Agar
Agar agar is a jelly like substance obtained from red algae present in marine water. Other common name of agar agar is Agar, China grass, Japanese isinglass, kanten, Japanese gelatin, Ceylon moss and Jaffna Moss etc.
Source- It is extracted from cell wall of various red algae/seaweed mainly from Gelidum and Gracilaria. These species are cultivated in Asia, South America and Africa. Red algae not only used to extract agar but also used as food source.
Habitat- Red algae are abundant in marine habitats but are relatively rare in freshwaters. They have the ability to survive at a greater depth in sea than other algae because the pigment Phycoerythrin absorbs the blue light that can penetrate deeper than any other light wave. This allows red algae to carry out photosynthesis at a greater depth.
History- Agar Agar was discovered in 17th century by Japanese inner keeper named Minoya Tarazaemon. A Japanese legend described the first preparation of agar:
“A Japanese Emperor and his Royal Party were lost in the mountains during a snow storm and arriving at a small inn, they were ceremoniously treated by the innkeeper who offered them a seaweed jelly dish with their dinner. Maybe the innkeeper prepared too much jelly or the taste was not so palatable but some jelly was thrown away, freezing during the night and crumbling afterwards by thawing and draining, leaving a cracked substance of low density. The innkeeper took the residue and, to his surprise, found that by boiling it up with more water the jelly could be remade”.
After discovery of agar people started using it in different cuisines. Its industrialization as a dry and stable product started, in Japan, at the beginning of the 18th century. Later, in the first years of the 20th century, it was introduced into the West and its production by industrial freezing was initiated.
Production of Agar- red algae plucked out from depth of sea then it is washed to remove foreign matters. After that it is allowed to heat in hot water for several hours. The agar dissolves in hot water and mixture is filtered to remove debris and residues. Once filtrate is cooled, it forms a jelly. If necessary, jelly is washed with water to remove other impurities. Then the water is removed using freeze drying method or pressure is applied to squeeze out water. Then hot air oven is used to remove remaining water. Dried agar cut in different shapes and sizes or converted into powder form and then it is sent to sell.
Properties of Agar- it is colorless, it has no odor and taste. Agar is insoluble in cold water but it absorbs as much as 20 times its own weight. Agar is soluble in boiling water it remains liquid till 420 C, at 370 C it becomes solid as firm gel.
Structure- in its natural form agar occurs as cell wall constituent of algae. Agar is a complex heterosaccharide, made up of two different polysaccharides known as agarose and agaropectin.
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Agarose is neutral galactose polymer and is responsible for the gel property of agar. It consists of D-galactose and L-galactose unit.
Agaropectin is responsible for the viscosity of agar solution. Agaropectin is consists of sulphonated polysaccharide in which galactose and uronic acid are partly esterified with sulphuric acid. Though structure of agaropectin in not yet completely known.
Agar in microbiology field- The idea of culturing bacteria was first introduced by Robert Koch. He used fresh, cleaned and boiled potato to grow bacteria but bacteria could not grow well on it. After that he used gelatin but it also could not work as it is consumed by bacteria. Later better alternative was provided by Fanny Hesse, she suggested to use agar in culture media which was an absolute success.
After this discovery petri plates are prepared by using Agar which provide appropriate environment for growth of microorganisms including bacteria and fungi. Agar can not be digested by microbes, therefore microbial growth remained unaffected. Different types of ingredients are mixed with agar to prepare various types of culture media.
e.g. Mackonkey agar composed of agar, peptone, proteose peptone, lactose monohydrate, bile salts, sodium chloride, neutral red, crystal violet, agar and distilled water.
Bismuth sulfite agar composed of Agar, peptone, meat extract, dextrose, disodium phosphate, ferrous sulphate, bismuth sulphite indicator, brilliant green indicator
Culture media state is also decided by percentage of agar. Solid media contains 1–2% of agar. Semisolid agar has 0.5% of agar while liquid media has 0% of agar.
Uses- In medical field agar is used to treat chronic constipation, as a laxative, sus-pending agent, an emulsifier and as surgical lubricant. It is also used as a tablet excipient, disintegrant, in production of medicinal encapsulation and ointment and as dental impression mold base. In field of microbiology to prepare different types of media. It is used as gel in cosmetic industry, as thickening agent in food especially confectionaries and dairy products, in meet canning; sizing for silk and paper; in dying and printing of fabrics and textiles; and in adhesive.
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