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Gracilaria brevis

Gracilaria brevis (Rhodophycae) from Baker, A.L. et al.  2012.  Phycokey -- an image based key to Algae (PS Protista), Cyanobacteria, and other aquatic objects. University of New Hampshire Center for Freshwater Biology. http://cfb.unh.edu/phycokey/phycokey.htm 17 May 2014

Agar

Agar dissolves in hot water to form gels on cooling.

 

V Source
V Structural unit
V Molecular structure
V Functionality

Source

Agar (E406) is scaffolding polysaccharide prepared from the same family of red seaweeds (Rhodophycae) as the carrageenans. It is commercially obtained from species of Gelidium and Gracilariae

Structural unit

-(1-3)-beta-D-galactopyranose-(1-4)-3,6-anhydro-alpha-L-galactopyranose unit

-(1-3)-beta-D-galactopyranose-(1-4)-3,6-anhydro-alpha-L-galactopyranose unit

 

Agar consists of a mixture of agarose and agaropectin. Agarose is a linear polymer, of relative molecular mass (molecular weight) about 120,000, based on the -(1->3)-β-D-galactopyranose-(1->4)-3,6-anhydro-α-L-galactopyranose unit; the major differences from carrageenans being the presence of L-3,6-anhydro-α-galactopyranose rather than D-3,6-anhydro-α-galactopyranose units and the lack of sulfate groups. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts. Their structures are similar but slightly branched and sulfated, and they may have methyl and pyruvic acid ketal substituents. They gel poorly and are simply removed from the excellent gelling agarose molecules by using their charge. Alkaline treatment, that converts of any L-galactose-6-sulfate to 3,6-anhydro-L-galactose mproves the quality of the agar.  [Back to Top to top of page]

Molecular structure

The gel network of agarose contains double helices formed from left-handed threefold helices. The presence of water molecules bound to the inside of the double helical cavity stabilizes these double helices [508]. Exterior hydroxyl groups allow aggregation of up to 10,000 of these helices to form suprafibers.  [Back to Top to top of page]

Functionality

Agar is insoluble in cold water but dissolves to give random coils in boiling water. Gelation is reported to follow a phase separation process [501a] (although these findings are disputed [501b]) and association on cooling (≈ 35 °C), forming gels with up to 99.5% water and remaining solid up to about 85 °C. Agar has a significant use in microbiological media as it is not easy for microorganisms to metabolize and forms clear, stable and firm gels but, in the food area, it finds use in icings, glazes, processed cheese, jelly sweets, and marshmallows. It may be used in tropical countries and by vegetarians as a substitute for gelatin. Agar gel can be used to selectively clean the surface of sensitive artworks and historical artifacts by the gradual removal of any soiling layer through the slow release of water (syneresis), diffusion of water into the porous substrate, and the withdrawal of the dirty water (evaporation, capillarity, and osmosis) [3343].


Interactive structures are available (Jmol).  [Back to Top to top of page]

 

 

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This page was established in 2003 and last updated by Martin Chaplin on 26 September, 2018


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