What Is AGAR-AGAR?
Agar-agar is a hydrocolloid extracted from red seaweeds that is widely used as a gelling agent in the food industry. In its gelling power, agar is outstanding among the hydrocolloids. Among its major properties one can mention its high gel strength at low concentrations, low viscosity in solution, high transparency in solution, thermo-reversible gel and sharp melting/setting temperatures. Agar-agar may come in several forms: powdered, flakes, bars and threads. Besides its use as a food additive, it is also used on a lesser scale in other industrial applications.
Agar-agar is extracted from several types and species of red seaweeds belonging to the Rhodophyceae class. These agar-containing seaweeds are called agarophytes and the major commercial species are Gracilaria and Gelidium. The agar content of seaweeds varies according to the conditions of seawater. Carbon dioxide concentration, oxygen tension, water temperature and intensity of solar radiation can have significant influence.
Seaweeds are usually harvested manually by fishermen in low depths at low tides or by diving using appropriate equipment. After being harvested, seaweeds are placed under the sun to dry until they reach a humidity level that is ideal for processing. Gelidium is obtained from natural seaweed beds mainly in Morocco, Spain, Portugal, Japan and South Korea, as attempts to cultivate it have not been successful. On the other hand, Gracilaria seaweeds have been successfully cultivated on a commercial scale, particularly in China, Indonesia and Chile.
In 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 units. 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.
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 is accomplished by the Gel Press method by pressing the agar gel. Agar-agar in bar and strip forms is manufactured through a more traditional production method by freezing and thawing the agar gel.
Industrial Method – Gel Press
Traditional Manual Method
Agar-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 to 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. Special types of agar-agar that passes through additional processes are soluble at lower temperatures between 85º to 90º C. They are marketed as Quick Soluble Agar or Instant Agar.
The ability to form reversible gels by simply cooling hot, aqueous solutions is the most important property of agar-agar. This gel-forming ability has led to the large number of practical applications where agar is used as a food additive or in other applications in microbiology, biochemistry or molecular biology, as well as in industrial applications.
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.
The 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.
An 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.
Functionality and applications
An agar-agar solution in hot water forms a characteristic gel after setting, with a melting point between 85º to 95º C, and a gelling point between 32º a 45º C. This physical property makes the gel very useful as an additive when used in many applications in the food industry.
Ice Creams, Yogurts, Milk desserts, Puddings, Processed cheeses
Sweets and Confectionary
Candy bars, Jelly candies, Marshmallows, Jams and Jellies, Marmalades, Fruit Jelly Dessert, Meringues
Clarifying and refining of juices, beers and wines
Sugar Icings, Pie fillings, Bread dough, Chiffon pies, Cake glaze
Other industrial applications
Agar-agar gel has the interesting property of inhibiting the characteristic liquefying that occurs in the enzyme action of microorganisms. This property finds a wide variety of applications in the medical and pharmaceutical industries. Agar-agar is used as a substratum in preparing bacteria cultures in microbiology, as laxatives and therapeutic agents in the treatment of malfunctions of the digestive tract, as a retarding agent and carrier in the management of medicines, antibiotics, vitamins, as a barium sulfate suspension agent in radiology, as a stabilizer in cholesterol solutions, and as a suspension agent in several types of emulsions. Agar-agar has other industrial applications as well where a gelling agent is needed, such as in dental prosthetics, photographic emulsions, differentiation of proteins through electrophoresis, chromatography through exclusion of sizes, molding of materials and as plant culture tissues in biotechnology.
Medical and Pharmaceutical Industry
Microbiology, Laxative, Therapeutic agent, Suspension agent of emulsion, Stabilizing agent of solution
Dental prosthetics, Electrophoresis, Chromatography, Impression materials, Plant Biotechnology
|Particle size||Mesh 80|
|Moisture content||Max. 18 %|
|Water absorption||Max. 75 c.c.|
|Acid insoluble ash||Max. 0,5 %|
|Total ash||Max. 6,5 %|
|Foreign insoluble material||Max 1,0 %|
|PH||6.5 to 7.5|
|Gel Strength (sol 1.5 % at 20ºC)||700 to 1.100 g/cm2|
|Viscosity (sol 1.5% at 60ºC)||10 to 100 cps|
|Melting Point||85 to 95 ºC|
|Setting Point||32 to 45 ºC|
|Arsenic||Max. 3 ppm|
|Lead||Max. 5 ppm|
|Total Plate Count||< 5000 CFU / g|
|Mold and Yeast||< 500 CFU / g|