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Alginic Acid
Sources and Extraction
Alginate is a component of the membrane of various brown algae (Phaeophyceteae). The main raw material sources are Macrocystis pyrifera (Pacific coast, California, Canada, Australia, New Zealand), Laminaria digitata (North Atlantic coasts; Ireland, Scotland, Norway, France), species of Eklonia and Sargassum (Japan). The alginate content of the algae is 15 to 40% of the dry substance. To extract alginate from Macrocystis pyrifera, the algae are gathered, hackled and washed. The wet or dried algae are leached with alkali. By precipitation with Ca-ions or acid, the alginate is isolated from the purified extract and bleached. After being treated with alkali, the alginic acid is transferred to water soluble alginate, dried and ground.
With Laminaria species, the neutral polysaccharide Laminarin must first be removed by treatment with diluted aqueous CaCl2 solution and subsequent wash-out of the salt with diluted acid. Only afterwards the extraction with alkali is effected. By variation of the manufacturing process, alginates with different grades of polymerisation can be produced. The annual production for the food industry is estimated at about 10 000 tons.
Properties
Appearance
Alginate is a colourless to slightly yellowish powder.
Composition
Alginate is the salt of a linear polyuronide with the cations Na, K, NH4 or Ca. The alginic acid is composed of D-mannuronic acid and L-guluronic acid, which are linked together by β-(1-4) and α-(1-4) bond respectively. The proportion of mannuronic acid to guluronic acid ranges between 0.4 und 1.9, depending on the algae type. Besides the alternate polymers (M-G), the macromolecule also contains block polymers which are composed only of mannuronic acid units (M-M) or of guluronic acid units, respectively. The medium molecular weights rage between 32 000 und 250 000.
Solubility
Alginic acid and Ca-alginate are water-insoluble, whereas the alkaline salt and the sal ammoniac dissolve easily. The solution being high viscous already in low concentrations are very stable when heated in the neutral pH range. In the acid pH range (from pH 5), the viscosity decreases strongly. Traces of Ca-ions increase the viscosity considerably.
Application properties
At room temperature, alginate solutions form irreversible gels in the presence of Ca-ions. Alginates with a high content of guluronic acid block polymers produce more solid and less elastic gels than alginates with a high content of mannuronic acid block polymers. This is due to the more favourable layout of the guluronic acid molecules to forming a bridge between carboxyl and OH groups over Ca-ions. In the presence of acids and Ca-ions, Alginates are much more delicate than low esterified pectines. Therefore, the production of gels requires the use of Ca-salts as hardly soluble as possible in a system that releases hydrogen ions (fumaric acid, glucono-δ-lacton) only slowly, or else Ca-binding substances such as polyphosphates are added. By adecuate modifications, the properties of alginate gels can be adapted to particular requirements. Due to the high viscosity and the heat resistance in the neutral environment, alginates are well suited to be used as thickening and binding agents for tinned meat and vegetables, soups, sauces and syrups. They are also used for the stabilisation of mayonnaises, salad dressings, egg white foams or naturally cloudy fruit juices as well as for the prevention of crystallisation in ice creams. As gelling agent, alginates are applied in (onfectionery products such as glazes, decor fruits or baking stable jams as well as for the production of puddings and gels. The ability of alginates to form films is used for the production of edible food coatings. Alginates are also widely used for pharmaceutical and cosmetic purposes and in the paper, textile and rubber industries.
Toxicologic Evaluation
In many regions, seaweed has been used as food ingredient already for a long time. Alginate is not being absorbed in the human digestive tract. It can reduce the resorption rate of Ca, Fe, et al.
The results of toxicologic tests existing so far have not evidenced any health problems. A limitation of the daily intake does not appear necessary.