STRUCTUREIn the natural state, agar occurs as structural carbohydrate in the cell walls of agarophytes algae, probably existing in the form of its calcium salt or a mixture of calcium and magnesium salts. It is a complex mixture of polysaccharides composed of two major fractions - agarose, a neutral polymer, and agaropectin, a charged, sulfated polymer.
Agarose, the gelling fraction, is a neutral linear molecule essentially free of sulfates, consisting of chains of repeating alternate units of β-1,3-linked- D-galactose and α-1,4-linked 3,6-anhydro-L-galactose. Agaropectin, the non gelling fraction, is a sulfated polysaccharide (3% to 10% sulfate), composed of agarose and varying percentages of ester sulfate, D-glucuronic acid, and small amounts of pyruvic acid. The proportion of these two polymers varies according to the species of seaweed. Agarose normally represents at least two-thirds of the natural agar-agar.
PRODUCTION PROCESSAgar-agar may come in several forms: powdered, flakes, bars and threads. Powdered agar-agar is a product mostly used for industrial applications. Flakes, bars and threads are mostly used in cooking. The manufacture of powdered and flake-like agar-agar may be accomplished through two methods: Gel Press or Precipitation in solvents. However, the later is not much used nowadays, due to its high cost and low efficiency. Agar-agar in bar and thread forms is produced through a more traditional manufacturing system.
1. SOLUBILITYAgar-agar is insoluble in cold water, but it swells considerably, absorbing as much as twenty times its own weight of water. It dissolves readily in boiling water and sets to a firm gel at concentrations as low as 0.50%. Powdered dry agar-agar is soluble in water and other solvents at temperatures between 95º and 100º C. Moistened agar flocculated by ethanol, 2-propanol or acetone, or salted out by high concentrations of electrolytes, is soluble in a variety of solvents at room temperature.
2. GELLINGThe gelling portion of agar-agar has a double helical structure. Double helices aggregate to form a three-dimensional structure framework which holds the water molecules within the interstices of the framework. Thus, thermo-reversible gels are formed. The gelling property of agar-agar is due to the three equatorial hydrogen atoms on the 3,6-anhydro-L-galactose residues, which constrain the molecule to form a helix. The interaction of the helixes causes the formation of the gel.
Regarding its gelling power, agar-agar is outstanding among other hydrocolloids. Agar-agar gels can be formed in very dilute solutions, containing a fraction of 0.5% to 1.0% of agar-agar. These gels are rigid, brittle, have well defined shapes, as well as sharp melting and gelling points. Moreover, they clearly demonstrate the interesting phenomenon of syneresis (spontaneous extrusion of water through the surface of the gel), and hysteresis (temperature interval between melting and gelling temperatures). Gelling occurs at temperatures far below the gel melting temperature. A 1.5% solution of agar-agar forms a gel on cooling to about 32º to 45º C that does not melt below 85º C. This hysteresis interval is a novel property of agar-agar that finds many uses in food applications. The gel strength of the agar-agar is influenced by concentration, time, pH, and sugar content. The pH noticeably affects the strength of the agar gel; as the pH decreases, the gel strength weakens. Sugar content has also a considerable effect over agar gel. Increasing levels of sugar make gels with harder but less cohesive texture.
3. VISCOSITYThe viscosity of agar solutions varies widely and is markedly dependent upon the raw material source. The viscosity of an agar solution at temperatures above its gelling point is relatively constant at pHs 4.5 to 9.0, and is not greatly affected by age or ionic strength within the pH range 6.0 to 8.0. However, once gelling starts viscosity at constant temperature increases with time.
4. STABILITYAn agar-agar solution is slightly negatively charged. Its stability depends upon two factors: hydration and the electric charge. The removal of both factors result in flocculation of the agar-agar. Prolonged exposure to high temperatures can degrade solutions of agar-agar, resulting in a lower gel strength after temperature decrease and gel formation. The effect is accelerated by decreasing pH . Therefore, it should be avoided to expose agar-agar solutions to high temperatures and to pHs lower than 6.0 for prolonged periods of time. Agar-agar in the dry state is not subject to contamination by microorganisms. However, agar-agar solutions and gels are fertile media for bacteria and/or molds and appropriate precautions should be taken to avoid the growth of microorganisms.
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