Agar is a gelatinous substance derived from red algae or seaweed, used as a culture medium in petri dishes and as a thickener in candies and other foods. It consists of two substances, agarose and agaropectin, and is solid at room temperature but easily melted. Agar is extracted from seaweed by washing and heating it in water, then increasing its concentration through a freeze-thaw technique or squeezing water under pressure. It is used in microbiology as a growth medium for microorganisms and in cooking as a gelling agent. Agarose, extracted from agar, is used in electrophoresis to separate protein and DNA fragments based on their size and electrical charge.
Agar-agar, usually shortened to agar, is a gelatinous substance derived from certain types of red algae or seaweed. There are a number of uses for it, although most people are familiar with it as a culture medium in petri dishes. This substance is also perfectly edible, and in addition to appearing in regional cuisine, it is also used as a thickener in candies and other foods. As it is derived from plant material, it has the advantage of being suitable for vegetarians, unlike gelatin, which is derived from animal sources.
Composition and properties
Agar actually consists of two substances, called agarose and agaropectin. Agarose is a polysaccharide, a type of polymer in which a large number of small carbohydrate units are linked together to form much larger molecules. Agaropectin is also a polysaccharide, but is made up of smaller molecules that also contain non-carbohydrate components, such as sulfates.
The substance is solid at room temperature, but is easily melted and resolidified. It doesn’t dissolve in cold water, but absorbs large amounts, swelling in the process. Agar dissolves in water when it is near the boiling point and forms a gel, even at very low concentrations.
Production and supply
Two types of algae, Gelidium and Gracilaria, are harvested to make agar. They are found in many parts of Asia and on the west coast of the United States. Gelidium provides a product that is superior for certain uses, such as growing bacteria.
To extract the substance, the seaweed is washed and then heated in water, causing it to dissolve. Once cooled it forms a gel with only a low concentration of product. The concentration is increased by a freeze-thaw technique or by squeezing the water under pressure. The rest of the water is then removed by hot air drying, leaving the solid product, which can then be cut into blocks or ground to produce granules, flakes or powder. Some can be further processed to separate agarose, which has some more specialized applications in biochemistry.
Many grocery stores sell agar suitable for human consumption, often in the same area as vegetarian alternatives. The laboratory grade product is sold through scientific supply companies. Although food-grade versions can sometimes be used in the laboratory, those intended for scientific use should not be consumed if potentially harmful substances have been added.
it is used
Agar has numerous uses in the fields of microbiology, biochemistry and molecular biology. In microbiology, it is one of the most important and widely used materials. Since most microbes are unable to digest it, it can, with the addition of proper nutrients, act as a growth medium for bacteria, fungi, and other microorganisms. The microbes feed on the added nutrients, but cannot digest the agar, so it remains intact, allowing colonies of organisms to be easily observed and studied.
Laboratory agar normally comes in the form of a very pure powder, as special measures must be taken to ensure that it is free from microorganisms, spores and any chemicals that could impede or interfere with the growth of the organisms to be cultured. It is dissolved in hot water and cooled, then a suitable nutrient and possibly other chemicals are added depending on what is to be grown. The mixture is poured into Petri dishes, where it sets into a gel, and then the materials to be tested are deposited onto the surface, usually in streaks. Petri dishes can then be incubated at a suitable temperature and observed over a period of days, to check what is growing on them. Agarose, extracted from agar, is primarily used as a gel for electrophoresis, a technique that separates different protein and DNA fragments based on their size and electrical charge.
Agar also has many uses in cooking and the food industry. In parts of Asia, it is used as an ingredient in soups and jellies. Outside of Asia, the substance appears in a wide range of foods, along with two other plant-based jellies, carrageenan and alginate. It is often used in foods that need to be gelled, but behaves slightly differently from protein-based jellies, tending to be slightly slimier and sometimes creating an odd texture. For vegetarians, it makes foods like marshmallows, jellies, and gummy bears edible.
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