Agar plate with colonies

Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori ('' Gracilaria'') and "tengusa" ('' Gelidiaceae''). As found in nature, agar is a mixture of two components, the linear polysaccharide agarose and a heterogeneous mixture of smaller molecules called agaropectin. It forms the supporting structure in the cell walls of certain species of algae and is released on boiling. These algae are known as agarophytes, belonging to the Rhodophyta (red algae) phylum. 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 substrate to contain culture media for microbiological work. Agar can be used as a laxative; an appetite suppressant; a vegan substitute for 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.

Etymology

The word "agar" comes from agar-agar, the Malay name for red algae ('' Gigartina'', '' Gracilaria'') from which the jelly is produced. It is also known as Kanten ( ja, 寒天) (from the phrase ''kan-zarashi tokoroten'' () or “cold-exposed agar”), Japanese isinglass, China grass, Ceylon moss or Jaffna moss. ''Gracilaria lichenoides'' is specifically referred to as agal-agal or Ceylon agar.

History

Agar may have been discovered in Japan in 1658 by Mino Tarōzaemon (), 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. Over the following centuries, agar became a common gelling agent in several Southeast Asian cuisines.

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''.

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's laboratory, on the suggestion of his wife Fanny Hesse. 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.

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. However, with the outbreak of World War II, many nations were forced to establish domestic agar industries in order to continue microbiological research. Around the time of World War II, approximately 2,500 tons of agar were produced annually. By the mid-1970s, production worldwide had increased dramatically to approximately 10,000 tons each year. Since then, production of agar has fluctuated due to unstable and sometimes over-utilized seaweed populations.

Composition

Agar consists of a mixture of two polysaccharides: agarose and agaropectin, with agarose making up about 70% of the mixture. Agarose is a linear polymer, made up of repeating units of agarobiose, a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts, and is made up of alternating units of D-galactose and L-galactose heavily modified with acidic side-groups, such as sulfate and pyruvate.

Agar exhibits hysteresis, solidifying at about 32–40 °C (305–313 K, 90–104 °F) but melting at 85 °C (358  K, 185 °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.

Uses

Culinary

Agar-agar is a natural vegetable gelatin counterpart. It is white and semi- translucent when sold in packages as washed and dried strips or in powdered form. It can be used to make jellies, puddings, and custards. 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 molds to be served as desserts and vegetable aspics or incorporated with other desserts such as a layer of jelly in a cake.

Agar-agar is approximately 80% dietary fiber, so it can serve as an intestinal regulator. Its bulking quality has been behind fad diets in Asia, for example the ''kanten'' (the Japanese word for agar-agar) 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 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

It can be used as addition to or as a replacement for 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 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.

Microbiology

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 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. Other ingredients are added to the agar to meet the nutritional needs of the microbes. Many microbe-specific formulations are available because some microbes prefer certain environmental conditions over others. Agar is often dispensed using a sterile media dispenser.

Types of agar in microbiology

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." Polyacrylamide polymerization is an irreversible process, and the resulting products are known as chemical gels.

Given below is a list of different types of agar that support the different strains of bacterial growth:

# Blood Agar : A bacterial growth media is blood agar. It is mostly used to cultivate pathogenic bacteria such as Streptococci. Ordinary growth media does not support the development of such organisms. They can only grow in blood agar because it contains inhibitors for several bacterial families
# Luria Bertani (LB) Agar : are nutrient-rich plates used for bacterial growth. They are frequently utilized in cloning operations to increase the number of antibiotic-resistant, competent microorganisms. The same ingredients, without the agar, can be used to make liquid LB medium.
# Chocolate Agar : Chocolate Agar (CAP or CHOC) is a nonselective improved medium used to detect and isolate finicky pathogens.Chocolate agar is created by heating blood agar, which subsequently splits the red blood cell (RBC), releasing nutrients that help in the development of fastidious bacteria such as Haemophilus and Neisseria species.The term comes from the fact that the absence of RBC results in the hue chocolate brown.Chocolate agar is very identical to blood agar, except that the red blood cells are destroyed during the manufacturing process when they are introduced to the molten agar foundation. This causes cell lysis, which releases intracellular nutrition such as hemoglobin, Hemin ("X" factors), and the coenzyme nicotinamide-adenine dinucleotide (NAD and "V" factor) into the agar, which is then utilized by fastidious bacteria.Chocolate agar is very identical to blood agar, except that the red blood cells are destroyed during the manufacturing process when they are introduced to the molten agar foundation. This causes cell lysis, which releases intracellular nutrition such as hemoglobin, Hemin ("X" factors), and the coenzyme nicotinamide-adenine dinucleotide (NAD and "V" factor) into the agar, which is then utilized by fastidious bacteria
# MacConkey Agar : Alfred Theodore MacConkey produced the first solid differential medium, (MAC), in the twentieth century. MacConkey agar is a selective and differential media used for the isolation and differentiation of non-fastidious gram-negative rods, notably members of the Enterobacteriaceae and Pseudomonas genera. MacConkey Agar Applications: Gram-negative intestinal bacteria are isolated using MacConkey agar. It is used to distinguish lactose fermenting gram-negative bacteria from lactose non-fermenting gram-negative bacteria. It's used to isolate coliforms and intestinal pathogens from water, dairy products, and biological samples
# Nutrient Agar : Nutrient Agar is a basic culture medium that is often used for the cultivation of non-fastidious microorganisms, as well as for quality control and purity checking prior to biochemical or serological testing. By supplementing the medium with serum or blood, nutrient media may also be utilized to cultivate picky microorganisms .Nutrient Agar is an excellent medium for demonstration and teaching because it allows cultures to survive at room temperature for longer periods of time without the risk of overgrowth that occurs with more nutritional mediums .This medium has a very basic formula that has been preserved and is still commonly employed in the microbiological evaluation of a wide range of materials, as well as being recommended by standard procedures .Nutrient agar is a general-purpose medium that is primarily used for regular culture or to assure microorganism survival .It is one of the most essential and widely used non-selective medium for regular microorganism culture. Many bacteria that are not particularly fastidious have been grown and counted on nutrient agar .By adding different biological fluids such as horse or sheep blood, serum, egg yolk, and so on, the media can be made suitable for the cultivation of other fastidious organisms.
# Neomycin Agar : Neomycin blood agar is a popular selective medium for isolating vancomycin-resistant enterococci from feces; however, not all isolates are recovered using this medium, perhaps due to excessive neomycin concentrations.

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

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 & Skoog (MS) nutrient mix and Gamborg's B5 vitamin mix in general are used. A 1.0% agar/0.44% MS+vitamin dH₂O solution is suitable for growth media between normal growth temps.

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. 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 – agar or Gelrite – does influence phytohormone sensitivity of the plant cell culture.

Other uses

Agar is used:
* As an 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
* To make salt bridges and gel plugs for use in electrochemistry.
* In formicariums 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.
* As a substrate for precipitin reactions in immunology.
* At different times as a substitute for gelatin in photographic emulsions, arrowroot in preparing silver paper and as a substitute for fish glue in resist etching.
* As an MRI elastic gel phantom to mimic tissue mechanical properties in Magnetic Resonance Elastography
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 system called Agar Plasticity, intended as a replacement for oil-based plastic packaging.

See also

* Agarose gel electrophoresis
* Algaculture
* Alginic acid
* Asepsis
* Carrageenan
* Gellan gum
* Immunodiffusion
* Immunoelectrophoresis
* Lima bean agar
* Ouchterlony double immunodiffusion
* R2a agar
* Radial immunodiffusion
* SEAgel
* Tokoroten

References

External links

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{{Authority control

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