316
14 Cellulose
14.3.1 Decomposition of Cellulose
Cellulose has a very simple linear polymer structure made up of glucose repeating
units. The absence of branching gives cellulose a very stable crystalline structure
making it resistant to degradation. This makes it an excellent structural component
of the cell wall. While in plants cellulose plays mainly structural role and not utilized
for energy or cell metabolism, bacteria are able to decompose cellulose and use
as a carbon source. Humans do not produce any known enzyme for digestion of
cellulose. However, cellulose is important as a fiber in the human diet. Degradation of
cellulose by cellulose-degrading organisms involves a cocktail of enzymes working
in synergy. These enzymes are referred to as cellulases (Beguin and Aubert 1994).
Industrial production of enzymes for degradation of cellulose involves isolation of
these enzymes from cellulose degrading organisms. Ruminant animals are able to
digest cellulose at relatively faster rate due to symbiotic relationship with bacteria
present in the rumen of these animals. The rate is further assisted by the process
of regurgitation, a process whereby the food is returned to the mouth and further
particle size reduction is achieved by chewing before being returned to the rumen
for digestion (Russel et al. 2009). Cellulose can also be degraded by thermochemical
process through the use of high temperature and pressure in the absence of oxygen.
This is used in the production of biofuel from cellulose in the process of pyrolysis.
The chemical structure of cellulose from both aquatic and terrestrial sources is
identical. Therefore, the degradation process is expected to follow the same mechanism. The difference in the degradation of cellulose within aquatic biomass lies in the
difference in the other components of the cell walls. In the isolated form, degradation
depends on the level of modification and the form in which it exists in the product.
14.4 Availability of Raw Materials
The growth rate of most aquatic plants is much faster than most non-edible and
edible crops being used or considered for biofuel production. They can therefore
generate biomass at a faster rate than the terrestrial crops. One of the limitations
facing commercial utilization of cellulosic biomass for biofuel production is the
availability of raw materials. With the faster biomass accumulation rate of aquatic
plants and algae, availability of cellulosic biomass can be significantly increased.
Cultivated Azolla plant in aquaculture can grow at a rate of 2.9–5.8 g/m
2 in dry
weight per day, and in the wild, it can grow at a rate of 25.6–27.4 g dry weight per m
2
per day (93.4–100 t/ha-year dry weight). Azolla is one of the fastest-growing aquatic
plants with the ability to double its weight within 5–6 days. On average, cellulose
content in algae is between 20 and 30% dry weight (Mihranyan 2010), and however,
there have been reports of cellulose content in filamentous algae as high as 45%.
Duckweeds can grow at a rate of between 39.2 and 44 tonnes per hectare annually.
Duckweed contains up to 45.7% starch and for over two decades has been explored
14 Cellulose
14.3.1 Decomposition of Cellulose
Cellulose has a very simple linear polymer structure made up of glucose repeating
units. The absence of branching gives cellulose a very stable crystalline structure
making it resistant to degradation. This makes it an excellent structural component
of the cell wall. While in plants cellulose plays mainly structural role and not utilized
for energy or cell metabolism, bacteria are able to decompose cellulose and use
as a carbon source. Humans do not produce any known enzyme for digestion of
cellulose. However, cellulose is important as a fiber in the human diet. Degradation of
cellulose by cellulose-degrading organisms involves a cocktail of enzymes working
in synergy. These enzymes are referred to as cellulases (Beguin and Aubert 1994).
Industrial production of enzymes for degradation of cellulose involves isolation of
these enzymes from cellulose degrading organisms. Ruminant animals are able to
digest cellulose at relatively faster rate due to symbiotic relationship with bacteria
present in the rumen of these animals. The rate is further assisted by the process
of regurgitation, a process whereby the food is returned to the mouth and further
particle size reduction is achieved by chewing before being returned to the rumen
for digestion (Russel et al. 2009). Cellulose can also be degraded by thermochemical
process through the use of high temperature and pressure in the absence of oxygen.
This is used in the production of biofuel from cellulose in the process of pyrolysis.
The chemical structure of cellulose from both aquatic and terrestrial sources is
identical. Therefore, the degradation process is expected to follow the same mechanism. The difference in the degradation of cellulose within aquatic biomass lies in the
difference in the other components of the cell walls. In the isolated form, degradation
depends on the level of modification and the form in which it exists in the product.
14.4 Availability of Raw Materials
The growth rate of most aquatic plants is much faster than most non-edible and
edible crops being used or considered for biofuel production. They can therefore
generate biomass at a faster rate than the terrestrial crops. One of the limitations
facing commercial utilization of cellulosic biomass for biofuel production is the
availability of raw materials. With the faster biomass accumulation rate of aquatic
plants and algae, availability of cellulosic biomass can be significantly increased.
Cultivated Azolla plant in aquaculture can grow at a rate of 2.9–5.8 g/m
2 in dry
weight per day, and in the wild, it can grow at a rate of 25.6–27.4 g dry weight per m
2
per day (93.4–100 t/ha-year dry weight). Azolla is one of the fastest-growing aquatic
plants with the ability to double its weight within 5–6 days. On average, cellulose
content in algae is between 20 and 30% dry weight (Mihranyan 2010), and however,
there have been reports of cellulose content in filamentous algae as high as 45%.
Duckweeds can grow at a rate of between 39.2 and 44 tonnes per hectare annually.
Duckweed contains up to 45.7% starch and for over two decades has been explored