72
4 Alginates
fibrils which can be seen in the extract but not visible in the intact seaweed due to
the presence of the other components such as lipids, minerals and other non-alginate
carbohydrates. The appearance of distinct fibrils can also therefore be used as an
additional characterization of alginate extract (Fertah et al. 2017).
The color of alginate can vary from whitish to brown. The brown pigment indicates
the presence of fucoxanthin pigment and varies for different types of alginophytes
(Mushollaeni 2011). Another main carbohydrate present in seaweed is laminarin, a
storage polysaccharide which is also discussed in a separate chapter in this book.
4.3.4 Decomposition of Alginate
The decomposition of alginate is important for two reasons: Firstly, if the biodegradable polymer is going to degrade within the body or environment, the safety of the
products of degradation is as important as the safety of the product itself. For example, when used as a thickener in food what are the products likely to be formed in the
body after consumption. Secondly, the degradation product could be a useful way
to extend the usage of the product after the first use, for instance, using an alginate
packaging film for the production of organic acids after their use as food packaging.
According to its structure, alginate can be broken down into smaller chains of
either purely mannuronic or guluronic acid oligomers or alternating chains of mannuronic and guluronic acid units using enzymes which are specific to the respective
sites. For example, an alginate lyase enzyme, poly-1 → 4-alpha-l-guluronate lyase
enzyme extracted from the mollusk Lamis sp., catalyzes the cleavage of the 1 → 4
glycosidic bond of alpha-l-guluronic acids (Sil’chenko et al. 2013).
The degradation of alginate salts and gels is particularly important as these are
the forms in which alginate exists in many products. Dense alginate hydrogels have
a half-life of 4–6 days in model tissues (Shkand et al. 2016). Under hydrothermal
conditions, alginate degrades into its monosaccharides, mannuronic and guluronic
acids through the hydrolysis of glycosidic bonds. Lactic acids and glycolic acids as
well as sodium carbonate are also produced as products of alginate decomposition
(Aida et al. 2010). Controlled depolymerization of alginates can be carried out to
obtain other organic acids (Aida et al. 2012) and alginate lyase enzymes (Zhu and Yin
2015). The products of the depolymerized alginates are safe and in some cases are
of commercial importance such as the alginate oligosaccharides which are discussed
in the section on applications of alginate.
Alginate is also produced by some species of bacteria such as pseudomonas and
Azotobacter (Chen and Long 2018). These bacteria also produce the enzyme alginate
lyases to break down the alginate for use. The alginate oligosaccharides derived
from the action of alginate lyases on alginate has potential applications such as
protection against disease-causing organisms (An et al. 2009), growth-promoting
agents in plants (Iwasaki and Matsubara 2000) and antioxidants (Falkeborg et al.
2014; Nagarajan et al. 2016; Chen and Long 2018).
4 Alginates
fibrils which can be seen in the extract but not visible in the intact seaweed due to
the presence of the other components such as lipids, minerals and other non-alginate
carbohydrates. The appearance of distinct fibrils can also therefore be used as an
additional characterization of alginate extract (Fertah et al. 2017).
The color of alginate can vary from whitish to brown. The brown pigment indicates
the presence of fucoxanthin pigment and varies for different types of alginophytes
(Mushollaeni 2011). Another main carbohydrate present in seaweed is laminarin, a
storage polysaccharide which is also discussed in a separate chapter in this book.
4.3.4 Decomposition of Alginate
The decomposition of alginate is important for two reasons: Firstly, if the biodegradable polymer is going to degrade within the body or environment, the safety of the
products of degradation is as important as the safety of the product itself. For example, when used as a thickener in food what are the products likely to be formed in the
body after consumption. Secondly, the degradation product could be a useful way
to extend the usage of the product after the first use, for instance, using an alginate
packaging film for the production of organic acids after their use as food packaging.
According to its structure, alginate can be broken down into smaller chains of
either purely mannuronic or guluronic acid oligomers or alternating chains of mannuronic and guluronic acid units using enzymes which are specific to the respective
sites. For example, an alginate lyase enzyme, poly-1 → 4-alpha-l-guluronate lyase
enzyme extracted from the mollusk Lamis sp., catalyzes the cleavage of the 1 → 4
glycosidic bond of alpha-l-guluronic acids (Sil’chenko et al. 2013).
The degradation of alginate salts and gels is particularly important as these are
the forms in which alginate exists in many products. Dense alginate hydrogels have
a half-life of 4–6 days in model tissues (Shkand et al. 2016). Under hydrothermal
conditions, alginate degrades into its monosaccharides, mannuronic and guluronic
acids through the hydrolysis of glycosidic bonds. Lactic acids and glycolic acids as
well as sodium carbonate are also produced as products of alginate decomposition
(Aida et al. 2010). Controlled depolymerization of alginates can be carried out to
obtain other organic acids (Aida et al. 2012) and alginate lyase enzymes (Zhu and Yin
2015). The products of the depolymerized alginates are safe and in some cases are
of commercial importance such as the alginate oligosaccharides which are discussed
in the section on applications of alginate.
Alginate is also produced by some species of bacteria such as pseudomonas and
Azotobacter (Chen and Long 2018). These bacteria also produce the enzyme alginate
lyases to break down the alginate for use. The alginate oligosaccharides derived
from the action of alginate lyases on alginate has potential applications such as
protection against disease-causing organisms (An et al. 2009), growth-promoting
agents in plants (Iwasaki and Matsubara 2000) and antioxidants (Falkeborg et al.
2014; Nagarajan et al. 2016; Chen and Long 2018).