Agar. U. S. Agar.
"The dried mucilaginous substance extracted from Gracilaria (Sphaerococcus) lichenoides Greville and other marine algae growing along the eastern coast of Asia, particularly several species of Gelidium, or Gloiopeltis (Class Rhodophyceae)." U. S.
Quite a number of the algae belonging to the Rhodophyceae, growing on the coast of Southern and Eastern Asia, contain large quantities of mucilage which is extracted and sold under the name of agar-agar. The most important species are those recognized by the U. S. Pharmacopoeia. The algae are collected, spread out upon the beach until they are bleached and then dried. They are then boiled with water and the mucilaginous solution strained through a cloth. The filtrate is allowed to harden and thoroughly dry in the sun. The algae are usually collected during the summer and fall, bleached and dried, but the process of the manufacture of agar-agar does not take place until cold weather and usually extends from November to February.
The following varieties of agar are known:
- —Ceylon Agar-agar, consisting chiefly of Gracilaria lichenoides, Ag., the alga used by the Hirundo esculenta in the formation of its edible nest.
- —Macassar Agar-agar, coming from the straits between Borneo and Celebes, consisting of impure Eucheuma spinosum, Ag., incrusted with salt.
- —Japanese Agar-agar, known as Japanese isinglass, derived from several algae, especially Sphaerococcus compressus, Ag., Gloiopeltis tenax, J. Ag., Gelidium corneum, Lam. and G. cartilagineum Gaill. It occurs in European commerce either in transparent pieces, two feet long and as thick as a straw, prepared in Singapore by putting the algae named in hot water, or, more frequently, in yellowish-white masses, a foot long and upward of an inch in width.
It is the latter kind of agar-agar that is suitable for the culture of bacteria, and is employed in medicine. (P. J., 1885, 188.)
Morin has investigated the gelose of Payen, contained in the agar-agar. When a solution of gelose is cooled, even that of 1 in 500 parts of water, a colorless, transparent, and stiff jelly is obtained, which, when heated with moderately strong nitric acid, yields mucic and oxalic acids. It dissolves on heating with acidulated water without yielding a jelly on cooling.
Gelose leaves 3.88 per cent. of ash, and when air-dried contains 22.85 per cent. of moisture. When dissolved there also separates out a flocculent mass amounting to 1.9 per cent. Alcohol precipitates gelose, but it cannot be obtained pure in this manner, as the precipitate contains some ash. (C. E. A. S., No. 90, 921-926.)
Under the name of gelosine a mucilaginous substance, extracted from a Japanese alga, has entered commerce in the form of dry, whitish leaves. Gelosine is soluble in alcohol and water, and is said when wet to gradually contract and expel water and the medicinal substances which it may contain. It has been proposed as a pharmaceutical basis for various preparations for local use. (See B. M. J., vol. ii, 1886.) Glycerin suppositories have been made with agar-agar as a vehicle, but they contain only 70 per cent. of glycerin as compared to 90 per cent. in the official suppositories made with sodium oleate.
Properties.—Agar-agar occurs "mostly in bundles from 4 to 6 dm. in length, consisting of thin, translucent, membranous, agglutinated pieces from 4 to 8 mm. in width; externally yellowish-white or brownish-white; tough when damp, brittle when dry; odor slight; taste mucilaginous. A fragment mounted in water and examined under the microscope gradually becomes more transparent, showing a granular structure and a few diatoms, notably the frustules of Arachnoidiscus Ehrenbergii Baillon, which are disk-shaped and from 0.1 to 0.2 mm. in diameter, and also fragments of the spiculas of sponges; upon the addition of iodine some of the granules or hyphal portions are colored bluish-black. Insoluble in cold water, but slowly soluble in hot water. A solution made by boiling 0.1 Gm. of Agar in 100 mils of water, upon cooling yields no precipitate upon the addition of tannic acid T.S. (gelatin), and does not produce a blue color upon the addition of iodine T.S. (starch). Boil 1 part of Agar for about ten minutes with 100 parts of water, and replace the water lost by evaporation; it yields a stiff jelly upon cooling. The powder is pale buff; when mounted in water and examined under the microscope it shows transparent, more or less granular, striated angular fragments, occasionally containing frustules of diatoms; with iodine T.S., fragments for the most part are colored bright red, certain more less definite areas being stained bluish-black. Agar yields not more than 5 per cent. of ash." U. S.
To detect agar in jams, jellies, etc., in which it is often used as an adulterant, it is usually considered necessary to ash the sample and examine the acid insoluble ash for presence of the characteristic diatoms. Albert Schneider (Pac. Pharm., 1912, p. 35) states that the ashing is unnecessary and often destructive of the diatoms and that they may be collected by dissolving 10 Gm. of the sample in 200 mils of distilled water and centrifuging for thirty minutes, after which the sediment may be placed on a microscopic slide and examined.
Uses.—Under the name of agar-agar a jelly-like substance has been used as a culture medium by bacteriologists for many years. (See Diagnostical Reagents, Part III.) Although agar contains sixty per cent. of carbohydrates, according to Saiki (J. B. C., 1906), the human digestive tract is able to utilize but a very small percentage of the food value. Its therapeutic importance depends upon the ability of the dry agar to absorb and retain moisture. Being indigestible, it passes through the intestinal tract, swelling up somewhat, owing to the absorption of water from the stomach, and gives bulk to the intestinal contents. In other words, it acts mechanically in an analogous manner to the cellulose of vegetable foods, and aids in maintaining the regularity of the bowel movements. It has been widely used in the treatment of chronic constipation. Of itself, when there is more or less atony of the intestinal muscles, it does not originate peristaltic movements, and therefore is frequently combined with small doses of cascara, or one of the other vegetable cathartics. It is best administered cut up in small pieces, and eaten like a cereal with the addition, if desirable, of cream and sugar. Ordinarily, from two to four drachms (7.7-15.5 Gm.) of the dry agar may be administered once a day.