When oil of peppermint is cooled to -4° C., it sometimes deposits colourless hexagonal crystals of Peppermint Camphor, C₁₀H₁₉OH, called also Menthol. We have never observed it, nor are we aware that menthol has been noticed in America, but it is largely afforded by eastern mints, and found in commerce under the name of Chinese or Japanese Oil of Peppermint,[1770] either liquid, and easily depositing the camphor, or also forming a crystalline mass impregnated with the liquid oil.
Pure menthol has the exquisite odour and taste of peppermint; it forms hexagonal crystals, melting at 42° C., and boiling at 212° C. By distilling menthol with P₂O₅ it yields menthene, C₁₀H₁₈, a levogyrate liquid, boiling at 163°, the peculiar odour of which reminds of peppermint.[1771] The Chinese crystallized oil of peppermint has sometimes a bitterish after-taste and an odour similar to that of spearmint, but by recrystallization it assumes the pure flavour.
The liquid part of the oil of peppermint has not yet been thoroughly investigated; it appears to consist chiefly of the compound C₁₀H₁₈O. Upon the liquid portions depend the remarkable colorations which the oil of peppermint is capable of assuming. If 50 to 70 drops of the crude oil are shaken with one drop of nitric acid, sp. gr. about 1·2, the mixture changes from faintly yellowish to brownish, and, after an hour or two, exhibits a bluish, violet or greenish colour; in reflected light, it appears reddish and not transparent. The colour thus produced lasts a fortnight. We have thus examined the various samples of peppermint oil at our command, and may state that the finest among them assume the most beautiful coloration and fluorescence, which, however, shows very appreciable differences. An inferior oil of American origin was not coloured; and a very old sample of an originally excellent English oil was likewise not coloured by the test. Menthol is not altered when similarly treated.[1772] The nitric acid test is not capable of revealing adulterations of peppermint oil, for the coloration takes place with an oil to which a considerable quantity of oil of turpentine has been added.
Remarkable colorations of a different hue are also displayed by the various kinds of oil of peppermint if other chemical agents are mixed with it. Thus green or brownish tints are produced by means of anhydrous chloral; the oil becomes bluish or greenish or rose-coloured if shaken with a concentrated solution of bisulphite of sodium. It is worthy of note that oils of different origin, which cannot be distinguished by means of nitric acid, exhibit totally different colorations if mixed with either of the liquids just named, or with vapour of bromine. This behaviour may be of some use in the examination of commercial sorts of peppermint oil.
As to bisulphite of sodium, it yields a solid compound with certain kinds of peppermint oil, which we have not yet examined.
Production and Commerce—In several parts of Europe, as well as in the United States, peppermint is cultivated on the large scale as a medicinal plant.
In England the culture is carried on in the neighbourhood of Mitcham in Surrey, near Wisebeach in Cambridgeshire, Market Deeping in Lincolnshire, and Hitchin in Hertfordshire.
At Mitcham in 1850 there were about 500 acres under cultivation; in 1864 only about 219 acres.[1773] At Market Deeping there were in 1871 about 150 acres cropped with peppermint. The usual produce in oil may be reckoned at 8 to 12 lb. per acre. The fields of peppermint at Mitcham are level, with a rich, friable soil, well manured and naturally retentive of moisture. The ground is kept free from weeds, and in other respects is carefully tilled. The crop is cut in August, and the herb is usually allowed to dry on the ground before it is consigned to the stills. These are of large size, holding 1000 to 2000 gallons, and heated by coal; each still is furnished with a condensing worm of the usual character, which passes out into a small iron cage secured by a padlock, in which stands the oil separator. The distillation is conducted at the lowest possible temperature. The water that comes over with the oil is not distilled with another lot of herb, but is for the most part allowed to run away, a very little only being reserved as a perquisite of the workmen. The produce is very variable, and no facilities exist for estimating it with accuracy.[1774] It is however stated that a ton of dried peppermint yields from 2½ to 3½ pounds of oil, which equals 0·11 to 0·15 per cent. But we have been assured by a grower at Mitcham that the yield is as much as 6 pounds from a ton, or 0·26 per cent.
At Mitcham and its neighbourhood two varieties of peppermint are at present recognized, the one being known as White Mint, the other as Black Mint, but the differences between the two are very slight. The Black Mint has purple stems; the White Mint, green stems, and as we have observed, leaves rather more coarsely serrated than those of the Black. The Black Mint is more prolific in essential oil than the White, and hence more generally cultivated; but the oil of the latter is superior in delicacy of odour and commands a higher price. White Mint is said to be principally grown for drying in bundles, or as it is termed “bunching.”
Peppermint is grown on a vastly larger scale in America, the localities where the cultivation is carried on being Southern Michigan, Western New York, and Ohio. In Michigan where the plant was introduced in 1855, there were in 1858 about 2100 acres devoted to its growth, all with the exception of about 100 acres being in the county of St. Joseph, where there are about 100 distilleries. The average produce of this district was estimated in 1858 at 15,000 lb.; but the yield fluctuates enormously, and in the exceptionally fine season of 1855 it was reckoned at 30,000 lb. We must suppose that it is sometimes much larger, for we have been informed by Mr. H. G. Hotchkiss, of Lyons, Wayne County, State of New York, one of the most well-known dealers, in a letter under date Oct. 10, 1871, that the quantity sent out by him in the previous year reached the enormous amount of 57,365 lb. It is further stated by the official statistics of Hamburg for the year 1876 that this port received 25,840 lb. of peppermint oil from the United States and 14,890 lb. from Great Britain.