Page 85 - Refining Biomass Residues for Sustainable Energy and Bioproducts

60 Refining Biomass Residues for Sustainable Energy and Bioproducts

The cultivation temperature (20 C 31 C) was considered an optimum for the forma-

tion of EPS molecules (Gandhi et al., 1997). Sutherland (2001a,b) stated that the decrease
in the cultivation temperature by 10 C below optimum level obstructs the biosynthesis of

exopolysaccharides by microbial cells.
3. Growth phase: It is very evident that the growth phase affects the EPS production while
the literature survey presents controversial results. For different bacterial genera the corre-
lation between the production of EPS and cell growth cycle is different and both are
closely correlated (More et al., 2014). The content of EPS increased during the exponen-
tial phase while in the late stationary phase, it decreased with the cultivation time. The
high productivity of EPS has been noted during the late logarithmic and early stationary
phases. Sheng et al. (2006) reported the increase in the production rate during the expo-
nential phase while it was decreasing during the stationary cycle. Some concluded no
effects of stationary phase. Similar variations were visible for solid retention time.
The development in technology has facilitated the exploration of the microbial biopo-
lymers in various environmental, industrial, and medical applications. Biopolymers are
considered as a potential alternative for the conventional chemical polymers because they
are easily biodegradable, nontoxic, nonsecondary pollutant, and highly efficient. These
biosynthesized EPS have been stated to have various utilizations in areas, such as the cos-
metic industry, drug delivery, dye removal, probiotics, tissue engineering, wound dress-
ing, wastewater flocculation, wastewater phosphorus removal, and wastewater
bioremediation processes.
4. Substrate: The type of substrate has a substantial grip over the microbial community and
its metabolism. For example, according to Sheng et al. (2006), bacteria were able to gen-
erate more EPS when supplied with benzoate as compared to butyrate or propionate.
Different investigations have concluded that a higher amount of EPS is favored under
stress conditions.
5. Other influential parameters: EPS have a high-binding capacity so everything it binds to
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has an influence over it. For example, an increased Ca concentration will support higher
EPS generation. Similarly, in the presence of toxic compounds, it increases in order to
protect the cell (Priester et al., 2006). The availability of oxygen also judges the concen-
tration of saccharides in EPS which increases under aerobic conditions.

3.3 Microbially synthesized polymers

Almost all types of bacteria can produce biopolymers, such as extracellular and
intracellular inclusions. There are various pathways to create biopolymers. We have
concentrated on the microbial pathways for the production of PHA and EPS. The
microbial synthesis pathways are shown in Figs. 3.2 and 3.3.

3.3.1 Bacterial synthesis of polyhydroxyalkanoate
PHAs signify a group of biodegradable and bio-based polymers, recognized similar
to petroleum-based biopolymers (Morais, 2013). Many bacterial species, such as
Azotobacter sp., Bacillus sp., Burkholderia sp., Cupriavidus sp., Pseudomonas sp.,
and recombinant E. coli, utilize PHAs as an intracellular source of carbon and
energy, amassing these biopolyesters of HAs as granules in the cytoplasm of cells