150
7 Agar
products during heat treatment of agar-containing foods could be reduced by using
stronger gels.
Upon thermal degradation, agar forms 3,6-anhydropyran galactopyranose and
galactopyranose. These then further degrade 3,6-anhydropyranose galactopyranose units break down into furyl hydroxymethyl ketone, a potentially toxic compound to humans. The galactopyranose breaks down into 3,4-atrosan d-allose
and two other potentially toxic compounds, furfural and 5-(hydroxymethyl)-2furancarboxaldehyde (Ouyang et al. 2018). Therefore, although generally considered a safe and approved product, care should be taken during the process of food
treatment to avoid the formation of toxic compounds either from partial or complete
degradation, especially during sterilization.
The initial melting of agar-based gel varies with factors such as concentration,
presence of compounds such as sugars and urea and the agarose/agaropectin content. Earlier studies have shown that at different agarose concentration, the melting
temperature of the gel remains constant at 75 °C. However, the enthalpy of transformation from gel to sol increases as the concentration of agarose decreases from
12 w/w to 2 w/w (Watase et al. 1990).
7.3.6 Biological Degradation of Agar
Biological degradation is a relatively economic degradation method, whereby a substance is broken down into its smaller units by naturally existing organisms which
produce the enzymes for digesting, hence degrading the substances which they can
then use for their metabolism and energy production. These enzymes can be extracted
from such organism for controlled degradation of the substance without the contamination of microbes. The understanding of degradation of biopolymers is important in
determining their safety and better understanding of their bioactivity and applications.
Agar is naturally present in the cell walls and intracellular structures of red algae
which occur in the aquatic environment; an ideal source of the organisms which
degrade agar would therefore be in the same aquatic environment. These microbes
either exist freely in the environment or within the bodies of other bigger organisms
which feed on the red algae as these organisms will require the enzymes which
break down all components of the red algae, which include the agarose, into smaller
products which can then be used for metabolism.
Agar-degrading bacteria include Pseudomonas, Cellulophaga, Acinetobacter,
Agarivorans, Microbulbifer, Pseudoalteromonas, Saccharophagus, Bacillus, Paenibacillus, Streptomyces and Zobellia (Kwon et al. 2019). There are more marine-based
bacteria which degrade agar than the freshwater-based ones. This is expected since
most of the red algae are found mostly in the marine environment; freshwater red
algae are rarer. The enzyme which degrades agarose is called agarase. These break
down agarose into cooligomers of 2–4 repeating units of galactose and anhydrogalactose; further cleavage of the glycosidic bonds breaks down these neoagarose units
into neoagarobiose, dimers of agar which is consisting of one unit of galactose and
7 Agar
products during heat treatment of agar-containing foods could be reduced by using
stronger gels.
Upon thermal degradation, agar forms 3,6-anhydropyran galactopyranose and
galactopyranose. These then further degrade 3,6-anhydropyranose galactopyranose units break down into furyl hydroxymethyl ketone, a potentially toxic compound to humans. The galactopyranose breaks down into 3,4-atrosan d-allose
and two other potentially toxic compounds, furfural and 5-(hydroxymethyl)-2furancarboxaldehyde (Ouyang et al. 2018). Therefore, although generally considered a safe and approved product, care should be taken during the process of food
treatment to avoid the formation of toxic compounds either from partial or complete
degradation, especially during sterilization.
The initial melting of agar-based gel varies with factors such as concentration,
presence of compounds such as sugars and urea and the agarose/agaropectin content. Earlier studies have shown that at different agarose concentration, the melting
temperature of the gel remains constant at 75 °C. However, the enthalpy of transformation from gel to sol increases as the concentration of agarose decreases from
12 w/w to 2 w/w (Watase et al. 1990).
7.3.6 Biological Degradation of Agar
Biological degradation is a relatively economic degradation method, whereby a substance is broken down into its smaller units by naturally existing organisms which
produce the enzymes for digesting, hence degrading the substances which they can
then use for their metabolism and energy production. These enzymes can be extracted
from such organism for controlled degradation of the substance without the contamination of microbes. The understanding of degradation of biopolymers is important in
determining their safety and better understanding of their bioactivity and applications.
Agar is naturally present in the cell walls and intracellular structures of red algae
which occur in the aquatic environment; an ideal source of the organisms which
degrade agar would therefore be in the same aquatic environment. These microbes
either exist freely in the environment or within the bodies of other bigger organisms
which feed on the red algae as these organisms will require the enzymes which
break down all components of the red algae, which include the agarose, into smaller
products which can then be used for metabolism.
Agar-degrading bacteria include Pseudomonas, Cellulophaga, Acinetobacter,
Agarivorans, Microbulbifer, Pseudoalteromonas, Saccharophagus, Bacillus, Paenibacillus, Streptomyces and Zobellia (Kwon et al. 2019). There are more marine-based
bacteria which degrade agar than the freshwater-based ones. This is expected since
most of the red algae are found mostly in the marine environment; freshwater red
algae are rarer. The enzyme which degrades agarose is called agarase. These break
down agarose into cooligomers of 2–4 repeating units of galactose and anhydrogalactose; further cleavage of the glycosidic bonds breaks down these neoagarose units
into neoagarobiose, dimers of agar which is consisting of one unit of galactose and