Agar or
agar-agar is a
gelatinGelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
ous substance derived from a
polysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
that accumulates in the
cell wallThe cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
s of
agarophyteAn agarophyte is a seaweed, typically a red alga, that produces the hydrocolloid agar in its cell walls. This agar can be harvested commercially for use in biological experiments and culturing. In some countries , the harvesting of agarophytes, either as natural stocks or a cultivated crop, is of...
red algaeThe red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds...
. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid
substrateIn biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
to contain
culture mediumA growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....
for
microbiologicalMicrobiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...
work. The gelling agent is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the
generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
GelidiumGelidium is a genus of thalloid alga comprising 124 species. Its members are known by a number of common names.Recorded common names are tengusa, makusa, genso, niu mau tsai, japansche scheleiachtige mos, steen-or klipbloem, hay tsay, olus marinus, sajur laut, tschintschau, tschoo-hoae, onikusa,...
and
GracilariaGracilaria is a genus of red algae notable for its economic importance as an agarophyte, as well as its use as a food for humans and various species of shellfish...
, or
seaweedSeaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...
(
Sphaerococcus euchema). For commercial purposes, it is derived primarily from
Gelidium amansiiGelidium amansii is an economically important species of red algae commonly found in the shallow coast of many East and Southeast Asian countries. This alga is used to make agar, as well it is sometimes served as part of a salad in producing regions....
.
Agar (agar-agar) can be used as a
laxativeLaxatives are foods, compounds, or drugs taken to induce bowel movements or to loosen the stool, most often taken to treat constipation. Certain stimulant, lubricant, and saline laxatives are used to evacuate the colon for rectal and/or bowel examinations, and may be supplemented by enemas under...
, a vegetarian gelatin substitute, a thickener for
soupSoup is a generally warm food that is made by combining ingredients such as meat and vegetables with stock, juice, water, or another liquid. Hot soups are additionally characterized by boiling solid ingredients in liquids in a pot until the flavors are extracted, forming a broth.Traditionally,...
s, in fruit preserves,
ice creamIce cream is a frozen dessert usually made from dairy products, such as milk and cream, and often combined with fruits or other ingredients and flavours. Most varieties contain sugar, although some are made with other sweeteners...
, and other desserts, as a clarifying agent in
brewingBrewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...
, and for
sizingSizing or size is any one of numerous specific substances that is applied to or incorporated in other material, especially papers and textiles, to act as a protecting filler or glaze....
paper and fabrics.
In chemical terms, agar is a
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
made up of subunits of the sugar
galactoseGalactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
. Agar polysaccharides serve as the primary structural support for the algae's cell walls.
Names
The word "agar" comes from
agar-agar, the
MalayMalay is a major language of the Austronesian family. It is the official language of Malaysia , Indonesia , Brunei and Singapore...
name for the cell (
Gigartina,
Gracilaria) from which the jelly is produced. It is also known as
kanten,
China grass, or
Japanese isinglass. The various species of alga or seaweed from which agar is derived are sometimes called
Ceylon moss or
Jaffna moss.
Gracilaria lichenoides is specifically referred to as
agal-agal or
Ceylon agar.
Structure
Agar consists of a mixture of agarose and agaropectin. The predominant component agarose 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 acidic molecules that gel poorly.
Properties
Agar exhibits
hysteresisHysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
, melting at 85 °C (358 K, 185 °F) and solidifying from 32-40 °C (305-313 K, 90-104 °F). 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.
Microbiology
Agar is used throughout the world to provide a solid surface containing
mediumA growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....
for the growth of
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
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 thalianaArabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
is standard across most experimental conditions.
Murashige & SkoogMurashige and Skoog medium or is a plant growth medium used in the laboratories for cultivation of plant cell culture. MSO was invented by plant scientists Toshio Murashige and Folke K. Skoog in 1962 during Murashige's search for a new plant growth regulator...
(MS) nutrient mix and Gamborg's B5 vitamin mix in general are used. A 1.0% agar/0.44% MS+vitamin dH
2O 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
autoclaveAn autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around 15–20 minutes depending on the size of the load and the contents. It was invented by Charles Chamberland in 1879, although a precursor known as the...
.
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
mossMosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
Physcomitrella patensPhyscomitrella patens is a moss used as a model organism for studies on plant evolution, development and physiology.-Model organism:...
, however, have shown that choice of the gelling agent — agar or
GelriteGellan gum is a water-soluble polysaccharide produced by Pseudomonas elodea, a bacterium.-Chemical structure:The repeating unit of the polymer is a tetrasaccharide, which consists of two residues of D-glucose and one of each residues of L-rhamnose and D-glucuronic acid...
- does influence phytohormone sensitivity of the
plantPlants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
cell cultureCell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
.
Molecular biology
Agar is a heterogeneous mixture of two classes of polysaccharide:
agaropectin and
agarose. Although both polysaccharide classes share the same galactose-based backbone,
agaropectin is heavily modified with acidic side-groups, such as sulfate and pyruvate.
The neutral charge and lower degree of chemical complexity of
agarose make it less likely to interact with biomolecules, 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
immunodiffusionImmunodiffusion is a diagnostic test which involves diffusion through a substance such as agar.Two commonly known forms are Ouchterlony double immunodiffusion and radial immunodiffusion....
and
immunoelectrophoresisImmunoelectrophoresis is a general name for a number of biochemical methods for separation and characterization of proteins based on electrophoresis and reaction with antibodies. All variants of immunoelectrophoresis require immunoglobulins, also known as antibodies reacting with the proteins to be...
, as the agarose fibers functions as an anchor for immunocomplexes. Agarose is used generally as the medium for analytical scale
electrophoreticElectrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
separation in agarose gel electrophoresis and for column-based preparative scale separation as in
gel filtration chromatographySize-exclusion chromatography is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight . It is usually applied to large molecules or macromolecular complexes such as proteins and industrial polymers...
and
affinity chromatographyAffinity chromatography is a method of separating biochemical mixtures and based on a highly specific interaction such as that between antigen and antibody, enzyme and substrate, or receptor and ligand.-Uses:Affinity chromatography can be used to:...
.
Culinary
Agar-agar is a natural vegetable
gelatinGelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
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
custardCustard is a variety of culinary preparations based on a cooked mixture of milk or cream and egg yolk. Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce , to a thick pastry cream used to fill éclairs. The most common custards are used as...
s. 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
moldsMolding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
to be served as desserts and vegetable
aspicAspic is a dish in which ingredients are set into a gelatin made from a meat stock or consommé. Non-savory dishes, often made with commercial gelatin mixes without stock or consommé, are usually called gelatin salads....
s, or incorporated with other desserts, such as a jelly layer in a
cakeCake is a form of bread or bread-like food. In its modern forms, it is typically a sweet and enriched baked dessert. In its oldest forms, cakes were normally fried breads or cheesecakes, and normally had a disk shape...
.
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) 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.
In Philippine cuisine, it is used to make the jelly bars in the various
gulamanGulaman, in Filipino cuisine, refers to the bars of dried seaweed used to make jellies or flan, as well as the desserts made from it. Agarose or agar is made of processed seaweed, mostly from Gelidium corneum--one of the most common edible alga, dehydrated and formed into foot-long dry bars which...
refreshments or desserts such as
sago gulaman (aka
gulaman at sago),
buko pandan,
agar flan,
halo-haloHalo-halo is a popular Filipino dessert that is a mixture of shaved ice and evaporated milk to which are added various boiled sweet beans and fruits, and served in a tall glass or bowl....
, the various
Filipino fruit salads,
black gulaman, and
red gulaman. One use of agar in
Japanese cuisineJapanese cuisine has developed over the centuries as a result of many political and social changes throughout Japan. The cuisine eventually changed with the advent of the Medieval age which ushered in a shedding of elitism with the age of shogun rule...
is
anmitsuis a Japanese dessert that has been popular for many decades.It is made of small cubes of agar jelly, a white translucent jelly made from red algae or seaweed. The agar is dissolved with water to make the jelly...
, 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
Indian cuisineIndian cuisine consists of thousands of regional cuisines which date back thousands of years. The dishes of India are characterised by the extensive use of various Indian spices, herbs, vegetables and fruit. Indian cuisine is also known for the widespread practice of vegetarianism in Indian society...
, agar agar is known as "China grass" and is used for making desserts. In Burmese cuisine, a sweet jelly known as
kyauk kyaw (ေကျာက်ေကြာ tɕauʔtɕɔ́) is made from agar. In Russia, it is used in addition or as a replacement to
pectinPectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
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
meringueMeringue is a type of dessert made from whipped egg whites and sugar, occasionally some recipes may call for adding an acid such as cream of tartar or a small amount of vinegar and a binding agent such as cornstarch found in icing sugar which may be added in addition to the corn starch which...
) used as a cake filling or chocolate-glazed as individual sweets.
Other uses
Agar is used:
- As an impression material
A dental impression is an imprint of hard and/or suft tissues, formed with specific types of impression materials that is used in different area of dentistry including prosthetics , restorative, diagnosis and Oral and Maxillofacial surgery for both intra oral and or extra oral aims.The required...
in dentistryDentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...
.
- To make salt bridge
A salt bridge, in chemistry, is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell , a type of electrochemical cell...
s for use in electrochemistryElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
.
- In formicariums as a transparent substitute for sand and a source of nutrition.
- By Kanten Clay Studio
Kanten Clay Studio is a brand of safe modelling compound or clay for children, composed of agar , natural food coloring and brine. Distributed by BorneLund Ltd., a Japan-based company that imports and sells educational toys, it is made in Japan with a patented formula, and emphasizes its...
as a natural ingredient to form modelling clayYou can use modelling clay to create items with it. The material compositions and production processes vary considerably. -Ceramic clay:...
for young children to play with.
See also
- Agarose gel electrophoresis
- Algaculture
Algaculture is a form of aquaculture involving the farming of species of algae.The majority of algae that are intentionally cultivated fall into the category of microalgae...
- Anmitsu
is a Japanese dessert that has been popular for many decades.It is made of small cubes of agar jelly, a white translucent jelly made from red algae or seaweed. The agar is dissolved with water to make the jelly...
- Asepsis
Asepsis is the state of being free from disease-causing contaminants or, preventing contact with microorganisms. The term asepsis often refers to those practices used to promote or induce asepsis in an operative field in surgery or medicine to prevent infection...
- Carrageenan
Carrageenans or carrageenins are a family of linear sulfated polysaccharides that are extracted from red seaweeds. There are several varieties of carrageen used in cooking and baking. Kappa-carrageenan is used mostly in breading and batter due to its gelling nature...
- Gulaman
Gulaman, in Filipino cuisine, refers to the bars of dried seaweed used to make jellies or flan, as well as the desserts made from it. Agarose or agar is made of processed seaweed, mostly from Gelidium corneum--one of the most common edible alga, dehydrated and formed into foot-long dry bars which...
- Halo-halo
Halo-halo is a popular Filipino dessert that is a mixture of shaved ice and evaporated milk to which are added various boiled sweet beans and fruits, and served in a tall glass or bowl....
- Immunodiffusion
Immunodiffusion is a diagnostic test which involves diffusion through a substance such as agar.Two commonly known forms are Ouchterlony double immunodiffusion and radial immunodiffusion....
, ImmunoelectrophoresisImmunoelectrophoresis is a general name for a number of biochemical methods for separation and characterization of proteins based on electrophoresis and reaction with antibodies. All variants of immunoelectrophoresis require immunoglobulins, also known as antibodies reacting with the proteins to be...
- Kanten Clay Studio
Kanten Clay Studio is a brand of safe modelling compound or clay for children, composed of agar , natural food coloring and brine. Distributed by BorneLund Ltd., a Japan-based company that imports and sells educational toys, it is made in Japan with a patented formula, and emphasizes its...
- Petri dish
A Petri dish is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells or small moss plants. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch...
- R2a agar
R2A agar is a culture medium developed to study bacteria which normally inhabit potable water. These bacteria tend to be slow-growing species and would quickly be suppressed by faster-growing species on a richer culture medium....
- Rhodophyta
- Robert Koch
Heinrich Hermann Robert Koch was a German physician. He became famous for isolating Bacillus anthracis , the Tuberculosis bacillus and the Vibrio cholerae and for his development of Koch's postulates....
- SEAgel
SEAgel is one of a class of high-tech foam materials known as aerogels. It is an excellent thermal insulator and among the least dense solids known. SEAgel was invented by Robert Morrison at the Lawrence Livermore National Laboratory in 1992...
- Walter Hesse
Walther Hesse is best known for his work in microbiology, specifically his work in developing Agar as a medium for culturing microorganisms.-Biography:...