1911 Encyclopædia Britannica/Peppermint

PEPPERMINT, an indigenous perennial herb of the natural order Labiatae, and genus Mentha (see Mint), the specific name being Mentha piperita, is distinguished from other species of the genus by its stalked leaves and oblong-obtuse spike-like heads of flowers. It is met with, near streams and in wet places, in several parts of England and on the European continent, and is also extensively cultivated for the sake of its essential oil in England,^[1] in several parts of continental Europe, and in the United States. Yet it was only recognized as a distinct species late in the 17th century, when Dr Eales discovered it in Hertfordshire and pointed it out to Ray, who published it in the second edition of his Synopsis stirpium britannicarum (1696). The medicinal properties of the plant were speedily recognized and it was admitted into the London Pharmacopoeia in 1721, under the name of Mentha piperitis sapore.

Fig. 1.—Mentha piperita.

a, Flowering branch (about 1/2 nat. size); b, flower showing form of
calyx teeth (enlarged).

Two varieties are recognized by growers, the white and the black mint. The former has purplish and the latter green stems; the leaves are more coarsely serrated in the white. The black is more generally cultivated, probably because it is found to yield more oil, but that of the white variety is considered to have a more delicate odour, and obtains a higher price. The white is the kind chiefly dried for herbalists. The flavour varies to a slight extent even with particular plots of land, badly drained ground being known to give unfavourable results both as to the quantity and quality of the oil. That of the Japanese and Chinese oil also differs slightly from the English, and is thus distinguishable by experts. In America the oil is liable to be injured in flavour by aromatic weeds which grow freely among the crop, the most troublesome of these being Erigeron canadense, and Erechthites hieracifolia. When pure the oil is nearly colourless and has an agreeable odour and powerful aromatic taste, followed by a sensation of cold when air is drawn into the mouth. It has a specific gravity of 0·84 to 0·92, and boils at 365° F. Mitcham oil, when examined by polarized light in a column 50 mm. long, deviates from 14·2° to 10·7° to the left, the American 4·3°. When oil of peppermint is cooled to 4° C. it sometimes deposits colourless hexagonal prisms of menthol, C₁₀H₂₀O, which are soluble in alcohol and ether, almost insoluble in water, and fusible at 92° F. The oil consists chiefly of menthol and a terpene called menthene, C₁₀H₁₈. Oil of peppermint is often adulterated with a third part of rectified spirit, which may be detected by the milkiness produced when the oil is agitated with water. Oil of rosemary and rectified oil of turpentine are sometimes used for the same purpose. If the oil contains turpentine it will explode with iodine.

Fig. 2.-Mentha arvensu, var. piperascens.

a, Flowering branch reduced), b, calyx showing form of teeth (enlarged).

If quite pure it dissolves in its own weight of rectified spirits of wine. Peppermint oil is largely distilled at Canton, a considerable quantity being sent to Bombay, also a large quantity of menthol. The species cultivated in the neighbourhood of Canton, is Mentha arvensis, var. glabrata. Peppermint is chiefly cultivated in the province of Kiang-si; and according to native statements as much as 40 piculs of oil of peppermint are sent annually to ports on the coast. In Japan also the distillation of oil of peppermint forms a considerable industry, the plant cultivated being M. arvensis, var. piperascens. The oil, under the name of hakka no abura, is exported from Hiogo and Ozaka, but is said to be frequently adulterated. The menthol is obtained by subjecting the oil to a low temperature, when it crystallizes out and is separated. The two varieties of M. arvensis just named yield much more menthol than M. piperita. It is remarkable, however, that the M. arvensis, var. javanica, growing in Ceylon, has not the flavour of peppermint but that of garden mint, while typical form of M. arvensis grown in Great Britain has an odour so different from peppermint that it has to be carefully removed from the field lest it should spoil the flavour of the peppermint oil when the herb is distilled. M. incana cultivated near Bombay as a herb, also possesses the flavour of peppermint. In the form in Which menthol is imported it bears some resemblance to Epsom salts, with which it is sometimes adulterated.

The volatile oil of Mentha piperita is a valuable and widely used drug. Its chief constituents are menthol and menthene, which is a liquid terpene. The British pharmacopoeia contains two preparations of this oil, the Aqua menthae piperitae and the Spiritus menthae piperitae. The oil has the characters of its class, with certain special features. Its local anaesthetic action is exceptionally strong it is also powerfully antiseptic. These two properties make it valuable in the relief of toothache and in the treatment of carious cavities in the teeth. They also render the drug valuable in certain forms of dyspepsia and in colic generally, “soda-mint lozenges” being a familiar form. The characteristic anti-spasmodic action of the volatile oils is perhaps more marked in this than in any other oil, and greatly adds to its power of relieving pains arising 1n the alimentary canal. The volatile oil of spearmint is also official in Great Britain and the United States, being given in the same doses and for the same purposes as oil of peppermint. It is of less value medicinally, not containing any appreciable quantity of menthol, the place of which is taken in the oleum menthae viridis—the pharmacopoeia name—by carvone, C₁₀H₁₄O, found in caraway oil, and isomeric w1th thymol.

The following mode of cultivation of peppermint is adopted at Market Deeping. A rich friable soil, retentive of moisture, is selected, and the ground is well tilled 8 to 10 in. deep. The plants are propagated in the spring, usually in April and May. When the young shoots from the crop of the previous year have attained a height of about 4 in. they are pulled up and transplanted into new soil. They grow vigorously the first year, and throw out numerous stolons on the surface of the ground. After the crop has been removed these are allowed to harden or become woody, and then farm-yard manure is scattered over the field and ploughed in. In this way the stolons are divided into numerous pieces, and covered with soil before the frost sets in. If the autumn is wet they are liable to become sodden, and rot, and the next crop fails. In the spring the fields are dressed with Peruvian guano. In new ground the peppermint requires hand-weeding two or three times, as the hoe cannot be used without injury to the plants. Moist heavy weather in August is apt to cause the foliage to drop off and leave the stems almost bare. In these circumstances rust (Puccinia menthae) also is liable to attack the plants. This is prevented to a certain extent by a rope being drawn across the plants, by two men walking in the furrows, so as to remove excessive moisture. The average yield of peppermint is about 165 cwt. per acre. The first year’s crop is always cut with the sickle to prevent injury to the stolons. The herb of the second and third year is cut with scythes, and then raked by women into loose heaps ready for carting. The field is then gleaned by boys, who add what they collect to the heaps. The plants rarely yield a fourth crop on the same land. The harvest usually commences in the beginning or middle of August, or as soon as the plants begin to flower, and lasts for six weeks, the stills being kept going night and day. The herb is carted direct from the field to the stills, which are made of copper, and contain about 5 cwt. of the herb. Before putting the peppermint into the still water is poured in to a depth of about 2 ft., at which height a false bottom is placed, and on this the herb is thrown and trodden down by men. The lid, which fits into a water-joint, is then let down by pulleys and fastened by two bars, any excess of pressure or temperature being indicated by the water that is ejected at the joint. The distillation is conducted by the application of direct heat at the lowest possible temperature, and is continued for about four and a half hours. When this operation is completed, the lid is removed and a rope is attached to a hook on the false bottom, which, as well as the herb resting on it, is raised bodily by a windlass and the peppermint carried away in the empty carts on their return journey to the fields, where it is placed in heaps and allowed to rot, being subsequently mixed with the manure applied in the autumn as above stated, The usual yield of oil, if the season be warm and dry, is said to be 1 oz. from 5 ℔ of the fresh flowering herb, but, if wet and unfavourable, the product is barely half that quantity. The yield of a charge of the still is estimated at from 1 ℔ 12 oz. to 5 ℔. The oil improves in mellowness even if kept as long as ten or fourteen years. The green colour sometimes present in the oil is stated to be due to a quantity of water larger than necessary having been used in the distillation on the other hand, if the herb be left in the still from Saturday to Monday, the oil assumes a brown tint.

In France peppermint is cultivated on damp rich ground at Sens, in the department of the Yonne. In Germany it is grown in the neighbourhood of Leipzig, where the little town of Colleda produces annually as much as 40,000 cwt. of the herb. In the United States peppermint is cultivated on a most extensive scale, chiefly in south-west Michigan, the west districts of New York state, and Ohio. The yield averages from 10 to 30 ℔ per acre. In Michigan the plant was introduced in 1855.

1. Near Mitcham in Surrey, Wisbech in Cambridgeshire, Market Deeping in Lincolnshire and Hitchin in Hertfordshire.