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Definition: The gelatinous fraction of the cell wall of certain red algae (Rhodophyta), comprising the polysaccharide agarose (B-1,3-linked D-galactose and 1,4-linked anhydro-L-galactose) and agaropectin (a sulphated galactan mixture) in a ratio of about 7:3. Once solidified, agar gel does not melt below 100 C. A 1.5% solution is clear and forms a solid but elastic gel on cooling to 32–39 C, not dissolving again at a temperature below 85 C. Comprises two polysaccharides, agarose and agaropectin, and is commercially extracted from such red algal species as Gelidium, Pterocladia, Acanthopeltis, Ahnfeltia and Gracilaria. Greatest use of agar occurs in the food and pharmaceutical industries for gelling and thickening purposes (e.g. canning of fish and meat, processed cheese, mayonnaise, puddings, jellies), emulsions, ointments and lotions. It is also widely used as a solidifying agent for culture media in bacteriology, mycology and phycology.
Source: Penguin Dictionary of Biology. London: Penguin, 2004. Credo Reference. Web. 17 August 2012.