Alginates

Alginates consist of alginic acid and its salts. The alginates originate mainly from the algal genera Laminaria and Macrocystis. Some bacteria are also capable of producing alginates. Pseudomonas aeruginosa, for example, forms a biofilm whose essential component consists of alginates. This mechanism is a major pathogenic factor (resistance to antibiotics) in cystic fibrosis.

To obtain alginate, the algae are first dried, washed and finally ground after harvesting. The alginate is then extracted and purified in several steps for further consumption.

Alginates are polysaccharides and are composed of the uronic acids (monosaccharides) alpha-1,4-guluronic acid and beta-D-mannuronic acid, which are glycosidically linked to form linear chains. This leads to the formation of conglomerates in which the two uronic acids are aggregated block-like. Depending on their composition, the aggregates are called "GG" (for guluronic acid) or "MM" (for mannuronic acid) blocks. The polysaccharide blocks form a leaflet structure. This structuring plays an essential role in gelation. Initiated by calcium ions, the leaflet of a further alginate complex fits exactly into the zigzag structure of the first leaflet. The nesting of several leaflets on top of each other results in a stable three-dimensional system, also known as an "eggbox model".

Note: This leaflet structure is similar to other biological leaflet models such as amyloid formation or plaque formation in Alzheimer's disease.

Alginates are mainly used in the food industry, where they are used as gelling agents, thickeners, stabilisers and emulsifiers. Alginates are not absorbed and are considered harmless. In the EU, alginic acid and the most commonly used sodium alginate (E 401) are approved for use in food processing. The following salts of alginic acid are also used: potassium alginate (E 402), ammonium alginate (E 403), calcium alginate (E 404) and propylene glycol alginate (PGA, E 405).

Alginates are found in bakery products, mayonnaises, salad dressings, ice cream, processed cheese, canned meat and vegetables and soups.

In medicine, alginate is used in the form of calcium alginate as a component of dressing material. By absorbing moisture it forms a gel with which wounds can be kept permanently moist. In animal experiments, alginates activate collagen synthesis in wounds.

Alginates also serve as gastric mucoprotective agents. Here, the polysaccharides together with calcium or potassium carbonate form a protective film which covers the stomach wall.

Alginates are also used as drug carriers from which an active ingredient is released in a protracted manner.

In dentistry, alginates are used as materials for dental impressions. As impression material, alginates are harmless and allow impressions of good accuracy.

Alginates are often found in adhesive materials for dental prostheses.

Bansal D et al (2016) Development of liposomes entrapped in alginate beads for the treatment of colorectal cancer. Int J Biol Macromol 82:687-695.

Chu J et al (2016) Poly (L-lacticacid) porous scaffold-supported alginate hydrogel with improved mechanical properties and biocompatibility. Int J Artif organ 39:435-443.

Jin Y et al(2016) Granular gel support-enabled extrusion of three-dimensional alginates and cellular structures. Biofabrication 8:025016.

McCaslin CA et al (2015) Impact of alginate-producing Pseudomonas aeruginosa on alveolar macrophage apoptotic cell clearance. J Cyst Fibros 14:70-77.

Momoh FU et al. (2015) Development and functional characterization of alginate dressing as potential protein delivery system for wound healing. Int J Biol Macromol 81:137-150.

Tan J et al (2014) A conformational landscape for alginate secretion across the outer membrane of Pseudomonas aeruginosa. Acta Crystallogr D Biol Crystallogr 70:2054-2068.

Wang T et al (2015) Calcium alginate enhances wound healing by up-regulating the ratio of collagen types I/III in diabetic rats. Int J Clin Exp Pathol 8:6636-6645.