This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The name agar-agar is now usually applied to a gelatinous substance prepared in Japan from various species of Gelidium viz. G. elegans, Kutz., G. Amansii, Kutz. (N.O. Rhodophyceoe). The name is also applied to various algae, e.g. Gracilaria lichenoides, Greville (Ceylon agar-agar), Eucheuma spinosum, Agardh (Macassar agar-agar), etc.
Commercial agar-agar is made by boiling the carefully washed algae with water, straining with pressure, cooling, cutting the jelly into strips and slowly drying.
Japanese agar-agar occurs in transparent strips about 6 dm. long and about as thick as a straw, or in yellowish white pieces about 3 dm. long, 2.5 to 25 mm. thick and more than 25 mm. wide. The decoction (1 to 200) solidifies on cooling to a jelly which melts at a much higher temperature than a similar jelly prepared from gelatin. In cold water it swells but does not dissolve.
Agar-agar consists chiefly of the carbohydrate gelose which is converted by boiling with dilute sulphuric acid into galactose. It always contains skeletons of diatoms (e.g. Arachnoidiscus ornatus) by which its presence in preserves can be readily detected.
Agar-agar is largely used for the preparation of bacteriological culture media, the high melting-point (40°) of the jelly rendering it for this purpose particularly suitable. It passes through the intestinal canal almost unchanged, but absorbs water during its passage and promotes peristalsis.