Alginate, as mentioned before, is found in all types of Phaeophycaea and apart from being a structural polysaccharide and providing rigidity to the algae it also helps prevent the dessication of those plants expose to air at low tide. Alginate occurs in brown seaweeds in the intercellular mucilage and algal cell wall as an insoluble mixed salt. Not all brown seaweeds are useful as commercial sources of alginate. However of those that are one of the principal sources is Macrocystis Pyrifera harvested off the coast of southern California and Australia. European and Scandinavian alginate prodcution relies mainly on laminaria hyperborea, laminaria digitata and ascophyllum nodosum. The countries involved in alginate production at present are the USA, Great Britain, France, Norway and Japan. However there are sufficient resources fro a large increase in seaweed harvesting and processing.
Macrocystis pyrifera is harvested by purpose built boats cutting and collecting all the growth from the top three feet of the ocean surface. The weed is then dried and milled to a coarse granular consistency. After milling the weed is treated with a preservative and mixed with hot water to form a paste, the paste is transferred to a maturation vat where hot water and sodium carbonate are added. During the alkali treatment, which is usually about three hours in duration, the seaweed swells up, looses its structure and forms a viscous solution. The solution is diluted and pumped to a slurry vat where the slurry is stirred for one to one and half hours after which it is pumped to a settling tank where a polyelectrolyte is added to aid the flocculation of the insoluble residues. After 18-24 hours the alginate containing liquid fraction is decanted off from the residue, chlorine is added as a bleach and the liquor is treated with calcium chloride to form precipitate of calcium alginate. The precipitate is made to float by blowing air through the solution, the calcium alginate fibre can then be skimmed off. The solid is then pressed to remove a large percentage of the water present. The pressed alginate is leached with hydrochloric acid followed by sulphuric acid to leave free alginic acid fibre. The free acid is usually pressed again to remove enough water to give 30-40% solids content. The free acid can be neutralised with an appropriate base to form a salt which is dried and milled to the required specification for sale (Kelco Undated, Thornley 1931, Clark 1936, Green 1936, Griffiths 1984). A modified preparation consists of precipitating the alginic acid directly from the filtered, dissolved seaweed by the addition of sulphuric acid and alcohol (le Gloahec 1938).
Additional information on alginate structure and bacterial alginate can be accessed using the arrows in the Further Reading box below.