Ol. Menth. Pip. [Peppermint Oil]
"A volatile oil distilled from the flowering plant of Mentha piperita Linné (Fam. Labiatae), rectified by steam distillation, and yielding not less than 5 per cent. of esters, calculated as menthyl acetate [C10H19C2H3O2 = 198.18], and not less than 50 per cent. of total menthol [C10H19OH = 156.16], free and as esters. Preserve it in well-stoppered, amber-colored bottles, in a cool place, protected from light." U. S. "Oil of Peppermint is the oil distilled from fresh flowering peppermint, Mentha piperita, Sm., and rectified, if necessary." Br.
Huile volatile de Menthe poivree, Fr. Cod.; Essence de Menthe poivree, Fr.; Oleum Menthae piperitae, P. G.; Pfefferminzöl, G.; Essenza di menta, It.; Esencia de menta piperita, Sp.
Peppermint varies exceedingly in the quantity of oil which it affords. Four pounds of the fresh herb yield, according to Baume, from a drachm and a half to three drachms of the oil. Zeiler gives as the product of the fresh herb, from 0.37 to 0.68 per cent., of the dried 1.14 per cent. The yield is generally less than 1 per cent. This oil is largely distilled in the States of Michigan, Indiana, and New York. For a valuable account of its method of production in Michigan, by A. M. Todd, see A. J. P., 1888, 328. The oil is also largely produced in Japan. (See illustrated paper, C. D., 1896, 601.) Gildemeister and Hoffmann (Aetherische Oele, p. 836) estimate the world's production of peppermint oil at 175,000 kilos, of which the United States furnishes 90,000 kilos, and Japan 70,000 kilos. It is officially described as "a colorless liquid, having a strong odor of peppermint, and a pungent taste, followed by a sensation of cold when air is drawn into the mouth. It is soluble in 4 volumes of 70 per cent. alcohol, showing not more than slight opalescence and no separation of oil globules. Specific gravity: 0.896 to 0.908 at 25° C. (77° F.). The optical rotation varies between -23° and -33° in a 100 mm. tube at 25° C. (77° F.). Distil about 1 mil from 25 mils of the Oil and pour the distillate on 5 mils of mercuric chloride T.S.; a white film does not form at the zone of contact within one minute (dimethyl sulphide found in non-rectified oils)." U. S.
Upon cooling or long standing it deposits a stearopten. Berzelius stated that at -13.3° C. (--8° F.) the oil deposits small capillary crystals. These are called peppermint camphor or menthol, C10H19OH. A complete summary of the composition of American peppermint oil, on the authority of Power and Kleber, given in Schimmel & Co.'s Report for April, 1897, shows the following constituents: acetaldehyde, isovaleraldehyde, dimethyl sulphide, amyl alcohol, isovaleric acid, pinene, phellandrene, cineol, limonene, menthone, C10H18O, menthol, the acetate and iso-valerate of menthyl, a-lactone, C10H16O2, and cadinene. Menthol, its oxidation product menthone, and the acetic and isovaleric esters of menthyl have been identified as present in English and French peppermint oils, which probably also contain most of the other constituents mentioned above. Menthol and its esters are regarded as the main constituents of oil of peppermint and an official assay for determining the percentage of menthol will be found above. The dimethyl sulphide and some of the other constituents are objectionable from the flavoring standpoint, and are largely removed by the steam distillation which is required for rectification. The polariscope has been shown to be an uncertain guide in determining the quality of oil of peppermint. (A. B. Stevens, Proc. A. Ph. A., 1888, p. 97.)
Flückiger discovered that from 50 to 70 drops of peppermint oil, shaken with one drop of nitric acid, sp. gr. 1.2, turn faintly yellowish brown, and after an hour or two a most beautiful blue-violet or greenish-blue by transmitted light, or copper color by reflected light. Either a greater amount of acid or heating hastens the reaction. The color is very persistent. The presence of 5 per cent. of turpentine does not interfere with the reaction. (P. J., Feb., 1871.)
Since menthol has been prepared largely from American peppermint oil, much of the latter has been put upon the market deprived of its menthol. To detect this fraud, Fritzsche Brothers recommend that a test tube partially filled with the oil and corked should be placed in a freezing mixture of ice and salt for ten or fifteen minutes. At the end of that time, if the oil has not been tampered with, it will have become cloudy, thick, or of a jelly-like consistence If then four or five small crystals of menthol be added, and the tube be replaced in the freezing mixture, the oil will in a very short time form a solid mass of crystals. For another method of testing dementholized oil, see a paper by E. C. Federer, Ph. Era, 1887, 36; also 1887, 97.
Power and Kleber first proposed (Ph. Rund., 1894, 157) to estimate the menthol in oil of peppermint by acetylizing a sample and saponifying an exactly weighed portion of the acetylized oil after washing, drying, and filtering the same. Kleber (A. J. P., 1897, 192) proposed to take instead a weighed quantity of the oil, acetylize it, and then saponify the product after washing and neutralizing it. In Schimmel & Co.'s Report for October, 1897, Kleber criticizes Kebler's modification, and claims greater accuracy for the original method. Of course, this determination gives the total menthol; a determination of the combined menthol is had by titrating with alcoholic alkali.
Peppermint oil has been variously mixed and adulterated with the following substances: alcohol, oil of turpentine, oil of copaiba, oil of erigeron (from the erigeron plant growing with the peppermint plant, and being collected with it for distillation), oil of eucalyptus, oil of pennyroyal, and others. Most of these are detectable by the official tests, or when present lower the assay strength, or are readily recognized by the odor or taste, which they communicate to the oil. For further details concerning some of these adulterants and their detection, see U. S. D., 19th ed., p. 854, 855.
Chinese Oil of Peppermint has not found its way to any extent into commerce, yet it has at times attracted much attention. According to John Mackey, some of it has reached London in cylindrical tin canisters, and it is to be obtained in San Francisco. Some specimens resemble the ordinary oil, but become solid in cold weather; others remain liquid and show no tendency to deposit a camphor, while others are solid crystalline masses at all temperatures. It seems probable that the original Chinese oil contains a great excess of menthol, and constitutes the variety first spoken of, and that the solid and liquid oils are prepared by separating it into its two constituents. The solid oil Fluckiger states not to differ from American menthol. (See P. J., v, 366, 825.)
Uses.—Oil of peppermint possesses the physiological properties and therapeutic virtues of menthol, and may be used for practically all those conditions in which menthol is of service. It is generally preferred for internal use because of its more pleasant taste. It is an excellent carminative and gastric stimulant, and is widely employed in flatulence, nausea, and gastralgia. It adds to a stimulating effect upon the alimentary canal an antiseptic and local anesthetic influence. As a local application for coryza it may be used in strengths of from five to ten minims to an ounce of liquid petrolatum or olive oil. Incorporated into a lozenge it offers a pleasant and efficient antiseptic and anesthetic in pharyngitis. Internally it may be given either rubbed up with sugar and then dissolved in water, or more commonly in the form of the spirit. It is extremely popular as a flavoring agent.
Dose, one to five minims (0.06-0.3 mil).