Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
15.6 ALGAL GUMS
Marine algae or seaweeds are an excellent source of hydrocolloids due to the
requirements for growth in marine environment. The gel-like hydrated and charged
polysaccharides of the seaweed cell wall mucilage assist the algae to cope with the
physical stresses of wave motion and the chemical stresses of the high salt concentration
of sea water. The polysaccharides are also a key protection for seaweeds
growing in the intertidal zone, where they are regularly exposed to the threat of
desiccation at low-tide, prevented only by the presence of the water-binding polysaccharide
gel around their tissues.
15.6.1 Alginates
Alginates are present in the walls of many brown seaweeds (Phaeophyta), which
grow in oceans throughout the world. The main commercial resources are found on
the coasts of temperate regions in both the Northern and Southern hemispheres.
Important commercial species are selected for yield of alginate and ease of growth
and collection, which is usually associated with the large size of the seaweed. The
most important algal resources are Laminaria digitata and L. hyberborea from
Northern Europe and Asia and Kelp (Macrocystis pyrifera) from the Pacific coast
of North America. Other species harvested in Northern Europe and America are
Ascophyllum nodusum and Fucus serratus.
Alginates are acidic polysaccharides, linear polymers of β-D-mannuronic acid
and α-L-guluronic acid, which are (1-4) linked. The two residues can be found
alternating in the chain but more commonly form a block copolymer structure with
regions containing only guluronic acid and regions with only mannuronic acid. The
final chain is then composed of stretches of pure guluronate or mannuronate with
some mixed regions. 92
Extraction of alginates from raw seaweed fronds commences with an acid wash
to remove acid-soluble impurities and convert alginates present to the acid form.
The alginate can then be extracted with alkali to give a soluble alginate salt, which
can be separated from other material by filtration before precipitation with acid.
Alginate can be neutralized with different bases to give the various soluble salts that
are used in the food industry, principally sodium, potassium, and ammonium alginate.
These salts are all readily cold-water soluble.
In solution, soluble alginates can act as effective thickeners to increase viscosity
at low concentration. The viscosity decreases with increasing temperature
and shows pseudoplastic behavior. The solutions are not stable at low pH, because
below pH 4 alginate is insoluble and forms a precipitate or acid gel. 93 In the
presence of Ca 2+ , alginates can form a thermostable gel by binding the cations
between two opposed blocks of guluronic acid. The guluronic acid regions of the
chain adopt a buckled conformation that provides a negatively charged pocket that
can accommodate a Ca 2+ ion and allow it to bind to charges on another guluronic
region on another chain. 94 Guluronate regions provide the junction zones whereas
mannuronate regions and mixed regions of the chain provide the nonbinding
regions.