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