Agar: Source, Production, Chemical Constituents, Chemical Tests, Uses and Adulterants

Source, Method of Production, Chemical Constituents, Chemical Tests, Uses and Adulterants of Agar

Objectives

• At the end of this lecture, student will be able to:

- Discuss the pharmacognostical aspects of agar

Agar

Synonym: Agar agar, Japanese Isinglass, vegetable gelatin

Family: Gelidiaceae

Source

• Dried gelatinous substance - red algae – Phaeophyceae

• Gelidium; G. elegans, G. amansii, G. polycladum

• Gracilaria lichenoides, Gracilaria conferoides

• Pterocladia species

Geographical source:

• Japan, Australia, New Zealand, USA and India

Agar - Method of preparation

• Poles driven into the sea bed - growth of algae

• Sea weeds collected

• Dried, beaten and shaken

• Boiled for 5-6 hours with 50 times faintly acidulated water

• Strained, wooden troughs, allowed to cool

• Jelly cut into rectangular pieces

• Forced through the netting - get narrow strips

• Dried, bleached in air and sunlight

• Water - removed by alternate freezing and thawing

Description

• Yellowish grey to nearly colourless

• Odourless

• Mucilagenous

• Sheets, flakes or coarse powder

• 45-60 cm long and 10-15 cm wide

Chemical constituents

• Calcium salts of sulphuric acid ester of a carbohydrate complex

• Sulphonated polysaccharides - galactose, uronic acid - partly esterified

• Agarose - gel strength of agar - galactose, anhydro L galactose, cellulose and nitrogen containing substance

• Agaropectin - viscosity of agar

• Traces of proteins

Chemical tests

• Cold water: Insoluble

• Hot water: dissolves and forms stiff gel on cooling

• Fehlings test- Red precipitate

• Sample + Dil Hcl, add Barium chloride solution: White precipitate

• With ruthemium red solution: Pink mucilage

• With tannic acid solution: No buff coloured precipitate

• With N/50 iodine solution: No blue or brown colour

• With alcoholic potassium hydroxide solution: Canary yellow colour

Uses

• Culture media

• Emulsifying agent

• Bulk laxative

• Jelly

Adulterants

• Gelatin

Substitutes

• Danish agar

• Indian agar

Summary

• Agar is the dried gelatinous substance from variuos red algae, contains agarose and agaropectin. Agar gives positive test with ruthenium red for mucilage

• Agarose is responsible for gel strength and agaropectin for viscosity