Agar: Difference between revisions

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** Gelidium: G. Arborescens, G.  Nudifrons, G. Pacificum (CA, USA)
** Gelidium: G. Arborescens, G.  Nudifrons, G. Pacificum (CA, USA)
** Gigartina: G. asperfolia, G. Coniliculata, G.Corymbifera (CA, USA)
** Gracillaria: G. Blodgetti, G. cervicornis (FL, USA)
** Gracillaria: G. Blodgetti, G. cervicornis (FL, USA)
** Gigartina: G. asperfolia, G. Coniliculata, G.Corymbifera
 
==Hazards==
==Hazards==
==Production==
==Production==

Latest revision as of 17:26, 2 September 2019

Agar is a mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin. It forms the supporting structure in the cell walls of certain species of algae, and is released on boiling. These algae are known as agarophytes, and belong to the Rhodophyta (red algae) phylum.

The gelling agent in agar is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from tengusa (Gelidiaceae) and ogonori (Gracilaria). For commercial purposes, it is derived primarily from ogonori. In chemical terms, agar is a polymer made up of subunits of the sugar galactose.

Uses

Justification Questioned

Other

  • Monad growth media
  • Salt bridges
  • Sizing for paper or fabric

Natural Occurrence

  • Agar does occur naturally in Seaweeds from several families.[1]
    • Gelidium: G. Arborescens, G. Nudifrons, G. Pacificum (CA, USA)
    • Gigartina: G. asperfolia, G. Coniliculata, G.Corymbifera (CA, USA)
    • Gracillaria: G. Blodgetti, G. cervicornis (FL, USA)

Hazards

Production

Extraction

A short and simplified description of the extraction of agar from seaweeds is that the seaweed is washed to remove foreign matter and then heated with water for several hours. The agar dissolves in the water and the mixture is filtered to remove the residual seaweed. The hot filtrate is cooled and forms a gel (jelly) which contains about 1 percent agar. The gel is broken into pieces, and sometimes washed to remove soluble salts, and, if necessary, it can be treated with bleach to reduce the colour. Then the water is removed from the gel, either by a freeze-thaw process or by squeezing it out using pressure. After this treatment, the remaining water is removed by drying in a hot-air oven. The product is then milled to a suitable and uniform particle size.

Gelidium is simply washed to remove sand, salts, shells and other foreign matter and is then placed in tanks for extraction with hot water. Gracilaria is also washed, but it must be treated with alkali before extraction; this alkaline pre-treatment causes a chemical change in the agar from Gracilaria, resulting in an agar with an increased gel strength. Without this alkaline pre-treatment, most Gracilaria species yield an agar with a gel strength that is too low for commercial use. For the alkali treatment, the seaweed is heated in 2-5 percent sodium hydroxide at 85-90°C for 1 hour; the strength of the alkali varies with the species and is determined by testing on a small scale. After removal of the alkali, the seaweed is washed with water, and sometimes with very weak acid to neutralize any residual alkali.

For the hot-water extraction, Gelidium is more resistant and extraction under pressure (105-110°C for 2-4 hours) is faster and gives higher yields. Gracilaria is usually treated with water at 95-100°C for 2-4 hours. The remainder of the process is the same for both types of raw material. The hot extract is given a coarse filtration to remove the seaweed residue, filter aid is added and the extract is pumped through a filter press equipped with a fine filter cloth. The extract is thick and will gel if allowed to cool, so it must be kept hot during the filtration processes. --UN FAO[2]

Purification

Testing

Storage

Disposal

See Also

  • Gelatin

References

  1.  (2010) "Gums, Adhesives & Sealants Technology (with Formulae & their Applications)".
    ASIA PACIFIC BUSINESS PRESS Inc
  2. McHughtitle, Dennis J. "AGAR"
    FAO FISHERIES TECHNICAL PAPER 441 
    link courtesy United Nations Food and Agriculture Organization.