View source for Agar

{{short description|Thickening agent used in microbiology and food}}
{{Other uses}}
[[File:GreenTeaYokan.jpg|thumb|Green tea-flavored [[yōkan]], a popular Japanese [[Red bean paste|red bean]] jelly made from agar]]
[[File:Agarplate redbloodcells edit.jpg|thumb|A [[blood agar]] plate used to culture bacteria and diagnose infection]]

'''Agar''' ({{IPAc-en|ˈ|eɪ|ɡ|ɑːr}} or {{IPAc-en|ˈ|ɑː|g|ər}}), or '''agar-agar''', is a [[Gelling agent|jelly-like substance]] consisting of [[polysaccharides]] obtained from the [[cell wall]]s of some species of [[red algae]], primarily from the ''[[Gracilaria]]'' genus (Irish moss, ogonori) and the [[Gelidiaceae]] family (tengusa).<ref>{{cite web |last=Shimamura |first=Natsu |date=August 4, 2010 |title=Agar |website=The Tokyo Foundation |url=http://www.tokyofoundation.org/en/topics/japanese-traditional-foods/vol.-4-agar |access-date=19 December 2016}}</ref><ref>{{cite book |year=2005 |title=Oxford Dictionary of English |edition=2nd |title-link=Oxford Dictionary of English}}</ref> As found in nature, agar is a mixture of two components, the linear polysaccharide [[agarose]] and a heterogeneous mixture of smaller molecules called [[agaropectin]].<ref name=Williams2000>{{cite book |last1=Williams |first1=Peter W. |last2=Phillips |first2=Glyn O. |year=2000 |title=Handbook of hydrocolloids |chapter=2: Agar |chapter-url=https://books.google.com/books?id=dXS7qnh-ZOEC&pg=PA28 |publisher=Woodhead |location=Cambridge, England |isbn=1-85573-501-6 |page=91 |quote=Agar is made from seaweed and it is attracted to bacteria.}}</ref>  It forms the supporting structure in the cell walls of certain species of algae and is released on boiling. These algae are known as [[agarophyte]]s, belonging to the [[Rhodophyta]] (red algae) phylum.<ref>{{cite book |last=Balfour |first=Edward Green |author-link=Edward Balfour |year=1871 |title=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 |chapter=agar |chapter-url=https://archive.org/details/cyclopdiaindiaa00unkngoog/page/n63 | publisher=Scottish and Adelphi Presses |page=50}}</ref><ref name=oxford>{{cite book |last=Davidson |first=Alan |date=2006 |title=The Oxford Companion to Food |publisher=Oxford University Press |isbn=978-0-19-280681-9 |url=https://books.google.com/books?id=JTr-ouCbL2AC}}</ref>  The processing of food-grade agar removes the agaropectin, and the commercial product is essentially pure agarose.

Agar has been used as an ingredient in [[dessert]]s throughout Asia and also as a solid [[Substrate (biology)|substrate]] to contain [[Growth medium|culture media]] for [[Microbiology|microbiological]] work. Agar can be used as a [[laxative]]; an [[appetite suppressant]]; a [[vegan]] substitute for [[gelatin]]; a thickener for [[soup]]s; in [[Fruit preserves#Jelly|fruit preserves]], ice cream, and other desserts; as a clarifying agent in [[brewing]]; and for [[sizing]] paper and fabrics.<ref>{{cite web |title=Showing Food Agar |website=The Metabolomics Innovation Centre (TMIC) |url=https://foodb.ca/foods/FOOD00284 |url-status=live |archive-url=https://web.archive.org/web/20210509052540/https://foodb.ca/foods/FOOD00284 |archive-date=2021-05-09}}</ref><ref>{{cite book |last=Balfour |first=Edward Green |year=1857 |title=Cyclopaedia of India and of Eastern and Southern Asia, commercial, industrial and scientific... |publisher=Scottish Press |page=13 |url=https://archive.org/details/b28708921_0002/page/13}}</ref>

== Etymology ==
The word ''agar'' comes from '''agar-agar''', the [[Malay language|Malay]] name for red algae (''[[Gigartina]]'', ''[[Eucheuma]]'',<ref>{{cite book |last1=Chapman |first1=V. J. |last2=Chapman |first2=D.J. |date=1980 |title=Seaweeds and their Uses |edition=third |publisher=Springer Netherlands |location=Dordrecht |isbn=978-94-009-5808-1 |page=148}}</ref> ''[[Gracilaria]]'') from which the jelly is produced.<ref>{{cite book |last=Balfour |first=Edward |year=1885 |title=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 |publisher=B. Quaritch |page=71 |url=https://archive.org/details/in.ernet.dli.2015.283118/page/n76}}</ref><ref name=Wilkinson>{{cite encyclopedia |last=Wilkinson |first=Richard James |date=1932 |title=agar |dictionary=A Malay-English dictionary (romanised) |publisher=Salavopoulos & Kinderlis |location=Mytilene, Greece |volume=I |page=9 |via=[[Trove|TROVE]], [[National Library of Australia]] |url=https://nla.gov.au/nla.obj-60272771/view?partId=nla.obj-435838799#page/n18/}}</ref> It is also known as '''kanten''' ({{langx|ja|寒天}}) (from the phrase '''''kan'''-zarashi [[Tokoroten|tokoro'''ten''']]'' ({{lang|ja|寒晒し心太}}) or "cold-exposed agar"), '''Japanese isinglass''', '''China grass''', '''Ceylon moss''', and '''Jaffna moss'''.<ref>[http://www.agar-agar.org/en/ Agar-Agar] {{Webarchive|url=https://web.archive.org/web/20110903170258/http://www.agar-agar.org/en/ |date=2011-09-03 }} at Agar-Agar.org</ref> ''[[Gracilaria edulis]]'' or its synonym ''G.&nbsp;lichenoides'' is specifically referred to as '''agal-agal''' or '''Ceylon agar'''.<ref>{{cite web |title=Agar-Agar |publisher=Botanical.com |url=http://www.botanical.com/botanical/mgmh/a/agara012.html |access-date=22 January 2017}}</ref>

== History ==
{{See also|Edible seaweed|Seaweed farming}}
[[File:Ogo.jpg|thumb|upright|[[Ogonori]], the most common red algae used to make agar]]
Macroalgae have been used widely as food by coastal cultures, especially in [[Southeast Asia]].<ref>{{cite book |last=Hopley |first=David |year=2010 |title=Encyclopedia of Modern Coral Reefs: Structure, Form and Process |publisher=Springer Science & Business Media |isbn=978-90-481-2638-5 |page=31 |url=https://books.google.com/books?id=5umXDDmqxwIC&q=agar+gelling+southeast+asia+dessert&pg=PA31}}</ref><ref name="Zaneveld"/> In the [[Philippines]], ''[[Gracilaria]]'', known as ''[[gulaman]]'' (also ''guraman'', ''gar-garao'', or ''gulaman dagat'', among other names) in [[Tagalog language|Tagalog]], 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 the ''[[Vocabulario de la lengua tagala]]'' (1754) by the [[Jesuit]] priests Juan de Noceda and Pedro de Sanlucar, where ''golaman'' or ''gulaman'' 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 for ''guinolaman'' to refer to food made with the jelly.<ref>{{cite journal |last=Wells |first=Albert H. |date=1916 |title=Possibilities of Gulaman Dagat as a Substitute for Gelatin in Food |journal=The Philippine Journal of Science |volume=11 |pages=267–271}}</ref><ref>{{cite book |last1=de Noceda |first1=Juan |last2=de Sanlucar |first2=Pedro |date=1754 |title=Vocabulario de la lengua Tagala |publisher=Imprenta de la compañia de Jesus |pages=101,215}}</ref><ref name=Zaneveld/>

[[Carrageenan]], derived from gusô (''[[Eucheuma]]'' spp.), which also congeals into a gel-like texture is also used similarly among the [[Visayan peoples]] and have been recorded in the even earlier ''Diccionario De La Lengua Bisaya, Hiligueina y Haraia de la isla de Panay y Sugbu y para las demas islas'' ({{Circa|1637}}) of the [[Augustinians|Augustinian]] missionary [[:es:Alonso de Méntrida|Alonso de Méntrida]] {{In lang|es}}. In the book, Méntrida describes gusô as being cooked until it melts, and then allowed to congeal into a sour dish.<ref>{{cite book |last1=de Mentrida |first1=Alonso |date=1841 |title=Diccionario De La Lengua Bisaya, Hiligueina Y Haraya de la isla de Panay |publisher=En La Imprenta De D. Manuel Y De D. Felis Dayot |page=380}}</ref>

In [[Ambon Island]] in the [[Maluku Islands]] of [[Indonesia]], agar is extracted from ''Graciliaria'' and eaten as a type of pickle or a sauce.<ref name="Zaneveld"/> Jelly seaweeds were also favoured and foraged by [[Malays (ethnic group)|Malay]] communities living on the coasts of the [[Riau Archipelago]] and [[Singapore]] in Southeast Asia for centuries. 19th century records indicate that dried ''Graciliaria'' were one of the bulk exports of [[British Malaya]] to China. Poultices made from agar were also used for swollen knee joints and sores in [[Johore]] and Singapore.<ref name=Zaneveld>{{cite journal |last=Zaneveld |first=Jacques S. |date=1959 |title=The Utilization of Marine Algae in Tropical South and East Asia |journal=Economic Botany |doi=10.1007/BF02859244 |jstor=4288011 |bibcode=1959EcBot..13...89Z |volume=13 |issue=2 |pages=89–131 }}</ref><ref>{{cite magazine |last=Johari |first=Khir |date=Oct–Dec 2021 |title=The Role of Foraging in Malay Cuisine |magazine=BiblioAsia |publisher=[[National Library Board]], Singapore |volume=17 |issue=3 |pages=20–23 |url=https://biblioasia.nlb.gov.sg/vol-17/issue-3/oct-dec-2021/}}</ref>

The application of agar as a food additive in Japan is alleged to have been discovered in 1658 by Mino Tarōzaemon ({{lang|ja|{{linktext|美濃}} {{linktext|太郎}}{{linktext|左|衞|門}}}}), an innkeeper in current [[Fushimi-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.<ref name=DifcoMan>{{cite book |editor1-last=Zimbro |editor1-first=Mary Jo |editor2-last=Power |editor2-first=David A. |editor3-last=Miller |editor3-first=Sharon M. |editor4-last=Wilson |editor4-first=George E. |editor5-last=Johnson |editor5-first=Julie A. |title=Difco & BBL Manual |edition=2nd |publisher=Becton Dickinson and Company |page=6 |url=http://www.bd.com/ds/technicalCenter/misc/difcobblmanual_2nded_lowres.pdf |access-date=2013-07-17 |archive-url=https://web.archive.org/web/20120606174455/http://www.bd.com/ds/technicalCenter/misc/difcobblmanual_2nded_lowres.pdf |archive-date=2012-06-06}}</ref> However, agar actually solidifies at room temperature and does not need to be frozen.<ref>{{cite web |title=All About Agar |url=https://www.sciencebuddies.org/science-fair-projects/references/grow-microbes-agar |website=Science Buddies |access-date=8 July 2025}}</ref>

Agar was first subjected to chemical analysis in 1859 by the French chemist [[Anselme Payen]], who had obtained agar from the marine algae ''Gelidium corneum''.<ref>Payen, Anselme (1859) [http://gallica.bnf.fr/ark:/12148/bpt6k3006f/f523.image.langEN "Sur la gélose et le nids de salangane"] (On agar and swiftlet nests), ''Comptes rendus'' ..., '''49''' : 521–530, appended remarks 530–532.</ref>

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 microbiologist [[Walther Hesse]], an assistant working in [[Robert Koch|Robert Koch's]] laboratory, on the suggestion of his wife [[Fanny Hesse]].<ref>Robert Koch (10 April 1882) [http://babel.hathitrust.org/cgi/pt?id=mdp.39015020075001;view=1up;seq=235 "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.)</ref><ref name=ln>{{cite magazine |last=Smith |first=A. |date=November 1, 2005 |title=History of the Agar Plate |magazine=Laboratory News |url=http://www.labnews.co.uk/features/history-of-the-agar-plate/ |access-date=November 3, 2012 |archive-url=https://web.archive.org/web/20121014060019/http://www.labnews.co.uk/features/history-of-the-agar-plate/ |archive-date=October 14, 2012}}</ref> 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.<ref name=hesse>{{cite journal |last=Hesse |first=W. |translator=Gröschel, D.H.M. |year=1992 |title=Walther and Angelina Hesse–Early Contributors to Bacteriology |journal=ASM News |volume=58 |issue=8 |pages=425–428 |url=http://www.asm.org/ccLibraryFiles/FILENAME/0000000227/580892p425.pdf |access-date=22 January 2017 |archive-url=https://web.archive.org/web/20170630173511/https://www.asm.org/ccLibraryFiles/FILENAME/0000000227/580892p425.pdf |archive-date=30 June 2017}}</ref>

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.<ref name=Loban>{{cite book |last1=Lobban |first1=Christopher S. |last2=Wynne |first2=Michael James |year=1981 |title=The Biology of Seaweeds |publisher=University of California Press |isbn=978-0-520-04585-9 |url=https://books.google.com/books?id=QG4tqjFPWJ0C&q=agar+gelling+malay+japan&pg=PA734 |pages=734–735}}</ref> However, with the outbreak of World War II, many nations were forced to establish domestic agar industries in order to continue microbiological research.<ref name=Loban/> Around the time of World War II, approximately 2,500 tons of agar were produced annually.<ref name=Loban/> By the mid-1970s, production worldwide had increased dramatically to approximately 10,000 tons each year.<ref name=Loban/> Since then, production of agar has fluctuated due to unstable and sometimes over-utilized seaweed populations.<ref>{{cite journal |last=Callaway |first=Ewen |date=8 December 2015 |title=Lab staple agar hit by seaweed shortage |journal=Nature |publisher=Nature News |doi=10.1038/528171a |doi-access=free |pmid=26659158 |bibcode=2015Natur.528..171C |volume=528 |issue=7581 |pages=171–172}}</ref>

== Chemical composition ==
[[File:Agarose polymere.svg|right|thumb|The structure of an [[agarose]] polymer]]
Agar consists of a mixture of two [[polysaccharide]]s: agarose and agaropectin, with agarose making up about 70% of the mixture, while agaropectin makes about 30% of it.<ref name=FAO3/> Agarose is a linear polymer, made up of repeating units of [[Agarose#Structure|agarobiose]], a disaccharide made up of [[D-galactose]] and 3,6-anhydro-L-galactopyranose.<ref name=FAO1/> 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 as [[sulfate]], [[glucuronate]], and [[pyruvate]].<ref>{{cite web |access-date=2023-03-21 |url=https://water.lsbu.ac.uk/water/agar.html |title=Agar |archive-url=https://web.archive.org/web/20220926200632/https://water.lsbu.ac.uk/water/agar.html |archive-date=2022-09-26 |website=London South Bank University}}</ref><ref name=FAO3>{{cite book|chapter-url=http://www.fao.org/docrep/field/003/AB730E/AB730E03.htm |title=Training manual on Gracilaria culture and seaweed processing in China |chapter=III: Properties, Manufacture, and Application of Seaweed Polysaccharides – Agar, Carageenan, and Algin |publisher=Food and Agriculture Organization, United Nations|date=August 1990 |access-date=2011-04-27}}</ref><ref name=FAO1>{{cite book|chapter-url=http://www.fao.org/docrep/x5822e/x5822e03.htm |title=Production and Utilization of Products from Commercial Seaweeds|chapter=Chapter 1 – Production, Properties and Uses of Agar |author1=Rafael Armisen |author2=Fernando Galatas |publisher=Food and Agriculture Organization, United Nations |date=1987 |editor=McHugh DJ |isbn=92-5-102612-2}}</ref>

== Physical properties ==
Agar exhibits a phenomenon known as [[hysteresis#Liquid–solid-phase transitions|hysteresis]] whereby, when mixed with water, it solidifies and forms a [[gel]] below about {{convert|32–42|C|K F}}, which is called the [[gel point]], and melts above {{convert|85|C|K F}}, which is the [[melting point]].<ref>{{cite web|url=https://www.sciencebuddies.org/science-fair-projects/references/grow-microbes-agar |title=Agar and Its Use in Chemistry and Science |last1=Liu |first1=Shijun |last2=Usinger |first2=Laurie |date=2008<!-- latest source access date --> |website=Science Buddies |access-date=21 March 2023 |archive-url= https://web.archive.org/web/20110603081846/http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_Agar.shtml| archive-date= 3 June 2011 | url-status= live}}</ref> [[Hysteresis]] is the property of having a difference between the gel point and melting point temperatures.<ref>{{Cite web |title=Hispanagar {{!}} Hysteresis, what is this peculiar word? |url=https://www.hispanagar.com/en/hysteresis-what-peculiar-word#:~:text=Hysteresis%20is,before. |access-date=2023-03-21 |website=www.hispanagar.com}}</ref> This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures.<ref>{{Cite journal |last1=Das |first1=N. |last2=Tripathi |first2=N. |last3=Basu |first3=S. |last4=Bose |first4=C. |last5=Maitra |first5=S. |last6=Khurana |first6=S. |date=2015-07-23 |title=Progress in the development of gelling agents for improved culturability of microorganisms |journal=[[Frontiers in Microbiology]] |volume=6 |issue=698 |page=698 |doi=10.3389/fmicb.2015.00698 |pmid=26257708 |pmc=4511835 |doi-access=free }}</ref> Since many scientific applications require incubation at temperatures close to human body temperature (37&nbsp;°C), agar is more appropriate than other solidifying agents that melt at this temperature, such as gelatin.<ref>{{Cite web |title=Agar-Agar: Definition, Producing and Uses |url=https://microbiologie-clinique.com/laboratory-agar.html#:~:text=Agar-Agar%20quickly%20supplanted,liquefied. |access-date=2023-03-21 |website=microbiologie-clinique.com}}</ref>

==Uses==

=== Culinary ===
[[File:Sago Gulaman.jpg|thumb|''[[Sago]] at [[gulaman]]'' in [[Filipino cuisine]] is made from agar (''[[gulaman]]''), pearl [[sago]], and sugar syrup flavored with [[Pandanus amaryllifolius|pandan]].|210x210px]]
Agar-agar is a natural vegetable [[gelatin]] counterpart.<ref name=":0">{{Cite book |last=Wings of Success |title=The Advantages of Being a Vegetarian: Selected Tips |pages=9–10}}</ref><ref>{{Cite book |last=Livlaid |first=Nele |title=Plant-Based Made Easy: The Complete Practical Guide to Transitioning to Healthy Whole Food Diet |publisher=Nutriplanet (Swing & Step OU) |year=2018 |isbn=978-9949-88-246-5}}</ref> It is white and semi-[[translucent]] when sold in packages as washed and dried strips or in powdered form.<ref name=":0" /><ref name=":1">{{Cite book |last=Stobart |first=Tom |title=Cook's Encyclopaedia |publisher=Grub Street Publishing |year=2016 |isbn=978-1-910690-83-3}}</ref> It can be used to make jellies,<ref>{{Cite book |last1=Zhang |first1=Louisa |title=Home Economics S2 Tb (nt) |last2=Seng |first2=Teo Kiok |last3=Hixson |first3=Sue |last4=Kwone |first4=Eileen |publisher=Longman |year=2022 |isbn=978-981-4079-47-1 |location=Singapore |pages=145}}</ref> [[puddings]], and [[custard]]s.<ref>{{Cite book |last=Ash |first=Michael |title=Handbook fo Fillers, Extenders, and Diluents |publisher=Synapse Information Resources |year=2007 |isbn=978-1-890595-96-8 |pages=233}}</ref> 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 into [[molding (process)|molds]] to be served as desserts and vegetable [[aspic]]s or incorporated with other desserts such as a layer of jelly in a [[cake]].<ref name=":1" />

Agar-agar is approximately 80% [[dietary fiber]], so it can serve as an intestinal regulator.<ref>{{Cite journal |last=Marden |first=Orison Swett |date=1921 |title=Constipation is a Crime |journal=The New Success: Marden's Magazine |publisher=Lowrey-Marden |volume=5 |page=113}}</ref> Its bulking quality has been behind [[fad diet]]s in Asia, for example the ''kanten'' (the Japanese word for agar-agar<ref name="oxford" />) diet. Once ingested, ''kanten'' triples in size and absorbs water. This results in the consumers feeling fuller.

==== Asian culinary ====
One use of agar in [[Japanese cuisine]] is in ''[[anmitsu]]'', a dessert made of small cubes of agar jelly and served in a bowl with various fruits or other ingredients.{{citation needed|date=April 2024}} It is also the main ingredient in {{Lang|ja-Latn|mizu yōkan}}, another popular Japanese food.{{citation needed|date=April 2024}} In [[Philippine cuisine]], it is used to make the jelly bars in the various [[gulaman]] refreshments like ''[[sago't gulaman]]'', ''[[samalamig]]'', or desserts such as ''[[buko pandan]]'', ''agar flan'', ''[[halo-halo]]'', ''fruit cocktail jelly'', and the black and red ''gulaman'' used in various fruit salads.{{citation needed|date=April 2024}} 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.{{citation needed|date=April 2024}} In [[Indian cuisine]], agar is used for making desserts.{{citation needed|date=April 2024}} In [[Burmese cuisine]], a sweet jelly known as ''kyauk kyaw'' is made from agar.{{citation needed|date=April 2024}} Agar jelly is widely used in Taiwanese [[bubble tea]].{{citation needed|date=April 2024}}

==== Other culinary ====
[[File:00114jfCuisine of Bulacan Food Cakes Delicaciesfvf 30.jpg|thumb|''[[Crema de fruta]]'', a traditional [[Filipino cuisine|Filipino]] 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 a [[Russian cuisine|Russian]] dish ''ptich'ye moloko'' ([[bird's milk]]), a rich jellified custard (or soft [[meringue]]) 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 a [[melon]] or [[watermelon]] fruit slice, and commonly covered with sugar. They are known in Spanish as ''Dulce 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.<ref>[http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5096516 Agar-agar Review Sheet] {{Webarchive|url=https://web.archive.org/web/20150620112844/http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5096516 |date=2015-06-20 }}, USDA Organic Materials Review, April 1995.</ref>

=== Microbiology ===
==== Agar plate ====
{{Main|Agar plate}}
[[Image:Agar Plate.jpg|right|thumb|{{cvt|100|mm|adj=on}} diameter [[Petri dish]]es containing agar gel for bacterial culture]]

An agar plate or [[Petri dish]] is used to provide a [[growth medium]] using a mix of agar and other nutrients in which microorganisms, including [[bacteria]] and [[fungi]], 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 the [[microbe]]s 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 sterile [[media 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 contains [[lysogeny broth]], a nutrient-rich medium used for bacterial growth.<ref>{{Cite journal |date=2009-03-01 |title=LB agar |url=http://cshprotocols.cshlp.org/content/2009/3/pdb.rec11683 |journal=Cold Spring Harbor Protocols |language=en |volume=2009 |issue=3 |article-number=pdb.rec11683 |doi=10.1101/pdb.rec11683 |issn=1940-3402|url-access=subscription }}</ref> Additionally, 2216 Marine Broth (MB) agar, with high salt content, is optimized for growing heterotrophic marine bacteria like those of the ''Vibrio'' genus, while Terrific Broth (TB) agar is used to non-selectively culture high yields of the bacterium ''E.&nbsp;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. 
[[File:Lactose fementing (LF), and non-lactose fermenting (NLF) colonies on MacConkey agar.jpg|thumb]]
Other fastidious organisms may require the addition of different biological fluids such as horse or sheep blood, serum, egg yolk, and so on.<ref>{{Citation |last1=Clark |first1=David P. |title=Chapter 7 – Cloning Genes for Synthetic Biology |date=2019-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128132883000070 |work=Molecular Biology (Third Edition) |pages=199–239 |editor-last=Clark |editor-first=David P. |publisher=Academic Cell |language=en |doi=10.1016/b978-0-12-813288-3.00007-0 |isbn=978-0-12-813288-3 |access-date=2022-12-12 |last2=Pazdernik |first2=Nanette J. |last3=McGehee |first3=Michelle R. |s2cid=91889487 |editor2-last=Pazdernik |editor2-first=Nanette J. |editor3-last=McGehee |editor3-first=Michelle R.|url-access=subscription }}</ref> 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.<ref>{{cite web |url=https://asm.org/Articles/2020/September/Why-Differential-Selective-Media-Are-Invaluable-To  |title=Why Differential & Selective Media Remain Invaluable Tools|date=September 25, 2020 |work=American Society for Microbiology}}</ref> 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.
[[File:Microorganisms-11-01566-g008.webp|thumb]]

==== Motility assays ====
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 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 ===
[[Image:Physcomitrella growing on agar plates.jpg|thumb|''[[Physcomitrella patens]]'' plants growing [[axenic]]ally [[in vitro]] on [[agar plate]]s. [[Petri dish]] is {{cvt|9|cm|adj=on}} in diameter.]]

Research grade agar is used extensively in [[plant]] 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 for ''[[Arabidopsis thaliana]]'' is standard across most experimental conditions. [[Murashige and Skoog medium|Murashige & Skoog]] (MS) nutrient mix and [[Oluf L. Gamborg|Gamborg]]'s B<sub>5</sub> vitamin mix in general are used. A 1.0% agar/0.44% MS+vitamin dH<sub>2</sub>O 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.<ref>{{cite book|last1=Kim|first1=Se-Kwon|title=Handbook of marine macroalgae: biotechnology and applied phycology|date=2011|publisher=John Wiley & Sons Inc.|location=Hoboken, NJ|isbn=978-0-470-97918-1|edition=1st imp.}}</ref> 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 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&nbsp;– agar or [[Gellan gum|Gelrite]] – does influence [[phytohormone]] sensitivity of the plant [[cell culture]].<ref name="Birgit Hadeler">
{{cite journal|title=Gelrite and agar differently influence cytokinin-sensitivity of a moss |journal=Journal of Plant Physiology |volume=146 |issue=3 |pages=369–371|author1 = Birgit Hadeler|author2 = Sirkka Scholz|author3=Ralf Reski |author-link3=Ralf Reski |doi=10.1016/s0176-1617(11)82071-7|year=1995 |bibcode=1995JPPhy.146..369H }}</ref>

=== Other uses ===
Agar is used:
* As an [[Dental impression|impression material]] in [[dentistry]].
* As a medium to precisely orient the tissue specimen and secure it by '''agar pre-embedding''' (especially useful for small endoscopy biopsy specimens) for [[histopathology]] processing<ref>{{cite journal|title = Improvised double-embedding technique of minute biopsies: A mega boon to histopathology laboratory |journal = Indian Journal of Pathology and Microbiology |url = https://www.researchgate.net/publication/272189364 |date = February 2015|volume = 58|issue =1|pages =12–6|doi =  10.4103/0377-4929.151156 |first1= Lokenddra |last1=Yadav|first2= Sarega |last2=Thomas|first3=Usha|last3= Kini|pmid = 25673584 |doi-access = free}}</ref>
* To make [[salt bridge]]s and gel plugs for use in [[electrochemistry]].
* In [[formicarium]]s as a transparent substitute for sand and a source of nutrition.
* As a natural ingredient in forming [[modeling clay]] for young children to play with.
* As an allowed biofertilizer component in organic farming.<ref>[https://books.google.com/books?id=icgrAQAAQBAJ&dq=agar+organic+farming&pg=PA184 Integrated Organic Farming Handbook], H. Panda, Asia Pacific Business Press Inc., Oct 4, 2013</ref>
* As a substrate for [[Precipitin#Precipitin reaction|precipitin reactions]] in [[immunology]].
* At different times as a substitute for [[gelatin]] in [[photographic emulsion]]s, [[arrowroot]] in preparing silver paper and as a substitute for [[Animal glue|fish glue]] in resist etching.<ref>{{Cite journal|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870000744.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870000744.pdf |archive-date=2022-10-09 |url-status=live|title=Black-and-White Photographic Chemistry|last=Walker|first=Ernie|date=June 1986|website=NASA Technical Reports Server}}</ref>
* As an MRI elastic gel phantom to mimic tissue mechanical properties in [[Magnetic Resonance Elastography]]<ref>{{cite journal |last1=McIlvain |first1=Grace |last2=Ganji |first2=Elahe |last3=Cooper |first3=Catherine |last4=Killian |first4=Megan L. |last5=Ogunnaike |first5=Babatunde A. |last6=Johnson |first6=Curtis L. |title=Reliable preparation of agarose phantoms for use in quantitative magnetic resonance elastography |journal=Journal of the Mechanical Behavior of Biomedical Materials |date=September 2019 |volume=97 |pages=65–73 |doi=10.1016/j.jmbbm.2019.05.001|pmid=31100487 |pmc=6699912 }}</ref>
* 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 commercial [[Packaging and labeling|packaging]] system called Agar Plasticity, intended as a replacement for oil-based plastic packaging.<ref>{{Cite web|url=https://www.good.is/articles/agar-plasticity-amam-araki-maetani-muraoka-packaging|title=New Seaweed-Based Material Could Replace Plastic Packaging|website=Good Magazine|access-date=2016-04-03|date=2016-03-09}}</ref><ref>{{Cite web|url=http://www.fastcompany.com/3055784/design-award/design-looks-ahead|title=Design Looks Ahead|website=Fast Company|date=26 January 2016|language=en-US|access-date=2016-04-03}}</ref>

== See also ==
{{portal|Food}}
{{div col|colwidth=25em}}
* {{annotated link|Agarose gel electrophoresis}}
* {{annotated link|Algaculture}}
* {{annotated link|Alginic acid}}
* {{annotated link|Asepsis}}
* {{annotated link|Bacterial cellulose}}
* {{annotated link|Carrageenan}}
* {{annotated link|Gellan gum}}
* {{annotated link|Immunodiffusion}}
* {{annotated link|Immunoelectrophoresis}}
* {{annotated link|Lima bean agar}}
* {{annotated link|Ouchterlony double immunodiffusion}}
* {{annotated link|R2A agar}}
* {{annotated link|Radial immunodiffusion}}
* {{annotated link|SEAgel}}
* {{annotated link|Tokoroten}}
{{div col end}}

== References ==
{{Reflist|30em}}

==External links==
*{{Wiktionary-inline|agar}}

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[[Category:Dietary fiber]]
[[Category:Edible thickening agents]]
[[Category:Microbiological gelling agent]]
[[Category:Dental materials]]
[[Category:Algal food ingredients]]
[[Category:Red algae]]
[[Category:Gels]]
[[Category:Polysaccharides]]
[[Category:Japanese inventions]]
[[Category:Food stabilizers]]
[[Category:Jams and jellies]]
[[Category:E-number additives]]
[[Category:Impression material]]