing end of cellulose. Here the processivity implies the ability of enzyme to continue repetitively its catalytic