Agar
Agar is an unbranched polysaccharide extracted from the cell walls of some species of red algae or seaweed and having major economic importance. Chemically, agar is a polymer made up of subunits of the sugar galactose, a monosaccharide.
Agar polysaccharides serve as the primary structural support for the algae's cell walls. Dissolved in hot water and cooled, agar becomes gelatinous. Its chief use is as a culture medium for microbiological work. Other uses are as a laxative; a thickener for soups; in jellies, ice cream and Japanese desserts such as anmitsu; as a clarifying agent in brewing; for paper sizing fabrics; and as a vegetarian gelatin substitute (since gelatin traces to the connective tissues of animals),
The word agar comes from the Malay word agar-agar (meaning jelly). It is also known as kanten or agal-agal (Ceylon agar).
Uses in microbiology
Nutrient agar is used throughout the world as a medium for the growth of bacteria and fungi. It is not used for viruses; however, a particular type of virus—bacteriophages—are often grown in bacteria that are growing on agar.
Though less than 1% of all known bacteria can be grown successfully, the basic agar formula can be modified to grow most of the microbes whose needs are known. More specific nutrient agars are available, because microbes can be selective. For example, blood agar, which is generally combined with horse blood, can be used to detect the presence of hemorrhagic micro-organisms such as E. coli O:157 H:7. The bacteria digest the blood, turning the plate clear.
Selective media is agar specially treated to apply a selective pressure to organisms growing on it—for example, to select for salt-tolerant, gram-positive, or gram-negative bacteria. To select for only gram negative organisms you would use MacConkey agar, which would also in turn reveal if the gram negative organism is a lactose fermenter or not, indicated by red colonies instead of translucent colonies of a non- lactose fermenter.
Differential media includes an indicator that causes visible, easily detectable changes in the appearance of the agar gel or bacterial colonies in a specific group of bacteria. For example, EMB (Eosin Methylene Blue) agar causes E. coli colonies to have a metallic green sheen, and MSA (Mannitol Salt Agar) turns yellow in the presence of mannitol fermenting bacteria.
Uses in molecular biology
Agar is a heterogeneous mixture of two classes of polysaccharide: agaropectin and agarose (Wu et al. 1990). Although both polysaccharide classes share the same galactose-based backbone, agaropectin is heavily modified with acidic side-groups, such as sulfate and pyruvate. The neutral charge and lower degree of chemical complexity of agarose make it less likely to interact with biomolecules, such as proteins. Gels made from purified agarose have a relatively large pore size, making them useful for size-separation of large molecules, such as proteins or protein complexes greater than 200 kilodaltons, or DNA fragments greater than 100 basepairs.
Agarose can be used for electrophoretic separation in agarose gel electrophoresis or for column-based gel filtration chromatography.
Uses in cooking
Agar is typically sold as packaged strips of washed and dried seaweed, or in powdered form. Raw agar is white and semi-translucent. For making jelly, it is boiled in water at a concentration of about 0.7-1% weight to volume (e.g. a 7 gram packet of powder into 1 liter of water would be 0.7%) until the solids dissolve, after which sweeteners, flavoring, coloring, and pieces of fruit may be added. The agar-agar may then be poured into molds or incorporated into other desserts, such as a jelly layer on a cake.
One of the latest fad diets in Asia is the Kanten Diet. Once ingested, kanten, a seaweed based thickener known as agar-agar, triples in size and absorbs water. This results in the consumer feeling more full. Recently this diet has received some press coverage in the United States as well. The agar diet has shown promise in obesity studies (Maeda et al. 2005), but agar/kanten has virtually no nutritional value. It is approximately 80% fiber, so part of the diet's effectiveness may be a result of it working as a laxative.
In Indian cuisine, agar agar is known as "China grass" and is used for making desserts.
Uses in plant biology
Research grade agar is used extensively in plant biology, as it is supplemented with a nutrient and vitamin mixture that allows for seedling germination in petri dishes under sterile conditions (given that the seeds are sterilized as well). Nutrient and vitamin supplementation for Arabidopsis thaliana is standard across most experimental conditions. Murashige & Skoog (MS) nutrient mix and Gamborg's B5 vitamin mix are generally used. A 1.0% agar/0.44% MS+vitamin dH20 solution is suitable for growth media between normal growth temps.
The solidification of the agar within any growth media (GM) is pH-dependent, with an optimal range between 5.4-5.7. Usually, the application of KOH is needed to increase the pH to this range. A general guideline is about 600 µl 0.1M KOH per 250 ml GM. This entire mixture can be sterilized using the liquid cyle of an autoclave.
This medium nicely lends itself to the application of specific concentrations of phytohormones and so forth to induce specific growth patterns. One can easily prepare a solution containing the desired amount of hormone, add it to the known volume of growth media, and autoclave to both sterilize and evaporate off any solvent may have been used to dissolve the often polar hormones in. This hormone/growth media solution can be spread across the surface of petri dishes sown with germinated and/or etiolated seedlings.
Other uses
Agar is used as an impression material in dentistry. It is also used to make salt bridges for use in Electrochemistry.
External links
- General agar information
- Adzuki Bean Agar-Agar Recipe
- Artist Daniel Dutton's recipe for "Superior Key Lime Pie" uses agar powder.
- Chemical structure of agarose
ReferencesISBN links support NWE through referral fees
- Maeda, H., R. Yamamoto, K. Hirao, and O. Tochikubo. 2005. Effects of agar (kanten) diet on obese patients with impaired glucose tolerance and type 2 diabetes. Diabetes, Obesity and Metabolism 7(1): 40-46.
- Wu, C., M. Ji, R. Li, et al. (eds.). 1990. Chapter III: Properties, manufacture and application of seaweed polysaccharides—agar, carrageenan and algin. In C. Wu et al., eds., Training Manual on Gracilaria Culture and Seaweed Processing in China. Food and Agriculatural Organization of the United Nations. Retrieved April 10, 2007.
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