AGAR

SYNONYMS: Agar-agar, Japan agar, Japanese Isinglass, Vegetable gelatin etc.

BIOLOGICAL SOURCE: It is the dried, hydrophilic, colloidal, hetero polysaccharide complex extracted from the agarocytes of various species of red algae of the class Rhodophyceae like Gelidium amansii, Gelidium pterocladia of the family Gelidaceae. The other species of red algae are pterocladia lucida, p.capillacea of the family Gelidaceae, and Gracilaria lichenoides, of the family gracilariaceae.

GEOGRAPHICAL SOURCE: Agar is produced commercially in Japan (first and the largest producers), Korea, South Africa, USA, Spain, Portugal, Mexico etc.

COLLECTION AND PREPARATION: Agar is produced on the coastal areas. For the quick and vigorous growth, supporting poles (bamboos) are planted in the sea. The algae grow vigorously on the poles. The poles are taken away from time to time to strip off the algae from it. The algae are collected from May to October.

                      The sea weeds are washed for 24 hrs in running water, beaten and shaken to remove sand and shells. It is then bleached by exposing to sun. Then it is extracted with dilute acids or acidified water for several hours .The hot mucilaginous decoction is filtered through the linen. On cooling a jelly is formed which is cut into bars.

                      The bars are converted to strips by passing through wire nettings. It is subjected to freezing and thawing to remove water. The striped material is dried in sunlight or at about 30˚c.Manufacture of agar is carried out in the months of winter from November to February.

CHARACTERS:  Shape-available in the form of strips, sheets, flakes or coarse powder. Colour- yellowish grey or white to nearly colourless, odourless Taste- mucilaginous and salty, insoluble in cold water, soluble in boiling water and insoluble in organic solvents.

Acid insoluble ash-NMT1%, Sulphated ash- NMT5%, FOM-NMT1%, loss on drying-NMT 18%, starch- negative with iodine solution

CHEMICAL CONSTITUENTS:  Agar is a hetero polysaccharide, consisting of two different polysaccharides namely agarose and agaropectin.

Agarose- It is a polymer of galactose disaccharide also known as agarobiose, responsible for gel strength of agar and is composed of D-galactose and 3, 6-anhydro-L- galactose units.

Agaropectin- It is responsible for the viscosity of agar solution. It is a sulphonated polysaccharide in which galactose and uronic acid are partly esterifies with sulphuric acid.

IDENTIFICATION TESTS:

1.      To the agar powder, a solution of Ruthenium red is added; Red colour is formed indicating the presence of mucilage.

2.      Boil about 1.5 gm agar with 100 ml water. Cool the solution to room temperature. It forms a stiff jelly.

3.      To 0.2% solution of agar in water add solution of tannic acid, no precipitate is formed. (Distinguishes from gelatin.)

4.      Aq. Solution of Agar gives no precipitate with millon’s test.                            (Distinguishes from  gelatin)

5.      Agar gives deep crimson to brown colour with N/20 solution of Iodine.

6.      On warming Agar with solution of KOH, canary yellow colour is produced.

7.      Agar gives positive test with Fehling’s solution test and Molisch’s test.

8.      Aq. solution of agar is hydrolysed with con Hcl by heating for 5 minutes. On addition of Barium chloride solution to the reaction mixture, a white precipitate of Barium sulphate is formed due to the presence of sulphate ions.

SUBSTITUENTS AND ADULTERANTS: Danish agar, Indian agar (Gracilaria species) are substituents. Gelatin is admixed as an adulterant.

USES: It is largely used in the preparation of nutrient media in bacteriological cultures, non- irritant bulk laxative, as an emulsifying and suspending agent, used as gelling agent  in the preparation of suppositories and pessaries, as a tablet excipient, encapsulating agent,  pure agarose is used for the electrophoresis of proteins.