Seaweeds as a source of agar

Agar, a general name for polysaccharides extracted from certain kinds of red algae, is built up of alternating D- and L- galactopyranose units. The name agar is derived from a Malaysian word "agar-agar", which literally means "seaweed". As the gelling agent "kanten", it is known from Japan since the 17th century; extracts from red seaweeds were carried up the mountains to freeze overnight so that water and other impurities could be extracted from the material. Agar finds its widest use as a solid microbiological culture substrate. Modern agar is a purified form consisting largely of the neutral fraction known as agarose; the non-ionic nature of the latter makes it more suitable for a range of laboratory applications. Agar in a crude or purified form also finds wide usage in the food industry where it is used in various kinds of ices, canned foods and bakery products.

The best quality agar is extracted from species of the red algal genera Pterocladia and Gelidium, which are harvested by hand from natural populations in Spain, Portugal, Morocco, the Azores, California, Mexico, New Zealand, South Africa, India, Chile, and Japan. Agars of lesser quality are extracted from Gracilaria and Hypnea species. Agar quality is seasonal in Pterocladia species, being low in the colder months and high in the warmer. At present, there is no commercial mariculture of agar-producing weeds carried out in the world, largely because techniques for growing weeds with good-quality agar have not been developed. About 10,000 t of agar are produced world-wide at present. There is currently a shortage of exploitable populations of agar-producing seaweeds and agar is consequently an expensive product. At present, more than 50% of all agar being produced is food-grade agar being produced from Gracilaria.