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Agar

[ah-gahr, ag-er]

Agar or agar agar is a gelatinous 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 to contain culture medium for microbiological work. The gelling agent is an unbranched polysaccharide obtained from the cell membranes of some species of red algae, primarily from the genuses 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 jellies, ice cream and Japanese desserts such as anmitsu, as a clarifying agent in brewing, and for paper sizing fabrics.

Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls.

Names

The word "agar" comes from the 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.

In Malay and 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 Mandarin Chinese as yángcài (洋菜) meaning "ocean vegetable" or "foreign vegetable." In 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, Apayao, Bikol, and Pangasinan, guraman in Ilokano and gurguraman in Sambali. In Thai it is known as wóon (วุ้น).

Usage

Microbiology

Culture Medium

Nutrient agar is used throughout the world to provide a solid surface containing 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 microbes 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 the 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. 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 electrophoretic separation in agarose gel electrophoresis or for column-based 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 & 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 molds to be served as desserts and vegetable aspics, 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.

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 material in dentistry. It is also used to make salt bridges for use in electrochemistry.

Agar is used in formicariums as a transparent substitute for sand and a source of nutrition.

Hysteresis

Hysteresis describes the phenomenon of the differing liquid-solid state 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)