View source for Biology:Agar

{{short description|Thickening agent used in microbiology and food}}
[[File:GreenTeaYokan.jpg|thumb|Green tea flavored yōkan, a popular Japanese 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 [[Chemistry:Gelling agent|jelly-like substance]] consisting of  polysaccharides obtained from the cell walls of some species of [[Biology:Red algae|red algae]], primarily from "ogonori" (''[[Biology:Gracilaria|Gracilaria]]'') and "tengusa" ([[Biology:Gelidiaceae|Gelidiaceae]]).<ref>{{cite web|last1=Shimamura |first1=Natsu |title=Agar |url=http://www.tokyofoundation.org/en/topics/japanese-traditional-foods/vol.-4-agar |website=The Tokyo Foundation |access-date=19 December 2016 |date=August 4, 2010}}</ref><ref>{{cite book|title=Oxford Dictionary of English |edition=2nd |year=2005 |title-link=Oxford Dictionary of English}}</ref> As found in nature, agar is a mixture of two components, the linear polysaccharide [[Chemistry:Agarose|agarose]] and a heterogeneous mixture of smaller molecules called [[Chemistry:Agaropectin|agaropectin]].<ref name="Williams2000">{{cite book |last1=Williams |first1=Peter W. |last2=Phillips |first2=Glyn O. |title=Handbook of hydrocolloids |year=2000  |chapter-url=https://books.google.com/books?id=dXS7qnh-ZOEC&pg=PA28 |chapter=2: Agar |location=Cambridge, England |publisher=Woodhead |page= 91 |isbn=1-85573-501-6 |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 [[Biology:Agarophyte|agarophyte]]s, belonging to the Rhodophyta (red algae) phylum.<ref>{{cite book|last1=Balfour |first1= Edward Green |chapter= agar |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-url = https://archive.org/details/cyclopdiaindiaa00unkngoog/page/n63 |year = 1871| publisher = Scottish and Adelphi Presses |page = 50}}</ref><ref name="oxford">{{cite book|last1= Davidson |first1= Alan |title = The Oxford Companion to Food |url = https://books.google.com/books?id=JTr-ouCbL2AC |date = 2006 |publisher = Oxford University Press |isbn = 978-0-19-280681-9 }}</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 desserts throughout Asia and also as a solid [[Biology:Substrate|substrate]] to contain culture media for [[Biology:Microbiology|microbiological]] work. Agar can be used as a [[Chemistry:Laxative|laxative]]; an appetite suppressant; a vegan substitute for [[Biology:Gelatin|gelatin]]; a thickener for soups; in 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|url=https://foodb.ca/foods/FOOD00284|url-status=live|website=The Metabolomics Innovation Centre (TMIC)|archive-url=https://web.archive.org/web/20210509052540/https://foodb.ca/foods/FOOD00284 |archive-date=2021-05-09 }}</ref><ref>{{cite book| author = Edward Green Balfour| title = Cyclopaedia of India and of Eastern and Southern Asia, commercial, industrial and scientific...| publisher = printed at the Scottish Press| url = https://archive.org/details/b28708921_0002| year = 1857| page = [https://archive.org/details/b28708921_0002/page/13 13] }}</ref>

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

== History ==
[[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 [[Place:Southeast Asia|Southeast Asia]].<ref>{{cite book|url=https://books.google.com/books?id=5umXDDmqxwIC&q=agar+gelling+southeast+asia+dessert&pg=PA31|title=Encyclopedia of Modern Coral Reefs: Structure, Form and Process|publisher=Springer Science & Business Media|isbn=9789048126385|page=31|language=en|last1=Hopley|first1=David|year=2010}}</ref> In the {{wipe|Philippines}}, ''[[Biology:Gracilaria|Gracilaria]]'', known as ''[[Chemistry:Gulaman|gulaman]]'' (or ''gulaman dagat'') in [[Social: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"'' ("an 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 |last1=Albert H. Wells |title=Possibilities of Gulaman Dagat as a Substitute for Gelatin in Food |journal=The Philippine Journal of Science |date=1916 |volume=11 |pages=267–271}}</ref><ref>{{cite book |last1=de Noceda |first1=Juan |last2=de Sanlucar |first2=Pedro |title=Vocabulario de la lengua Tagala |date=1754 |publisher=Imprenta de la compañia de Jesus |page=101, 215}}</ref> 

[[Chemistry:Carrageenan|Carrageenan]], derived from gusô (''[[Biology:Eucheuma|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'' (c.1637) of the Augustinian missionary 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 |title=Diccionario De La Lengua Bisaya, Hiligueina Y Haraya de la isla de Panay |date=1841 |publisher=En La Imprenta De D. Manuel Y De D. Felis Dayot |page=380}}</ref>

Jelly seaweeds were also favoured and foraged by Malay communities living on the coasts of the Riau Archipelago and [[Place:Singapore|Singapore]] in Southeast Asia for centuries.<ref>{{Cite magazine|url=https://biblioasia.nlb.gov.sg/vol-17/issue-3/oct-dec-2021/|title=The Role of Foraging in Malay Cuisine|author=Khir Johari|date=Oct–Dec 2021|magazine=BiblioAsia|volume=17|issue=3|pages=20–23|publisher=National Library Board, Singapore}}</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|url=http://www.bd.com/ds/technicalCenter/misc/difcobblmanual_2nded_lowres.pdf|title=Difco & BBL Manual|publisher=Becton Dickinson and Company|editor1=Mary Jo Zimbro |editor2=David A. Power |editor3=Sharon M. Miller |editor4=George E. Wilson |editor5=Julie A. Johnson|edition=2nd|page=6|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|url-status=dead}}</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 [[Biography: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|url=http://www.labnews.co.uk/features/history-of-the-agar-plate/|title=History of the Agar Plate|date=November 1, 2005|magazine=Laboratory News|author=Smith, A.|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|url-status=dead}}</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|year=1992|title=Walther and Angelina Hesse–Early Contributors to Bacteriology|url=http://www.asm.org/ccLibraryFiles/FILENAME/0000000227/580892p425.pdf|journal=ASM News|volume=58|issue=8|pages=425–428|author=Hesse, W.|translator=Gröschel, D.H.M.|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|url-status=dead|df=dmy-all}}</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|title=The Biology of Seaweeds|publisher=University of California Press|isbn=9780520045859|url=https://books.google.com/books?id=QG4tqjFPWJ0C&q=agar+gelling+malay+japan&pg=PA734|language=en |pages=734–735|year=1981}}</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|title=Lab staple agar hit by seaweed shortage |journal=Nature |volume=528 |issue=7581 |pages=171–172 |publisher=Nature News |author=Ewen Callaway |date=8 December 2015 |doi=10.1038/528171a |pmid=26659158 |bibcode=2015Natur.528..171C |doi-access=free }}</ref>

== Chemical composition ==
[[File:Agarose polymere.svg|right|thumb|The structure of an agarose polymer.]]
Agar consists of a mixture of two [[Chemistry:Polysaccharide|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 [[Chemistry: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 [[Chemistry:Sulfate|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 [[Hysteresis#Liquid–solid-phase transitions|hysteresis]] because when mixed with water, it solidifies and forms a [[Physics:Gel|gel]] at about {{convert|32–42|C|K F}}, which is called the [[Chemistry:Gel point|gel point]], and melts at {{convert|85|C|K F}}, which is the [[Physics:Melting point|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 of agar occurs due to the 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|''[[Chemistry:Sago|Sago]] at gulaman'' in Filipino cuisine is made from agar (''gulaman''), pearl sago, and sugar syrup flavored with pandan|210x210px]]
Agar-agar is a natural vegetable [[Biology:Gelatin|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=9789949882465}}</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=9781910690833}}</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=9789814079471 |location=Singapore |pages=145}}</ref> puddings, and custards.<ref>{{Cite book |last=Ash |first=Michael |title=Handbook fo Fillers, Extenders, and Diluents |publisher=Synapse Information Resources |year=2007 |isbn=9781890595968 |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 [[Engineering:Molding (process)|molds]] to be served as desserts and vegetable aspics 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 |pages=113}}</ref> Its bulking quality has been behind fad diets 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 (Wagashi) 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 ''mizu yōkan'', another popular Japanese food. In Philippine cuisine, it is used to make the jelly bars in the various [[Chemistry:Gulaman|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. 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 is used for making desserts. In Burmese cuisine, a sweet jelly known as ''kyauk kyaw'' is made from agar. Agar jelly is widely used in Taiwanese bubble tea.

==== Other culinary ====
[[File:00114jfCuisine of Bulacan Food Cakes Delicaciesfvf 30.jpg|thumb|''Crema de mangga'', a traditional 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 [[Chemistry:Pectin|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 a 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 [[Biology:Melon|melon]] or [[Biology:Watermelon|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 ====
[[Image:Agar Plate.jpg|right|thumb|100mm (4") diameter Petri dishes containing agar gel for bacterial culture]]
{{Main|Organization:Agar plate}}

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 [[Biology:Bacteria|bacteria]] and [[Biology:Fungi|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 microbes 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 [[Engineering:Media dispenser|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, [[Chemistry:Polyacrylamide|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 |pages=pdb.rec11683 |doi=10.1101/pdb.rec11683 |issn=1940-3402}}</ref> 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.}}</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>

==== 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 axenically [[In vitro|in vitro]] on agar plates (Petri dish, 9&nbsp;cm, 3½" diameter).]]

Research grade agar is used extensively in [[Biology:Plant|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 ''[[Biology:Arabidopsis thaliana|Arabidopsis thaliana]]'' is standard across most experimental conditions. [[Biology:Murashige and Skoog medium|Murashige & Skoog]] (MS) nutrient mix and Gamborg's B5 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=9780470979181|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 [[Organization:Autoclave|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 [[Biology:Moss|moss]] ''[[Biology:Physcomitrella patens|Physcomitrella patens]]'', however, have shown that choice of the gelling agent&nbsp;– agar or [[Chemistry:Gellan gum|Gelrite]] – does influence phytohormone sensitivity of the plant [[Biology:Cell culture|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 |doi=10.1016/s0176-1617(11)82071-7|year=1995 }}</ref>

=== Other uses ===
Agar is used:
* As an [[Medicine:Dental impression|impression material]] in [[Medicine:Dentistry|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 [[Medicine:Histopathology|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 [[Chemistry:Salt bridge|salt bridge]]s and gel plugs for use in [[Chemistry:Electrochemistry|electrochemistry]].
* In [[Biology:Formicarium|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 [[Biology:Precipitin#Precipitin reaction|precipitin reactions]] in [[Biology:Immunology|immunology]].
* At different times as a substitute for [[Biology:Gelatin|gelatin]] in [[Physics:Photographic emulsion|photographic emulsion]]s, [[Chemistry:Arrowroot|arrowroot]] in preparing silver paper and as a substitute for [[Chemistry: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>
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 [[Social: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 ==
{{div col|colwidth=25em}}
* {{annotated link|Biology:Agarose gel electrophoresis}}
* {{annotated link|Biology:Algaculture}}
* {{annotated link|Chemistry:Alginic acid}}
* {{annotated link|Biology:Asepsis}}
* {{annotated link|Chemistry:Carrageenan}}
* {{annotated link|Chemistry:Gellan gum}}
* {{annotated link|Biology:Immunodiffusion}}
* {{annotated link|Biology:Immunoelectrophoresis}}
* {{annotated link|Biology:Ouchterlony double immunodiffusion}}
* {{annotated link|Biology:Radial immunodiffusion}}
{{div col end}}

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

==External links==

{{Vegetarianism|state=collapsed}}

[[Category:Edible thickening agents]]
[[Category:Red algae]]
[[Category:Polysaccharides]]
[[Category:Food stabilizers]]
[[Category:E-number additives]]

{{Sourceattribution|Agar}}