Agar

Culinary usage

Scientific usage

Mizuyōkan (top)- a popular Japanese red bean jelly made from agar.

Red blood cells on an agar plate (bottom) are used to diagnose infection.

Agar or agar-agar is a gelatinous substance derived from red algae.^[1] Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work. The gelling agent is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the genera Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema). Commercially it is derived primarily from Gelidium amansii.

Agar (agar-agar) can be used as a laxative, a vegetarian gelatin substitute, a thickener for soups, in jellies, ice cream and other desserts, as a clarifying agent in brewing, and for paper sizing fabrics.

Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls.

Names

The word "agar" comes from the Malay word agar-agar (meaning jelly). 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. Gracilaria lichenoides specifically is referred to as agal-agal or Ceylon agar.^[2]

In Malay and Indonesian, it is known as agar-agar. In Japanese, it is known as kanten (寒天) meaning "cold weather," referring to the fact that it is harvested in the winter months. In Mandarin Chinese as hǎicài (海菜) meaning "ocean vegetable", hǎizàoqióngzhī (海藻瓊脂) or dòngfěn (凍粉). In Taiwanese Hokkien it is known as chhài-iàn (菜燕) meaning "vegetable swiftlet," i.e., similar in texture to the nest of the edible-nest swiftlet used in bird's nest soup. In Korea, it is known as hancheon (한천). In the Philippines, it is known as gulaman in Tagalog, Apayao, Bikol, and Pangasinan, guraman in Ilokano and gurguraman in Sambali.^[3] In Thai it is known as wóon (วุ้น). In Tamil and Telugu it's called paal kasuv.

Structure

The structure of an agarose polymer.

Agar consists of a mixture of agarose and agaropectin. 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. Its major differences from carrageenans are the presence of L-3,6-anhydro-α-galactopyranose rather than D-3,6-anhydro-α-galactopyranose units and the lack of sulfate groups. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts. Their structures are similar but slightly branched and sulfated, and they may have methyl and pyruvic acid ketal substituents. They gel poorly and may be simply removed from the excellent gelling agarose molecules by using their charge. The quality of agar is improved by toe treatment that converts of any L-galactose-6-sulfate to 3,6-anhydro-L-galactose.

The gel network of agarose contains double helices formed from left-handed threefold helices. These double helices are stabilized by the presence of water molecules bound inside the double helical cavity [508]. Exterior hydroxyl groups allow aggregation of up to 10,000 of these helices to form suprafibers.^[4]

Properties

Agar exhibits hysteresis, melting at 85 °C (358 K, 185 °F) and solidifying from 32-40 °C (305-313 K, 90-104 °F).^[5]

Microbiology

100mm diameter petri dishes containing agar jelly for bacterial culture

Nutrient agar is used throughout the world to provide a solid surface containing medium for the growth of bacteria and fungi. 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. The basic agar formula can be used to grow most of the microbes whose needs are known. More specific nutrient agars 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 only show growth 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.

Molecular biology

Agar is a heterogeneous mixture of two classes of polysaccharide: agaropectin and agarose.^[6] 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 immunodiffusion and immunoelectrophoresis as the agarose fibers functions as an anchor for immunocomplexes. Agarose is used generally as the medium for analytical scaleelectrophoretic separation in agarose gel electrophoresis and for column-based preparative scale separation as in gel filtration chromatography and affinity chromatography.

Plant biology

Physcomitrella patens plants growing axenically in vitro on agar plates (Petri dish, 9 cm diameter).

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 thaliana is standard across most experimental conditions. Murashige & Skoog (MS) nutrient mix and Gamborg's B5 vitamin mix are generally used. A 1.0% agar/0.44% MS+vitamin dH₂0 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 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.^[7]

Culinary

Agar-agar is a natural vegetable gelatin 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 custards. 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 molds to be served as desserts and vegetable aspics, or incorporated with other desserts, such as a jelly layer in a cake.

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^[1]) diet. Once ingested, kanten triples in size and absorbs water. This results in the consumer feeling more full. Recently this diet has received some press coverage in the United States as well. The diet has shown promise in obesity studies.^[8]

One use of agar in Japanese cuisine 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 Mizuyōkan, another popular Japanese food. (See very top image.) In Indian cuisine, agar agar is known as "China grass" and is used for making desserts. In Burmese cuisine, a sweet jelly known as kyauk kyaw (​ေကျာက်​ေကြာ [tʃaoʔtʃau]) is made from agar.

Other uses

Agar is used:

• As an impression material in dentistry.
• To make salt bridges for use in electrochemistry.
• In formicariums as a transparent substitute for sand and a source of nutrition.

See also

• Agarose gel electrophoresis
• Algaculture
• Anmitsu
• Asepsis
• Carrageenan
• Immunodiffusion, Immunoelectrophoresis
• Petri dish
• R2a agar
• Rhodophyta
• Robert Koch
• SEAgel
• Walter Hesse

References