Agar
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Agar is a galactose polymer (or agarose) obtained from the cell walls of some species of red algae or seaweed (Sphaerococcus euchema) and species of Gelidium and Gracilaria, chiefly from eastern Asia, Chile and California. It is also known as Kanten, Agar-Agar, or Agal-Agal (Ceylon Agar).
Chemically, agar is a polymer made up of subunits of the sugar galactose; it is a component of 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 vegetarian gelatin substitute — a thickener for soups, in jellies, ice cream and Japanese desserts such as anmitsu, as a clarifying agent in brewing, and for paper sizing fabrics.
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Uses in cooking
Agar-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% w/v (e.g. a 7 gram packet of powder into 1 litre of water would be 0.7%) until the solids dissolve, after which sweeteners, flavouring, colouring, 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 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 diet has shown promise in obesity studies, but it should be noted that 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.
Uses in microbiology
- Main article: Agar plate
Nutrient agar is used throughout the world as a medium for the growth of bacteria and fungi, but not viruses (however, viruses are often grown in bacteria that are growing on agar). Though less than 1% of all existing bacteria can be grown successfully, the basic agar formula can be used to grow most of the microbes, whose needs are known. More specific nutrient agars are available, because microbes can be picky. For example, blood agar, which is generally combined with horse blood, can be used to detect the presence of haemorrhagic micro-organisms such as E.coli O:157 H:7. The bacteria digest the blood, turning the plate clear.
Agarose is also used in Agarose gel electrophoresis.
Selective Media
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's Agar, which would also in turn tell you if the gram negative organism is a lactose fermenter or not indicated by red colonies instead of translucent (non- lactose fermenter).
Differential Media
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.
Hysteresis
Hysteresis describes the phenomenon of the differing liquid-solid state transition temperatures that agar exhibits. Agar melts at 85 °C and solidifies from 32-40 °C.
See also
External links
de:Agar fr:Agar-agar gl:Ágar-ágar id:Agar it:Agar agar he:אגר ja:寒天 pl:Agar-agar fi:Agar sv:Agar
