Oleoresins are those substances obtained from vegetable medicines by means of ether (sometimes alcohol, etc.), which consist principally of a fixed or volatile oil and a resin. In some cases the resin will be held in solution in the oil, while in others it will be deposited upon standing, and will require agitation to again diffuse and suspend it in the oil. A third case occurs in which the oil and resin form a more or less permanent mixture, having the consistence of a very soft extract. The resins in these preparations, like the essential oils, are generally mixtures of two or more resins, but which on account of their unequal solubility in different menstrua, may frequently be isolated from each other. Often the resins are formed by the oxidation of the essential oils contained in the plants, or of a certain portion of these oils. In many instances, especially with the oleoresins obtained from alcoholic tinctures, it will be better not to distill off the remaining third of the alcohol from the water, until the oleoresin has spontaneously precipitated and been separated from the liquid, because an elevated or too prolonged heat will injure the preparation. All these oleoresins should be kept in well-stopped vessels.
Mr. N. H. Rittenhouse, in the process for obtaining oleoresins, recommends the use of 1 ½ ounces of ether for each ounce of drug treated, followed by sufficient benzin to make the amount of percolate equal to the amount of ether employed, as both economical and satisfactory (Proc. Amer. Pharm. Assoc., 1866, p. 208). The expense may be considerably reduced by recovering a part of the ether distilled for employment in future operations. The suggestion of Mr. George M. Beringer (Amer. Jour. Pharm., 1892, p. 145), to use acetone in place of ether can not but be regarded favorably. It is equally as efficient, has a higher boiling point than ether, and is much less expensive.