The History of Alginate Chemistry – Bacterial

Bacterial alginate

Alginate can be produced by a microbial fermentation using bacteria such as Azobacter vinelandii and Pseudomonas eeruginosa (Linker and Jones 1964, Gorin and Spencer 1966). These bacteria produce a polysaccharide with a structure resembling alginate, differing only in that there are acetyl groups on a portion of the C2 and C3 hydroxyls. It is believed that the acetate groups are associated mainly with the D-mannuronic acid residues (Davidson 1977, Sutherland 1983, Paul 1986). The level of acetylation is variable as is the mannuronic and guluronic acid content. However the level of guluronic acid in the final polymer can be controlled to some extent by altering the level of calcium in the fermentation broth (Haug and Larsen 1971). The sequence structures and acetylation patterns of bacterial alginate, from different sources, have been studied with 2D COSY proton NMR techniques. The acetyl residues were found to be exclusively associated with the mannuronic acid residues with degrees of acetylation varying from 4-57%. one other interesting point noted was that alginate isolated from four different pseudomonas bacteria all showed a complete lack of consecutive guluronic acid segments, a necessary structural feature for the formation of calcium gels (Skjak-braek 1986).

One of the reasons for the interest in bacterial alginate was the potential development of products with a wide range of molecular weights and properties to compete against algal alginates. However the technology has never been commercialised and is unlikely to ever be competitive against seaweed based alginate production. It is believed that Tate and Lyle developed a bacterial alginate process up to pilot plant scale and then sold the process to Kelco (Paul 1986, Lawson and Sutherland, 1978). Propylene glycol alginate is produced by taking an alginic acid sample that has been pressed to contain between 30-40% solids and partially neutralise the acid with sodium carbonate to between 5-40% neutralised. The solids are then washed in acetone or propan-2-ol and centrifuged to remove the excess liquid. Finally a fibre with 60-87% solids is obtained. The fibre is then treated with 1,2 epoxypropane to form an ester. The levels of substitution of over 90% can be produced. (Uniroyal, 1978; Kelco Undated).

Additional information on alginate structure and production can be accessed using the arrows in the Further Reading box below.