Agar
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HollyWLambert
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[[Image:Youkan mizuyoukan.jpg|[[Yokan|Mizuyōkan]] - a popular Japanese red bean jelly made from agar|thumb|right|200px]]
'''Agar''' or '''agar agar''' is a [[gelatin]]ous substance derived from seaweed. Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Japan, but in the past century has found extensive use as a solid [[substrate (biology)|substrate]] to contain [[Growth medium|culture medium]] for [[microbiology|microbiological]] work. The gelling agent is an unbranched [[polysaccharide]] obtained from the [[cell membrane]]s of some species of [[red alga]]e, primarily from the [[genus]]es ''[[Gelidium]]'' and ''[[Gracilaria]]'', or [[seaweed]] (''[[Sphaerococcus euchema]]''). Commercially it is derived primarily from ''[[Gelidium amansii]]''.
Agar can be used as a [[laxative]], a [[vegetarian]] [[gelatin]] substitute, a thickener for soups, in [[Jelly (fruit preserves)|jellies]], [[ice cream]] and [[Japan]]ese desserts such as [[anmitsu]], as a clarifying agent in [[brewing]], and for [[sizing|paper sizing]] fabrics.
Chemically, agar is a [[polymer]] made up of subunits of the sugar [[galactose]]. Agar [[polysaccharide]]s serve as the primary structural support for the algae's cell walls.
==Names==
The word "agar" comes from the [[Malay language|Malay]] word '''agar-agar''' (meaning jelly). It is also known as '''kanten''', '''China grass''', or '''Japanese isinglass'''. The various species of alga or seaweed from which agar is derived are sometimes called '''Ceylon moss'''. ''[[Gracilaria lichenoides]]'' specifically is referred to as '''agal-agal''' or '''Ceylon agar'''.<ref>[http://www.botanical.com/botanical/mgmh/a/agara012.html Agar-Agar] at Botanical.com</ref>
In Malay and [[Indonesian language|Indonesian]], it is known as ''agar-agar''. In Japanese, it is known as ''kanten'' (寒天) meaning "cold weather," referring to the fact that it is harvested in the winter months. In [[Standard Mandarin|Mandarin Chinese]] as yángcài (洋菜) meaning "ocean vegetable" or "foreign vegetable." In [[Taiwanese (linguistics)|Taiwanese]] [[Minnan]] it is known as ''chhài-iàn'' (菜燕) meaning "vegetable [[swiftlet]]," i.e., similar in texture to the nest of the [[edible-nest swiftlet]] used in [[bird's nest soup]]. In [[Korea]], it is known as ''hancheon'' (한천). In [[China]], it is known as 海藻瓊脂 or 凍粉. In the [[Philippines]], it is known as ''gulaman'' in [[Tagalog language|Tagalog]], [[Apayao language|Apayao]], [[Bikol language|Bikol]], and [[Pangasinan language|Pangasinan]], ''guraman'' in [[Ilokano language|Ilokano]] and ''gurguraman'' in [[Sambali]].<ref>[http://www.bpi.da.gov.ph/Publications/mp/html/g/gulaman.htm Gulaman] at Bureau of Plant Industry website</ref> In [[Thai language|Thai]] it is known as ''wóon'' (วุ้น).
"Agar" name in the Holy Bible, King James Version uses Agar for the slave woman named Hagar. In most Bible Translations the use of Hagar is prefered over Agar.
==Usage==
===Microbiology===
====Culture Medium====
[[Image:Agar Plate.jpg|right|thumb|100mm diameter petri dishes containing agar jelly for bacterial culture]]
{{main|Agar plate}}
Nutrient agar is used throughout the world to provide a solid surface containing [[growth medium|medium]] for the growth of [[bacteria]] and [[fungi]]. Though less than 70% of all existing bacteria can be grown successfully, the basic agar formula can be used to grow most of the [[microbe]]s whose needs are known. More specific nutrient agars are available, because some microbes prefer certain environmental conditions over others.
===Motility assays===
As a gel, an agarose medium is porous and therefore can be used to measure microorganism motility and mobility. The gel's porosity is directly related to the concentration of agarose in the medium, so various levels of effective viscosity (from the cell's "point of view") can be selected, depending on the experimental objectives.
A common identification assay involves culturing a sample of the organism deep within a block of nutrient agar. Cells will attempt to grow within the gel structure. Motile species will be able to migrate, albeit slowly, throughout the gel and infiltration rates can then be visualized; whereas non-motile species will only show growth along then now-empty path introduced by the invasive initial sample deposition.
Another setup commonly used for measuring chemotaxis and chemokinesis utilizes the under-agarose cell migration assay whereby a layer of agarose gel is placed between a cell population and a chemoattractant. As a concentration gradient develops from the diffusion of the chemoattractant into the gel, various cell populations requiring different stimulation levels to migrate can then be visualized over time using microphotography as they tunnel upward through the gel against gravity along the gradient.
===Molecular biology===
Agar is a heterogeneous mixture of two classes of polysaccharide: '''agaropectin''' and '''agarose'''.<ref>[http://www.fao.org/docrep/field/003/AB730E/AB730E03.htm FAO agar manual]</ref> 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 >200 kilodaltons, or DNA fragments >100 basepairs. Agarose can be used for [[Electrophoresis|electrophoretic]] separation in [[agarose gel electrophoresis]] or for column-based [[Size exclusion chromatography|gel filtration chromatography]].
===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 and Skoog medium|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 cycle of an [[autoclave]].
This medium nicely lends itself to the application of specific concentrations of phytohormones etc. to induce specific growth patterns in that you can easily prepare a solution containing the desired amount of hormone, add it to the known volume of GM and autoclave to both sterilize and evaporate off any solvent you may have used to dissolve the often polar hormones in. This hormone/GM solution can be spread across the surface of petri dishes sown with germinated and/or etiolated seedlings.
===Culinary===
Agar-Agar is a natural vegetable [[gelatin]] counterpart originally eaten in Japan. White and semi-translucent, it is sold in packages as washed and dried strips or in powdered form. It can be used to make jellies, puddings and custards. For making jelly, it is boiled in water until the solids dissolve. One then adds sweetener, flavouring, colouring, fruit or vegetables, and pours the liquid into [[molding (process)|molds]] to be served as desserts and vegetable [[aspic]]s, or incorporated with other desserts, such as a jelly layer on a [[cake]].
Agar-agar is approximately 80% fiber, so it can serve as a great intestinal regulator. Its bulk quality is behind one of the latest fad diets in Asia, 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.{{Fact|date=March 2008}}
One use of agar in [[Japanese cuisine]] is [[anmitsu]], a dessert made of small cubes of agar jelly and served in a bowl with various fruits or other ingredients. In [[Indian cuisine]], agar agar is known as "China grass" and is used for making desserts. In [[Burmese cuisine]], a sweet jelly known as ''kyauk kyaw'' (ေကျာက်ေကြာ [tʃaoʔtʃau]) is made from agar.
===Other===
Agar is used as an [[Impression (Dental)|impression material]] in [[dentistry]]. It is also used to make salt bridges for use in [[electrochemistry]].
Agar is used in [[ant farm|formicariums]] as a transparent substitute for sand and a source of nutrition.
==Hysteresis==
[[Hysteresis]] describes the phenomenon of the differing liquid-solid state [[melting temperature|transition temperatures]] that agar exhibits. Agar melts at 85 °C (358 K, 185 °F) and solidifies from 32-40 °C. (305 - 313 K, 90-104 °F)
== See also ==
* [[Carrageenan]]
* [[Rhodophyta]]
* [[Asepsis]]
* [[Agarose gel electrophoresis]]
* [[Petri dish]]
* [[Robert Koch]]
* [[Walter Hesse]]
* [[R2a agar]]
==References==
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[[Category:Polysaccharides]]
[[Category:Edible thickening agents]]
[[Category:Microbiological gelling agent]]
[[Category:Dental materials]]
[[Category:Malay words and phrases]]
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