Green tea-flavoredyōkan, a popular Japanesered bean jelly made from agarAblood agar plate used to culture bacteria and diagnose infection
Agar (/ˈeɪɡɑːr/ or/ˈɑːɡər/), oragar-agar, is ajelly-like substance consisting ofpolysaccharides obtained from thecell walls of some species ofred algae, primarily from theGracilaria genus (Irish moss, ogonori) and theGelidiaceae family (tengusa).[1][2] As found in nature, agar is a mixture of two components, the linear polysaccharideagarose and a heterogeneous mixture of smaller molecules calledagaropectin.[3] It forms the supporting structure in the cell walls of certain species of algae and is released on boiling. These algae are known asagarophytes, belonging to theRhodophyta (red algae) phylum.[4][5] The processing of food-grade agar removes the agaropectin, and the commercial product is essentially pure agarose.
The wordagar comes fromagar-agar, theMalay name for red algae (Gigartina,Eucheuma,[8]Gracilaria) from which the jelly is produced.[9][10] It is also known askanten (Japanese:寒天) (from the phrasekan-zarashitokoroten (寒晒し心太) or "cold-exposed agar"),Japanese isinglass,China grass,Ceylon moss, andJaffna moss.[11]Gracilaria edulis or its synonymG. lichenoides is specifically referred to asagal-agal orCeylon agar.[12]
Ogonori, the most common red algae used to make agar
Macroalgae have been used widely as food by coastal cultures, especially inSoutheast Asia.[13][14] In thePhilippines,Gracilaria, known asgulaman (alsoguraman,gar-garao, orgulaman dagat, among other names) inTagalog, have been harvested and used as food for centuries, eaten both fresh or sun-dried and turned into jellies. The earliest historical attestation is from theVocabulario de la lengua tagala (1754) by theJesuit priests Juan de Noceda and Pedro de Sanlucar, wheregolaman orgulaman was defined as"una yerva, de que se haze conserva a modo de Halea, naze en la mar" ("a herb, from which a jam-like preserve is made, grows in the sea"), with an additional entry forguinolaman to refer to food made with the jelly.[15][16][14]
Carrageenan, derived from gusô (Eucheuma spp.), which also congeals into a gel-like texture is also used similarly among theVisayan peoples and have been recorded in the even earlierDiccionario De La Lengua Bisaya, Hiligueina y Haraia de la isla de Panay y Sugbu y para las demas islas (c. 1637) of theAugustinian missionaryAlonso de Méntrida(in Spanish). In the book, Méntrida describes gusô as being cooked until it melts, and then allowed to congeal into a sour dish.[17]
InAmbon Island in theMaluku Islands ofIndonesia, agar is extracted fromGraciliaria and eaten as a type of pickle or a sauce.[14] Jelly seaweeds were also favoured and foraged byMalay communities living on the coasts of theRiau Archipelago andSingapore in Southeast Asia for centuries. 19th century records indicate that driedGraciliaria were one of the bulk exports ofBritish Malaya to China. Poultices made from agar were also used for swollen knee joints and sores inJohore and Singapore.[14][18]
The application of agar as a food additive in Japan is alleged to have been discovered in 1658 by Mino Tarōzaemon (美濃太郎左衞門), an innkeeper in currentFushimi-ku, Kyoto, who, according to legend, was said to have discarded surplus seaweed soup (Tokoroten) and noticed that it gelled later after a winter night's freezing.[19] However, agar actually solidifies at room temperature and does not need to be frozen.[20]
Agar was first subjected to chemical analysis in 1859 by the French chemistAnselme Payen, who had obtained agar from the marine algaeGelidium corneum.[21]
Beginning in the late 19th century, agar began to be used as a solid medium for growing various microbes. Agar was first described for use in microbiology in 1882 by the German microbiologistWalther Hesse, an assistant working inRobert Koch's laboratory, on the suggestion of his wifeFanny Hesse.[22][23] Agar quickly supplanted gelatin as the base of microbiological media, due to its higher melting temperature, allowing microbes to be grown at higher temperatures without the media liquefying.[24]
With its newfound use in microbiology, agar production quickly increased. This production centered on Japan, which produced most of the world's agar until World War II.[25] However, with the outbreak of World War II, many nations were forced to establish domestic agar industries in order to continue microbiological research.[25] Around the time of World War II, approximately 2,500 tons of agar were produced annually.[25] By the mid-1970s, production worldwide had increased dramatically to approximately 10,000 tons each year.[25] Since then, production of agar has fluctuated due to unstable and sometimes over-utilized seaweed populations.[26]
Agar consists of a mixture of twopolysaccharides: agarose and agaropectin, with agarose making up about 70% of the mixture, while agaropectin makes about 30% of it.[27] Agarose is a linear polymer, made up of repeating units ofagarobiose, a disaccharide made up ofD-galactose and 3,6-anhydro-L-galactopyranose.[28] Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts, and is made up of alternating units of D-galactose and L-galactose heavily modified with acidic side-groups, such assulfate,glucuronate, andpyruvate.[29][27][28]
Agar exhibits a phenomenon known ashysteresis whereby, when mixed with water, it solidifies and forms agel below about 32–42 °C (305–315 K; 90–108 °F), which is called thegel point, and melts above 85 °C (358 K; 185 °F), which is themelting point.[30]Hysteresis is the property of having a difference between the gel point and melting point temperatures.[31] This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures.[32] 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.[33]
Agar-agar is a natural vegetablegelatin counterpart.[34][35] It is white and semi-translucent when sold in packages as washed and dried strips or in powdered form.[34][36] It can be used to make jellies,[37]puddings, andcustards.[38] When making jelly, it is boiled in water until the solids dissolve. Sweetener, flavoring, coloring, fruits and or vegetables are then added, and the liquid is poured intomolds to be served as desserts and vegetableaspics or incorporated with other desserts such as a layer of jelly in acake.[36]
Agar-agar is approximately 80%dietary fiber, so it can serve as an intestinal regulator.[39] Its bulking quality has been behindfad diets in Asia, for example thekanten (the Japanese word for agar-agar[5]) diet. Once ingested,kanten triples in size and absorbs water. This results in the consumers feeling fuller.
Crema de fruta, a traditionalFilipino fruitcake, is made with an agar layer on top to keep the fruit components in place.
It can be used as addition to (or as a replacement for)pectin in jelly, jam, or marmalade, 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 aRussian dishptich'ye moloko (bird's milk), a rich jellified custard (or softmeringue) used as a cake filling or chocolate-glazed as individual sweets.
Agar-agar may also be used as the gelling agent in gel clarification, a culinary technique used to clarify stocks, sauces, and other liquids.Mexico has traditional candies made out of Agar gelatin, most of them in colorful, half-circle shapes that resemble amelon orwatermelon fruit slice, and commonly covered with sugar. They are known in Spanish asDulce de Agar (Agar sweets)
Agar-agar is an allowed nonorganic/nonsynthetic additive used as a thickener, gelling agent, texturizer, moisturizer, emulsifier, flavor enhancer, and absorbent in certified organic foods.[40]
100 mm (3.9 in) diameterPetri dishes containing agar gel for bacterial culture
An agar plate orPetri dish is used to provide agrowth medium using a mix of agar and other nutrients in which microorganisms, includingbacteria andfungi, can be cultured and observed under the microscope. Agar is indigestible for many organisms so that microbial growth does not affect the gel used and it remains stable. 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. Nutrients are typically added to meet the nutritional needs of themicrobes organism, the formulations of which may be "undefined" where the precise composition is unknown, or "defined" where the exact chemical composition is known. Agar is often dispensed using a sterilemedia dispenser.
Different algae produce various types of agar. Each agar has unique properties that suit different purposes. Because of the agarose component, the agar solidifies. When heated, agarose has the potential to melt and then solidify. Because of this property, they are referred to as "physical gels". In contrast,polyacrylamide polymerization is an irreversible process, and the resulting products are known as chemical gels.
There are a variety of different types of agar that support the growth of different microorganisms. A nutrient agar may be permissive, allowing for the cultivation of any non-fastidious microorganisms; a commonly used nutrient agar for bacteria is the Luria Bertani (LB) agar which containslysogeny broth, a nutrient-rich medium used for bacterial growth.[41] Additionally, 2216 Marine Broth (MB) agar, with high salt content, is optimized for growing heterotrophic marine bacteria like those of theVibrio genus, while Terrific Broth (TB) agar is used to non-selectively culture high yields of the bacteriumE. coli. More generally, enriched media is an agar variety that is infused with the necessary nutrients required by fastidious organisms to grow. Despite the large diversity of agar mediums, yeast extract is a common ingredient across all varieties as it is a macronutrient that provides a nitrogen source for all bacterial cell types.
Other fastidious organisms may require the addition of different biological fluids such as horse or sheep blood, serum, egg yolk, and so on.[42] Agar plates can also be selective, and can be used to promote the growth of bacteria of interest while inhibiting others. A variety of chemicals may be added to create an environment favourable for specific types of bacteria or bacteria with certain properties, but not conducive for growth of others. For example, antibiotics may be added in cloning experiments whereby bacteria with antibiotic-resistant plasmid are selected.[43] In addition to antibiotic treated agar, other selective and indicator agar plates include TCBS agar and MacConkey agar. Thiosulfate citrate bile salts sucrose (TCBS) agar is used to differentiate Vibrio species based on their sucrose metabolism, since only some will metabolize the sucrose in the plate and change its pH. Indicator dyes included in the gel will display a visual change of the pH by changing the gel color from green to yellow. MacConkey agar contains bile salts and crystal violet to selectively grow gram-negative bacteria and differentiate between species using pH-indicator dyes that demonstrate lactose metabolism properties.
As a gel, an agar or 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 measuringchemotaxis 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.
Research grade agar is used extensively inplant biology as it is optionally supplemented with a nutrient and/or vitamin mixture that allows for seedling germination in Petri dishes under sterile conditions (given that the seeds are sterilized as well). Nutrient and/or vitamin supplementation forArabidopsis thaliana is standard across most experimental conditions.Murashige & Skoog (MS) nutrient mix andGamborg'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.
When using agar, within any growth medium, it is important to know that the solidification of the agar is pH-dependent. The optimal range for solidification is between 5.4 and 5.7.[44] Usually, the application of potassium hydroxide 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 anautoclave.
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.
As a medium to precisely orient the tissue specimen and secure it byagar pre-embedding (especially useful for small endoscopy biopsy specimens) forhistopathology processing[46]
In the Arts, for example in "microbial art" in which agar acts as canvas, and microbes as a form of paint
Gelidium agar is used primarily for bacteriological plates. Gracilaria agar is used mainly in food applications.
In 2016, AMAM, a Japanese company, developed a prototype for agar-based commercialpackaging system called Agar Plasticity, intended as a replacement for oil-based plastic packaging.[50][51]
^Williams, Peter W.; Phillips, Glyn O. (2000)."2: Agar".Handbook of hydrocolloids. Cambridge, England: Woodhead. p. 91.ISBN1-85573-501-6.Agar is made from seaweed and it is attracted to bacteria.
^Balfour, Edward Green (1871)."agar".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. Scottish and Adelphi Presses. p. 50.
^Wilkinson, Richard James (1932)."agar".A Malay-English dictionary (romanised). Vol. I. Mytilene, Greece: Salavopoulos & Kinderlis. p. 9 – viaTROVE,National Library of Australia.
^Wells, Albert H. (1916). "Possibilities of Gulaman Dagat as a Substitute for Gelatin in Food".The Philippine Journal of Science.11:267–271.
^de Noceda, Juan; de Sanlucar, Pedro (1754).Vocabulario de la lengua Tagala. Imprenta de la compañia de Jesus. pp. 101, 215.
^de Mentrida, Alonso (1841).Diccionario De La Lengua Bisaya, Hiligueina Y Haraya de la isla de Panay. En La Imprenta De D. Manuel Y De D. Felis Dayot. p. 380.
^Zimbro, Mary Jo; Power, David A.; Miller, Sharon M.; Wilson, George E.; Johnson, Julie A. (eds.).Difco & BBL Manual(PDF) (2nd ed.). Becton Dickinson and Company. p. 6. Archived fromthe original(PDF) on 2012-06-06. Retrieved2013-07-17.
^Robert Koch (10 April 1882)"Die Aetiologie der Tuberculose" (The etiology of tuberculosis),Berliner Klinische Wochenschrift (Berlin Clinical Weekly),19 : 221–230. From p. 225:"Die Tuberkelbacillen lassen sich auch noch auf anderen Nährsubstraten kultiviren, wenn letztere ähnliche Eigenschaften wie das erstarrte Blutserum besitzen. So wachsen sie beispielsweise auf einer mit Agar-Agar bereiteten, bei Blutwärme hart bleibenden Gallerte, welche einen Zusatz von Fleischinfus und Pepton erhalten hat." (The tubercule bacilli can also be cultivated on other media, if the latter have properties similar to those of congealed blood serum. Thus they grow, for example, on a gelatinous mass which was prepared with agar-agar, which remains solid at blood temperature, and which has received a supplement of meat broth and peptone.)
^Kim, Se-Kwon (2011).Handbook of marine macroalgae: biotechnology and applied phycology (1st imp. ed.). Hoboken, NJ: John Wiley & Sons Inc.ISBN978-0-470-97918-1.
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