Oleum Menthae Piperitae (U. S. P.)—Oil of Peppermint.

Preparations: Peppermint Water - Spirit of Peppermint
Related entries: Menthol (U. S. P.)—Menthol - Mentha Piperita (U. S. P.)—Peppermint - Mentha Viridis (U. S. P.)—Spearmint - Oleum Menthae Viridis (U. S. P.)—Oil of Spearmint

A volatile oil distilled from Mentha piperita, Smith (Nat. Ord.—Labiatae).
"It should be kept in well-stoppered bottles in a cool place"—(U. S. P.).

History and Preparation.—The three most important peppermint-growing countries are the United States, Japan, and England, the Japanese oil being derived from the species Mentha arvensis, Linné, var. piperascens, Holmes. Distillation in the United States began in 1816, in Wayne county, in the State of New York; since 1835, peppermint has been grown and oil distilled therefrom in Michigan. Until 1846 distillation was carried out by means of simple copper stills heated by direct fire; since that date distillation by steam has become the rule. Indiana also distills large quantities of oil. Much care is now taken to exclude from the peppermint fields such weeds as Erechtites (fireweed), Erigeron, and Hedeoma (pennyroyal), to prevent undesirable contamination with the oils from these plants. The total production of oil of peppermint has reached enormous proportions, the United States alone, in 1897, producing 251,000 pounds. Minor quantities of oil of peppermint are produced in Russia, Germany, Italy, Norway, etc. (For much interesting detail regarding the history and statistics of this oil, see Dr. Frederick Hoffmann, in Die Etherischen Oele, pp. 825-836.) The yield from German herb (fresh) is reported by Schimmel & Co. to be 0.1 to 0.25 per cent; from dried herb, 0.7 per cent.

Description.—The U. S. P. describes oil of peppermint as "a colorless, or yellowish, or greenish-yellow liquid, becoming darker and thicker by age and exposure to the air, having the characteristic, strong odor of peppermint, and a strongly aromatic, pungent taste, followed by a sensation of cold when air is drawn into the mouth. Specific gravity, 0.900 to 0.920 at 15° C. (59° F.). The oil does not fulminate with iodine. It forms a clear solution with an equal volume of alcohol, becoming turbid when somewhat further diluted, and is soluble in all proportions, in carbon disulphide, and in glacial acetic acid. The alcoholic solution of the oil is neutral to litmus paper"—(U.S. P.). The oil is optically laevo-rotatory varying in American oil from -25° to -33°. The better grades of oil from Wayne county, New York, do not form a clear solution with 3 to 5 volumes of 70 per alcohol, while the Michigan (western oils), the English and the Japanese oils do. Schimmel & Co. suggest that this difference may be due to the practice of previously drying the herb, which yields an oil not soluble in diluted alcohol, while that from fresh herb is soluble (Gildemeister and Hoffmann, loc. cit., p. 838). When American oil of peppermint is subjected to cold, crystals of menthol fall out (see Tests below). The Japanese oil is semisolid at ordinary temperature, owing to the large quantity of menthol it contains.

Tests.—Adulterations of the oil with alcohol and oil of turpentine are not infrequent; the latter may be known by the turpentine odor, by the imperfect solution it forms with alcohol, and by its fulmination when iodine is added to it; the former may be known by the formation of a dirty-white liquid when an equal volume of water is added to it. Sometimes adulteration is practiced by removing part of the menthol by freezing. The following are the official tests, which include the characteristic color reactions of the oil of peppermint: "If 5 drops of the oil be added to 1 Cc. of glacial acetic acid, and the mixture gently warmed, the liquid will assume a blue color, with a red fluorescence. If 2 Cc. of the oil be mixed with 1 Cc. of glacial acetic acid, and 1 drop of nitric acid added, the liquid will soon acquire a green, greenish-blue, blue, or violet tint with a copper-red fluorescence. If 1 Cc. of the oil be dissolved in 5 Cc. of alcohol, 0.5 Gm. of sugar, and 1 Cc. of hydrochloric acid added and the mixture gently heated, a deep-blue or violet color will gradually be produced. If to 5 Cc. of nitric acid 1 drop of the oil be added, and the mixture gently agitated, and allowed to stand for about 3 hours, it should have a yellowish, but not a bright red color (absence of oil of camphor and of oil of sassafras). If a portion of the oil, contained in a test-tube, be placed in a freezing mixture of snow (or pounded ice) and salt for 15 minutes, it should become cloudy and thick, and after the addition of a few crystals of menthol, being still exposed to cold, it should soon form a crystalline mass (distinction from dementholized oil). When heated on a water-bath, in a flask provided with a well-cooled condenser, the oil should not yield a distillate having the characters of alcohol"—(U. S. P.).

Chemical Composition.—The chief constituent of peppermint oil is menthol, known also as mint stearopten, or mint camphor (see Menthol). New York oil contains 50 to 60 per cent total menthol, of which 40 to 45 per cent are free, and the rest combined in the form of ester. It also contains 12 per cent of menthone. Michigan oil contains about 48 to 58 per cent total menthol, of which 43 to 50 per cent are free menthol. By distilling menthol with phosphoric anhydride, a colorless liquid of an agreeable odor, menthene (C10H18) results. This hydrocarbon has been said to occur in Russian oil of peppermint; the American oil does not contain it. According to a detailed research on American peppermint oil by F. B. Power and C. Kleber (Pharm. Rundschau, 1894, p. 157), this oil contains the following substances: (1) Acetaldehyde; (2) isovaleric aldehyde; (3) amyl alcohol; (4) free acetic, and (5) isovalerianic acids; (6) pinene; (7) phellandrene; (8) l-limonene; (9) cadinene; (10) cineol; (11) a lactone (C10H16O2); (12) menthone; (13) menthol; (14) menthyl-acetate; (15) menthyl-isovalerianate; (16) menthylester of an acid, C8H12O2; (17) dimethyl sulphide (S[CH3]2). Substances 3 and 17 were additionally found in the laboratory of Schimmel & Co., in 1894 and 1896.

English oil of peppermint, according to Umney (Pharm. Jour. Trans., 1896, Vol. II, p. 123, and Vol. III, p. 103), contained of total menthol, 63 to 66 per cent; menthol in the form of ester, 3 to 14 per cent; menthone, 9 to 11 per cent. Gildemeister and Hoffmann (loc. cit., p. 844) believe that the other constituents of the English oil will prove the same as those of the American oil above enumerated. The Japanese oil contains of total menthol, 70 to 91 per cent; of free menthol, 65 to 85 per cent. It does not give the above-described color reactions, or but very faintly, with glacial acetic acid.

Action, Medical Uses, and Dosage.—Oil of peppermint is a powerful diffusible stimulant, with carminative, antispasmodic, and antiemetic properties. It is much employed to relieve flatulence, gastrodynia, nausea, spasms of the stomach, and to cover the taste of other drugs. Externally, it is occasionally employed as a rubefacient and anodyne. It relieves the pain of burns, scalds, and toothache from carious teeth. In spray, it is useful to alleviate painful and inflamed fauces and tonsils, and by inhalation relieves many of the unpleasant symptoms of asthma and chronic bronchitis in old persons. It enters into a liniment, which at one time was a popular remedy for various painful local affections; it is composed as follows: Take of oil of olives, oil of peppermint, oil of turpentine, tincture of opium, alcohol, aqua ammoniae, each, 1 fluid ounce. Mix. To be applied 3 or 4 times a day. Peppermint oil is commonly used under the name of essence of peppermint, which is a tincture of it. The dose of the oil is 2 to 10 drops on sugar.

King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.