Page 83 - Refining Biomass Residues for Sustainable Energy and Bioproducts

58 Refining Biomass Residues for Sustainable Energy and Bioproducts

also vary depending on influencing parameters. The distribution of EPS is studied
using fluorescent dye staining under fluorescent or confocal laser microscope.
Its positioning differs, like in the middle of a floc center developed from
sludge, layer distribution in case of biofilms, or concentrating in the outer
layer of a granular sludge (De Beer et al., 1996; Zhang and Bishop, 2001;
Zhang and Fang, 2004). This shows the heterogeneous nature of EPS spatial
distribution, similar is with the components. The outer layer of granular sludge
contains saccharides and cells, whereas most of the proteins are concentrated
inside the core (McSwain et al., 2005). This concludes the heterogeneous
nature as well as the dependency of EPS distribution on the aggregate origin,
type, and structure.

3.2.2.3 Structure of EPS
In vitro, EPS is not essential to a microbial survival while under natural conditions,
its presence is helpful in reference to cell transport, protection, and structural integ-
rity. Its importance has been overlooked that’s why the presence of EPS was indi-
vidually studied after culturing in artificial media. It is visualized as a mass of
slimy macromolecule embedding the microbial aggregate. Conventional microscopy
used to misinterpret the void between the cells and the dehydrating conditions leads
to the collapse of glycocalyx (Comte et al., 2006). Later, it was solved with the
help of antioxidants after which the fiber-like structure (a colloidal aggregate) con-
taining EPS was identified.
Owing to the composition, bacteria are supposed to produce two types of EPS,
homopolysaccharide and heteropolysaccharide. Homopolysaccharides are catego-
rized according to the monomeric units and linkage bonds into α-D-glucans, β-D-
glucans, fructans, and polygalactan. Similarly, the repeating units comprising a
heteropolysaccharide are L-rhamnose, D-galactose, D-glucose, glucuronic acid
(GlcA), N-acetylgalactosamine, and/or N-acetylglucosamine. Some noncarbohy-
drate moieties, such as acetyl, phosphate, and glycerol, are also found (Ates, 2015).
The synthesis site and the enzymes involved in the production also vary according
to the type of EPS. Each component is playing a role and decide the functionality
of the produced EPS. Depending on the function served by an EPS, seven catego-
ries of EPS are formed: active, redox-active, informative, nutritive, surface-active
(with molecules having amphiphilic), structural or constructive (role in cell protec-
tion and water retention), and sorptive (with charged polymers). Following these
properties, EPS plays a significant role in stress conditions, adsorption adherence
and forms protective barriers. Superior physical and chemical properties of EPS,
such as dextran (glucose), gellan, curdlan, and xanthan (glucuronate, mannose, and
glucose), make them more useful polysaccharides in comparison to algal (alginate)
or plant saccharides (pectin). The traditional applications and value are higher for
such saccharides. Unique biological properties exhibited by EPS, such as wellan,
levan (fructose), and pullulan (glucose), are responsible for increasing their market
on commercial fronts (Ates, 2015).