Agar
Agar (/ˈeɪɡɑːr/ or /ˈɑːɡər/), 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" and "tengusa".
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.
[6][7] The word agar comes from agar-agar, the Malay name for red algae (Gigartina, Eucheuma,[8] Gracilaria) from which the jelly is produced.
[14] Jelly seaweeds were also favoured and foraged by Malay communities living on the coasts of the Riau Archipelago and Singapore in Southeast Asia for centuries.
[14][18] The application of agar as a food additive in Japan is alleged to have been discovered in 1658 by Mino Tarōzaemon (美濃 太郎左衞門), 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.
[24] However, with the outbreak of World War II, many nations were forced to establish domestic agar industries in order to continue microbiological research.
[27] 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.
[30] This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures.
[citation needed] 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 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.
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.
[40] 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. 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.
Other fastidious organisms may require the addition of different biological fluids such as horse or sheep blood, serum, egg yolk, and so on.
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.
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.
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.
