4.3 Chemistry of Alginates
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4.3.2 Rheological Properties
Polymers have the unique property of increasing the viscosity of a solvent when
they are dissolved in the solvent. Alginates increase the viscosity of the solution and
form irreversible heat-stable gels. This forms the basis of most of their applications
in food and pharmaceuticals. The viscosity-enhancing properties and the gelling
properties of alginates are dependent on the amount of mannuronic acid and guluronic
acid, respectively, within the polymer chain. Alginates with higher mannuronic acid
content have higher viscosity while those with more guluronic acid have higher
gel-forming properties.
Alginates produce non-thermoreversible ionic gels, once formed into gels, and
unlike for example gelatine, they retain these gel forms at different temperatures
unlike for example gelatine which losses its gel form when the temperature is
increased. The ability to form non-thermoreversible ionic gels serves as the basis
of much of the applications of alginates. Alginate forms a gel in the presence of bivalent cations through ion exchange. It does not form a gel with monovalent cations
and magnesium ion. Calcium, strontium and barium are the bivalent ions used due
to their non-toxicity. Sodium, potassium and ammonium alginates are water soluble
while the others are insoluble.
4.3.3 Characterization of Alginate
Molecular weight of extracted sodium alginate reported varies in the ten thousands
(40,680 g/mol using the chemical method with a yield of 44.32% (Helmiyati and
Aprilliza 2017). For chemical characterization of polymers, methods commonly used
include FTIR, XRD, DSC and SEM (Olatunji and Olsson 2016). Typical FTIR spectra of sodium alginate show peaks at 939 and 884 cm
−1 for uronic acid and mannuronic acid functional group, respectively. The alginate chains also include OH
functional group and the CH stretching which also appear in the FTIR scan at around
3200–3400 cm
−1 for OH and 2928 cm
−1 for the CH stretching. The absorbance and
transmittance depend on the composition of the functional groups within the alginate structure. Pure sodium alginate has a decomposition temperature of 251.12 °C
(Helmiyati and Aprilliza 2017). This is obtained from differential scanning calorimetry. Extracted sodium alginate should therefore show values close to this depending
on the level of purity achieved during extraction process.
Crystallinity of alginate affects properties such as solubility and thermal properties. Pure sodium alginate has a crystallinity of about 35.62%. Crystallinity of raw
extract of alginate is usually less than this, and the purer it is, the closer it gets to
the crystallinity value of pure form, for example, a crystallinity 29.292% is reported
in one study for sodium alginate extracted from brown algae (Helmiyati and Aprilliza 2017). Furthermore, when the SEM of alginate containing brown seaweed is
compared to that of extracted seaweed, a distinguishing factor is the visible alginate
71
4.3.2 Rheological Properties
Polymers have the unique property of increasing the viscosity of a solvent when
they are dissolved in the solvent. Alginates increase the viscosity of the solution and
form irreversible heat-stable gels. This forms the basis of most of their applications
in food and pharmaceuticals. The viscosity-enhancing properties and the gelling
properties of alginates are dependent on the amount of mannuronic acid and guluronic
acid, respectively, within the polymer chain. Alginates with higher mannuronic acid
content have higher viscosity while those with more guluronic acid have higher
gel-forming properties.
Alginates produce non-thermoreversible ionic gels, once formed into gels, and
unlike for example gelatine, they retain these gel forms at different temperatures
unlike for example gelatine which losses its gel form when the temperature is
increased. The ability to form non-thermoreversible ionic gels serves as the basis
of much of the applications of alginates. Alginate forms a gel in the presence of bivalent cations through ion exchange. It does not form a gel with monovalent cations
and magnesium ion. Calcium, strontium and barium are the bivalent ions used due
to their non-toxicity. Sodium, potassium and ammonium alginates are water soluble
while the others are insoluble.
4.3.3 Characterization of Alginate
Molecular weight of extracted sodium alginate reported varies in the ten thousands
(40,680 g/mol using the chemical method with a yield of 44.32% (Helmiyati and
Aprilliza 2017). For chemical characterization of polymers, methods commonly used
include FTIR, XRD, DSC and SEM (Olatunji and Olsson 2016). Typical FTIR spectra of sodium alginate show peaks at 939 and 884 cm
−1 for uronic acid and mannuronic acid functional group, respectively. The alginate chains also include OH
functional group and the CH stretching which also appear in the FTIR scan at around
3200–3400 cm
−1 for OH and 2928 cm
−1 for the CH stretching. The absorbance and
transmittance depend on the composition of the functional groups within the alginate structure. Pure sodium alginate has a decomposition temperature of 251.12 °C
(Helmiyati and Aprilliza 2017). This is obtained from differential scanning calorimetry. Extracted sodium alginate should therefore show values close to this depending
on the level of purity achieved during extraction process.
Crystallinity of alginate affects properties such as solubility and thermal properties. Pure sodium alginate has a crystallinity of about 35.62%. Crystallinity of raw
extract of alginate is usually less than this, and the purer it is, the closer it gets to
the crystallinity value of pure form, for example, a crystallinity 29.292% is reported
in one study for sodium alginate extracted from brown algae (Helmiyati and Aprilliza 2017). Furthermore, when the SEM of alginate containing brown seaweed is
compared to that of extracted seaweed, a distinguishing factor is the visible alginate