Pharmacognosy of Agar

                                      AGAR AGAR
Agar is derived from the polysaccharide agarose, which forms the supporting structure in the cell walls of certain species of algae, and which is released on boiling. These algae are known as agarophytes and belong to the Rhodophyta (red algae) phylum. Agar is actually the resulting mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin.

                                 

           FIG;-chemical structure of agarose

Synonyms: Agar-agar, Vegetable Gelatin, Japanese or Chinese Gelatin, Japanese Isinglass.

Biological Source: It is the dried colloidal concentrate prepared from the decoction of various species of the genus Gelidium, family Gelidaceae. The genus Gelidium provides about 35% of the total raw material. Japanese agar is obtained from Gelidium amansii.

It is also obtained from genus Gracilaria, family Gracilariaceae. Other Geographical Sources include Korea, South Africa, Atlantic and Pacific Coasts of the USA, Chile, Spain and Portugal. More than 6500 tonnes of agar is produced annually.

Collection and Preparation: In the coastal area of Japan, the algae are cultivated in special areas. The poles are planted in the sea to form supports for the development of algae. The poles are withdrawn from time to time and the algae are stripped off in the months from May to October. The algae are dried, beaten and shaken to remove any earthy material adhering to it. It is then bleached by watering and drying in the sun. The algae are then boiled with acidulated water for several hours. A mucilaginous decoction is formed, which is filteredwhile hot through a linen cloth. On cooling, a jelly is produced which is cut into bars and subsequently strips are produced.

The manufacturing of agar takes place only in winter season. The moisture is removed by freezing, thawing and drying at about 35 ° C.

Morphological Characters:

Form: occurs in two forms: 1) Coarse powder or flakes 2) bundles of translucent, and crumpled, strips, 2-5mm wide.

Color: colorless to pale yellow.

Fracture: Tough when damp and brittle when dry.

Odor: not distinct

Taste: mucilaginous

Chemical Constituents: It is a heterogeneous polysaccharide composed of two principal constituents: agarose and agaropectin. Agarose represents the gel strength and agaropectin is responsible for the viscosity of the agar solutions

Uses:

􀂾 The gels of pure agarose are used for the electrophoresis of proteins.

􀂾 Agar is used for the preparation of culture media

􀂾 It is used as an emulsifying agent

􀂾 In the treatment of constipation

􀂾 Used in affinity chromatography

Physical Characteristics:

Solubility:

Cold water: does not dissolve but swells to a gelatinous mass

Boiling water: dissolves and 1% solution gives a stiff jelly on cooling

Chemical Tests:

1. Warm a small quantity of drug with caustic potash solution, a canary-yellow color is produced.

2. To 5ml of 0.5% solution of drug in water, add 0.5ml of HCl and heat on a water bath for about 30 min. neutralize the solution and divide it into two portions. To one portion, add Fehling’s solution and heat on a water bath, a red precipitate is formed. To other portion, add solution of BaCl₂. A slight, white precipitate is formed

(tragacanth gives no precipitate) (on hydrolyzing, galactose and sulphate ions are produced, former reducing Fehling’s solution and the latter precipitating with BaCl₂)

3. Moisten the drug with N/50 iodine solution, a deep crimson color is produced

(distinction from Acacia and Tragacanth)

4 Moisten the drug with a solution of Ruthenium red, a pink color is produced.

5 Heat a little drug in a test tube with soda-lime. Test the vapours with litmus paper, no alkaline reaction (since no ammonia is produced)

6 Warm a little drug with acetic acid, solution occurs on prolonged heating.

7 Dissolve 0.2 g of the drug in 40ml of hot water. Divide the solution in three parts and treat it as follows:

a. Add a few drops of 10% solution of tannic acid. A white precipitate in the cold and on boiling, an opalescence is produced.

b. Add a few drops of Millon’s reagent. No white precipitate is produced

c. Add excess of saturated solution of trinitrophenol. No yellow precipitate is formed.

Note: Tests 4-6 differentiate it from gelatine

Adulterants:

􀂾 The powdered drug is adulterated with starch.

􀂾 Mount the drug in chloral-iodine solution and observe the starch grains.

                                                            

Note: Agar BP is required to comply for the absence of Escherichia coli and Salmonella.

The general microbial contamination should not exceed a level of 10³microorganisms/ gram by a plate count method.