agar
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(ā'gär', ä'gär') 
also a·gar-a·gar (ā'gär-ā'gär', ä'gär-ä'-)
n.
also a·gar-a·gar (ā'gär-ā'gär', ä'gär-ä'-)n.
- A gelatinous material derived from certain marine algae. It is used as a base for bacterial culture media and as a stabilizer and thickener in many food products.
- A culture medium containing this material.
[Short for Malay agar-agar.]
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Agar-agar
agar-agar
A transparent gelatinous substance obtained from certain species of red seaweed. Agar-agar can replace gelatin. Agar-agar makes foods firmer and more difficult to melt than gelatin.
Preparing Agar-agar is used in bar form or in small pieces. It absorbs water, softens and swells. Bring the liquid containing the agar-agar to a boil and melt it over a gentle heat. The quantity of agar-agar needed varies depending on the firmness desired and the product it is mixed with (the thicker or more acid the liquid, the larger the amount required). About ¹∕3 oz (10 g) of agar-agar is needed per 4 cups (1 l) of liquid. A jelly forms when cooled.
Serving ideas Agar-agar is used to make jellies using juice or purées. It is used in the food industry as a stabilizer.
Nutritional Information
| dry | |
| protein | 6 g |
| fat | 0.3 g |
| carbohydrate | 81 g |
| calories | 306 |
| per 100 g |
Buying Agar-agar is bought as powder, flakes, bars or threads.
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A major constituent of the cell walls of certain red algae, especially members of the families Gelidiaceae and Gracilariaceae. Extracted for its gelling properties, it is one of three algal polysaccharides of major economic importance, the others being alginate and carrageenan. Agar is composed of two similar fractions, agarose and agaropectin, in which the basic unit is galactose, linked alternately α-1,3-(D-galactose) and β-1,4-(α-L-galactose).
Agar is prepared by boiling the algae in water, after which the filtered solution is cooled, purified, and dried. It is an amorphous, translucent material that is packaged in granules, flakes, bricks, or sheets. One of its chief uses is as a gelling agent in media for culturing microorganisms. It is also used in making confections, as an emulsifier in cosmetics and food products, as a sizing agent, as an inert carrier of drugs in medicine, and as a laxative. See also Culture; Rhodophyceae.
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Dried extracts from various seaweeds, including Gelidium and Gracilaria spp. It is a partially soluble non-starch polysaccharide, composed of galactose units. It swells with water to form a gel, and is used in soups, jellies, ice-cream, and meat products. It is also used as the basis of bacteriological culture media, as an adhesive, for sizing silk, and as a stabilizer for emulsions. Also called agar-agar, Macassar gum, vegetable gelatine.
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agar; agar-agar
[AH-gahr; AY-gahr] Also called kanten and Japanese gelatin, this tasteless dried seaweed acts as a setting agent and is widely used in Asia. It is marketed in the form of blocks, powder or brittle strands and is available at Asian markets and natural food stores. Agar can be substituted for gelatin but has stronger setting properties (about 5 times greater) so less of it is required. Unlike gelatin, agar will set at room temperature.
Columbia Encyclopedia:
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agar; agar-agar
agar (ä'gär, ā'-, ăg'är), product obtained from several species of red algae, or seaweed, chiefly from the Ceylon, or Jaffna, moss (Gracilaria lichenoides) and species of Gelidium, harvested in eastern Asia and California. 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 boiling water and cooled, agar becomes gelatinous; its chief uses are as a culture medium (particularly for bacteria) and as a laxative, but it serves also as a thickening for soups and sauces, in jellies and ice cream, in cosmetics, for clarifying beverages, and for sizing fabrics. See also Rhodophyta.
Biology Q&A:
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What is agar?
Agar is a polysaccharide extract of red algae that is used as a
solidifying material in microbiological media. Agar was developed as a culture
media for bacteria by Robert Koch (1843-1910). Koch was interested in the
isolation of bacteria in pure culture. Because isolation was difficult in
liquid media, he began to study ways in which bacteria could be grown on solid
media. After sterile, boiled potatoes proved unsatisfactory, a better
alternative was suggested by Fannie E. Hesse (1850-1934), the wife of
Walther Hesse (1846-1911), who was one of Koch's assistants. She
suggested that agar, which she had used to thicken sauces, jams, and jellies,
be used to solidify liquid nutrient broth. Agar is generally inexpensive and,
once jelled, does not melt until reaching a temperature of 212°F (100°C). If
1-2 g of agar are added to 100 ml of nutrient broth, it produces a solid medium
that is not degraded by most bacteria.
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Source: Red algae, including Gelidium cartilagineum (L.) Gaill., Gelidium amansii Lamour., Gracilaria confervoides (L.) Grev., other Gelidium and Gracilaria species as well as species of the genera Pterocladia, Ahnfeltia, Acanthopeltis, and Suhria.
Common/vernacular names: Agar-agar, gelosa, gelose, layor carang, and vegetable gelatin; also, Chinese gelatin, colle du Japon, Japanese gelatin, and Japanese isinglass.
Agar is the dried hydrophilic, colloidal extract of various red algae (Class Rhodophyceae); the more commonly used red algae are Gelidium cartilagineum (L.) Gaill., Gelidium amansii Lamour., Gracilaria confervoides (L.) Grev., other Gelidium and Gracilaria species as well as species of the genera Pterocladia, Ahnfeltia, Acanthopeltis, and Suhria. Agar is extracted from the algae by boiling them in water at a neutral or slightly acidic pH. The hot liquor is filtered and on cooling forms a gel that is purified by freezing and thawing followed by drying.
The major agar producer has been and still is Japan. Other producing countries include the United States, Spain, Portugal, Chile, Taiwan, Korea, Morocco, New Zealand, Australia, Argentina, and Mexico.
Agar is insoluble in cold water but readily soluble up to 5% in boiling water. The solution (sol) on cooling to 35–40°C forms a firm, resilient gel that does not melt below 85°C. This ability to gel at a much lower temperature than the melting temperature of the gel, commonly called hysteresis lag, is uniquely long in agar, and many of its uses depend on this property. Agar gels also have the property of shrinking and exuding water from their surface (syneresis), particularly when broken. The gel strength of agar can be increased by addition of dextrose, sucrose, and locust bean gum, while it tends to weaken with gelatin, algin, starch, and karaya gum. The colorless, tasteless powder can absorb up to 200 times its volume of water when forming a gel.
Agar solutions have low viscosity; their degree of clarity and color (yellowish to colorless) depend on the quality and source of the agar, as do their gel strength, gelling temperature, and the degree of syneresis. Quality is largely affected by extraction procedures. Physical and rheological properties of agar that provide the greatest determinations of quality are the average molecular weight and molecular weight distribution.
Agar is insoluble in organic solvents and is precipitated from aqueous solution by alcohol and tannin.
| Aconite | |
| Alder Buckthorn |
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or agar-agar
a complex sulfated galactan extracted from certain seaweeds, especially Gelidium and related genera. The two main components are agarose and agaropectin. Agar forms an aqueous gel suitable for the solidification of microbiological culture media and for use as a support medium in zone electrophoresis or (immuno)diffusion techniques. It is not metabolized by most organisms. The gelling temperature varies from about 25 to 35 °C for different types of preparation. The gel is then stable to about 90 °C.
a complex sulfated galactan extracted from certain seaweeds, especially Gelidium and related genera. The two main components are agarose and agaropectin. Agar forms an aqueous gel suitable for the solidification of microbiological culture media and for use as a support medium in zone electrophoresis or (immuno)diffusion techniques. It is not metabolized by most organisms. The gelling temperature varies from about 25 to 35 °C for different types of preparation. The gel is then stable to about 90 °C.
| agamogeny, agammaglobulinemia, aflatoxin | |
| agar-diffusion method, agarobiose, agaropectin |
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A dried hydrophilic, colloidal substance extracted from various species of red algae. When suspended in a liquid medium and heated to 212°F (100°C), the agar dissolves. When it is allowed to cool to 110°F (43°C) the medium becomes a solid gel. It is used in culture media for bacteria and other microorganisms, in making emulsions, and as a supporting medium for immunodiffusion and immunoelectrophoresis. Because of its bulk it is also used in medicines to promote peristalsis and relieve constipation.
- birdseed a. — one containing Guizotia abyssinicia (Niger) seed and creatinine, used for growing Cryptococcus neoformans. Called also Niger agar.
- bismuth sulfite a. — a special preparation used for isolation of salmonellae from food.
- blood a. — a culture medium used for the growth of bacteria. Consists of agar and intact erythrocytes.
- brain heart infusion a. — used for cultivating the yeast phase of dimorphic fungi.
- brilliant green a. — used to cultivate salmonellae.
- chocolate a. — an enriched agar for the growth of Hemophilus, some Actinobacillus, and Taylorella spp. A molten agar and blood mixture is held at 122°F (50°C) prior to pouring plates. The additional nutrients supplied are hemin and NAD.
- a. diffusion test — see antimicrobial sensitivity test.
- eosin-methylene blue (EMB) a. — used for the identification of Eschericha coli.
- a. gel immunodiffusion test — see immunodiffusion tests.
- MacConkey a. — contains bile salts, lactose and neutral red indicator for isolation of enterobacteria.
- mannitol salt a. — selective for staphylococci.
- milk a. — contains skim milk and used to demonstrate casein digestion.
- Niger seed a. — see birdseed agar (above).
- nutrient a. — the basic growth medium for bacteria, composed of beef extract and peptone.
- potato dextrose a. — used in cultivating fungi; promotes sporulation and pigmentation.
- Sabouraud's dextrose a. — one used for isolation of fungi. See also dermatophyte test medium.
- a. sausage — see medium sausage.
- xylose lysine (XLD) a. — used to differentiate Enterbacteriaceae.
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categories related to 'agar'
- Tools and Techniques - agar: culture medium for growing bacteria
- PHARMACOLOGY - agar: polysaccharide derived from seaweed, used as culture medium for microorganisms; gelatinous natural laxative
- Additives and Thickening and Leavening Agents
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For other uses, see Agar (disambiguation).
Culinary usage
Mizuyōkan - a popular Japanese red bean jelly made from agar.
Scientific usage
A blood agar plate used to culture bacteria and diagnose infection.
Agar or agar-agar is a gelatinous substance derived by boiling[1] from a polysaccharide in red algae, where it accumulates in the cell walls of agarophyte and serve as the primary structural support for the algae's cell walls.[2][3] Agar is a mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin.
Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work. Agar (agar-agar) can be used as a laxative, a vegetarian gelatin substitute, a thickener for soups, in fruit preserves, ice cream, and other desserts, as a clarifying agent in brewing, and for sizing paper and fabrics.
The gelling agent is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the genera Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema). For commercial purposes, it is derived primarily from Gelidium amansii. In chemical terms, agar is a polymer made up of subunits of the sugar galactose.
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Contents
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Background
Agar consists of a mixture of agarose and agaropectin. Agarose, the predominant component of agar, is a linear polymer, made up of the repeating monomeric unit of agarobiose. Agarobiose is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts.[4]
Agar exhibits hysteresis, melting at 85 °C (358 K, 185 °F) and solidifying from 32-40 °C (305-313 K, 90-104 °F).[5] This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures. Since many scientific applications require incubation at temperatures close to human body temperature (37 °C), agar is more appropriate than other solidifying agents that melt at this temperature, such as gelatin.
The word "agar" comes from agar-agar, the Malay name for red algaes (Gigartina, Gracilaria) from which the jelly is produced.[6] It is also known as kanten, China grass,Japanese isinglass, Ceylon moss or Jaffna moss.[7] Gracilaria lichenoides is specifically referred to as agal-agal or Ceylon agar.[8]
Uses
Microbiology
Main article: Agar plate
Agar is used throughout the world to provide a solid surface containing medium for the growth of bacteria and fungi. Microbial growth does not destroy the gel structure because most microorganisms are unable to digest agar. Agar is typically sold commercially as a powder that can be mixed with water and prepared similarly to gelatin before use as a growth medium. Other ingredients are added to the agar to meet the nutritional needs of the microbes. Many specific formulations 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 show growth only 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.
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 in general are used. A 1.0% agar/0.44% MS+vitamin dH2O 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 one 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 that may have been used to dissolve the often-polar hormones. This hormone/GM solution can be spread across the surface of Petri dishes sown with germinated and/or etiolated seedlings.
Experiments with the moss Physcomitrella patens, however, have shown that choice of the gelling agent — agar or Gelrite - does influence phytohormone sensitivity of the plant cell culture.[9]
Molecular biology
Main article: Gel electrophoresis
Agar is a heterogeneous mixture of two classes of polysaccharide: agaropectin and agarose.[10] 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, and, therefore, agarose has become the preferred matrix for work with proteins and nucleic acids. Gels made from purified agarose have a relatively large pore size, making them useful for separation of large molecules, such as proteins and protein complexes >200 kilodaltons, as well as DNA fragments >100 basepairs. Agarose has been used widely for immunodiffusion and immunoelectrophoresis, as the agarose fibers functions as an anchor for immunocomplexes. Agarose is used generally as the medium for analytical scale electrophoretic separation in agarose gel electrophoresis and for column-based preparative scale separation as in gel filtration chromatography and affinity chromatography.
Culinary
Agar-agar is a natural vegetable gelatin counterpart. 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. Sweetener, flavouring, colouring, fruit or vegetables are then added and the liquid is poured into molds to be served as desserts and vegetable aspics, or incorporated with other desserts, such as a jelly layer in a cake.
Agar-agar is approximately 80% fiber, so it can serve as an intestinal regulator. Its bulk quality is behind one of the latest fad diets in Asia, the kanten (the Japanese word for agar-agar[2]) diet. Once ingested, kanten triples in size and absorbs water. This results in the consumer's feeling more full. This diet has recently received some press coverage in the United States as well. The diet has shown promise in obesity studies.[11]
In Philippine cuisine, it is used to make the jelly bars in the various gulaman refreshments or desserts such as sago gulaman (aka gulaman at sago), buko pandan, agar flan, halo-halo, the various Filipino fruit salads, black gulaman, and red gulaman. 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. It is also the main ingredient in Mizuyōkan, another popular Japanese food. (See very top image.) In Vietnamese cuisine, jellies made of flavored layers of agar agar, called thạch, are a popular dessert, and are often made in ornate molds for special occasions. 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 [tɕaʊʔtɕɔ́]) is made from agar. In Russia, it is used in addition or as a replacement to pectin in jams and marmalades, as a substitute to gelatin for its superior gelling properties, and as a strengthening ingredient in souffles and custards. Another use of agar-agar is in Ptich'ye moloko (Bird's Milk), a rich gellied custard (or soft meringue) used as a cake filling or chocolate-glazed as individual sweets.
Other uses
Agar is used:
- As an impression material in dentistry.
- To make salt bridges for use in electrochemistry.
- In formicariums as a transparent substitute for sand and a source of nutrition.
- By Kanten Clay Studio as a natural ingredient to form modelling clay for young children to play with.
See also
References
- ^ Cyclopædia of India and of eastern and southern Asia, commercial ..., Volume 2 (1871), edited by Edward Balfour
- ^ a b Davidson, Alan, and Tom Jaine. The Oxford companion to food. Oxford University Press, USA, 2006. 805. Print. Retrieved Aug. 08, 2010, from [1]
- ^ Williams, Peter W.; Phillips, Glyn O. (2000). Handbook of hydrocolloids. Cambridge: Woodhead. ISBN 1-85573-501-6.
- ^ Agar at lsbu.ac.uk Water Structure and Science
- ^ "All About Agar". Sciencebuddies.org. Archived from the original on 3 June 2011. http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_Agar.shtml. Retrieved 2011-04-27.
- ^ Balfour, Edward. (1885). The cyclopædia of India and of eastern and southern Asia: commercial, industrial and scientific, products of the mineral, vegetable, and animal kingdoms, useful arts and manufactures. B. Quaritch. p. 71.
- ^ Agar-Agar at Agar-Agar.org
- ^ Agar-Agar at Botanical.com
- ^ Birgit Hadeler, Sirkka Scholz, Ralf Reski. "Gelrite and agar differently influence cytokinin-sensitivity of a moss". Journal of Plant Physiology 146: 369–371.
- ^ "FAO agar manual". Fao.org. 1965-01-01. http://www.fao.org/docrep/field/003/AB730E/AB730E03.htm. Retrieved 2011-04-27.
- ^ Maeda H, Yamamoto R, Hirao K, Tochikubo O (January 2005). "Effects of agar (kanten) diet on obese patients with impaired glucose tolerance and type 2 diabetes". Diabetes, Obesity, and Metabolism 7 (1): 40–6. doi:10.1111/j.1463-1326.2004.00370.x. PMID 15642074.
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