Agar

• BPAgar
• JPAgar
• Ph. Eur.Agar
• USP–NFAgar

Agar-agar; Bengal isinglass; Ceylon isinglass; Chinese isinglass; gelose; Japanese isinglass; E406

Agar (polysaccharide mixture of agarose and agaropectin from red algae) · [9002-18-0]

Polymer, mixture or material of variable composition — no single molecular formula applies.

Gelling agent, viscosity modifier

Typical concentrations: — · Dosage forms: Liquid, Semi-solid

Used as a gelling, suspending, emulsion-stabilising and thickening agent, in suppositories and surgical lubricants, and occasionally as a tablet binder or disintegrant. It is also used as a bulk laxative, and it is the standard gelling agent for microbiological culture media (usually around 1–2%). In foods it is a gelling agent and stabiliser.

Supplied as colourless to pale yellow translucent strips, flakes or granules, or as a powder. It is tough when damp and brittle when dry, practically odourless or with a slight odour, and has a mucilaginous taste. Agar consists mainly of agarose, a neutral gelling galactan, together with agaropectin, a more charged sulfated fraction.

Gelling behaviour

dilute hot solutions (from about 0.5–1.5%) set to firm gels on cooling to about 32–40 °C; the gel does not melt again until heated to about 85 °C or above

Loss on drying

not more than 20% w/w (pharmacopoeial limit)

Solubility

soluble in boiling water; practically insoluble in cold water, in which it swells, and in ethanol

Swelling

absorbs many times its weight of cold water

Agar gels are stable at room temperature, but prolonged heating, especially at acidic pH, hydrolyses the polymer and weakens the gel. Aqueous preparations support microbial growth and need a preservative. Store in a well-closed container in a cool, dry place.

Precipitated from solution by ethanol and by tannic acid. High electrolyte concentrations partially dehydrate the gel and reduce viscosity, and strong acids degrade it on heating.

Extracted with hot water from red seaweeds (class Rhodophyceae), mainly species of Gelidium and Gracilaria. The extract is filtered, gelled, purified by freeze–thaw or pressing, then dried and milled.

Widely used in food and oral medicines and regarded as non-toxic; it is not digested and passes through the gut. Taken as a bulk laxative without enough water it can swell and cause oesophageal or intestinal obstruction.

Observe normal precautions for the quantity handled. Eye protection and a dust mask are recommended when handling the powder.

GRAS listed. Accepted as a food additive in Europe (E406). Used in non-parenteral medicines licensed in Europe.

Alginic acid; carrageenan; sodium alginate.

The marked hysteresis between gelling and melting temperatures is the property that makes agar useful: gels made with it stay solid at body and incubation temperatures. Gel strength varies with the seaweed source and grade, so the grade should be specified when gel firmness matters.