2.2.3.1 Algin
Synonyms Sodium alginate; Alginic acid sodium; Sodium polymannuronate; Kelgin; Minus; Protanal;
Biological Source Algin is a gelling polysaccharide extracted from the giant brown seaweed (giant kelp. Macrocystis pyrifera (L.) Ag., Lessoniaceae) or from horsetail kelp (Laminaria digitata (L.) Lamour, Laminariaceae) or from sugar kelp (Laminaria saccharina (L.) Lamour). Some other common species are Laminaria hyperborea and Ascophyllum nodosum
Geographical Source The different varieties of seaweeds are invariably found in the Pacific and Atlantic Oceans, more specifically along the coastal lines of USA, Canada, Scotland, Japan and Australia. In India the Western coast of Saurashtra is also a potential source of algin. However, USA, and UK are the largest producers of algin in the world.
Preparation The algin (or sodium alginate) is the sodium salt of alginic acid which is a purified carbohydrate extracted from brown seaweed (algae) by the careful treatment with dilute sodium hydroxide. The brown colour of the crude algin is due to the presence of a carotenoid pigment
associated with it which may be eliminated by treating the aqueous solution with activated carbon and spray drying the powder.
Description
Colour : Yellowish-white, cream coloured, buff coloured
Odour : Odourless
Taste : Tasteless
Solubility : Insoluble in alcohol, ether, chloroform and strong acids, freely soluble in water
Viscosity : A 1% (w/v) aqueous solution at 20°C may show a viscosity ranging between 20-400 centipoises.
Chemical Constituents Alginic acid is mainly comprised of D-mannuronic acid residues which on methylation and hydrolysis gives rise to the formation of 2,3 dimethyl-D- mannuronide. Therefore, the ring as well as bridge oxygen atoms involve C-4 and C-5 and the carboxyl groups are absolutely free to react (to form sodium salts), whereas the aldehydic moieties are duly utilized by the respective glycosidic linkages. It has been observed that these mannuronic acid entities are joined by β-1, 4-glycosidic linkages. The resulting structure could be either linear or very slightly branched.
Chemical Tests
1. The aqueous solution of algin gives an instant white copious precipitate with calcium chloride solution.
2. A 1% (w/v) aqueous solution of algin yields a heavy gelatinous precipitate with diluted sulphuric acid.
3. It is not precipitated by saturated ammonium sulphate solution (distinction from agar and tragacanth)
4. It gives effervescence (liberates CO2) with carbonates.
5. It readily reacts with compounds having ions of alkali metals (e.g., Na+, K+, Li+) or ammonium (NH4+) or magnesium (Mg2+ ) to produce their respective alginates (salts) that are water soluble and forms thick and viscous solutions characteristic of hydrophillic colloids.
Uses
1. It is extensively used in the manufacture of ice-creams where it serves as a stabilizing colloid, ensuring creamy texture thereby checking the growth of ice-flakes (or crystals).
2. It is also used in the flocculation of suspended solids in most water treatment plants.
3. It is employed as a stabilizing and thickening agent in food and pharmaceutical industry.
4. It is used as a film and film-forming agent in the rubber and paint industry.
5. It is widely used in the textile industry as absorbable haemostatic dressings.
6. It is employed as a binding and distintegrating agent for tablets and lozenges.
