Chemistry of Essential Oils

LABIATE 231

production is in the districts of Ewell, Cheam, Carshalton, and Mitcham;
smaller quantities are also obtained in certain parts of Sussex, Kent,
Hertfordshire, and Lincolnshire, In England the plants are grown from
roots, not from seed. The land having been ploughed and manured
during the winter, the plants are dibbled in in May. A plantation lasts
from four to five years, the best output being in the second year. After
four or five years the land is not used again for mint growing for some
years.
The harvest begins early in September, when a red rust or " snuff"
appears on the leaves. After some slight drying the plants are distilled
for about six hours. The oil-yield per acre ranges from 8 up to 25 Ib.
In general, English oils have a specific gravity of about 0

• 900 to
  0 912, and an optical rotation of - 23° to - 32°, and contain from 60
  to 70 per cent, of menthol, or rather more than most American oils.
  The refractive index of the oil varies from 14600 to 1*4640. The oil is
  usually quite soluble in 3 to 4 volumes of 70 per cent, alcohol. Umney
  has examined samples of " black " and " white " mint oils comparatively,
  and gives the following figures for them :—

Black.

White

Specific

Gravity.

•9036

•9058

Rotation.

- 23-5°

- -33°

Menthol—

as Esters. Free.

3-7

13-6

59-4

51-9

Menthone.

11-8

9-2

Fluckiger and Power
1
isolated two laevo-rotatory terpenes from
English peppermint oil, but their purity is doubtful. It is probable that
they were mixtures of pinene, phellandrene, and limonene. A dextro-
rotatory sesquiterpenei(cadinene?) is also present, as well as menthone,
and menthol, both in the free state and as acetate and isovalerianate.

ITALIAN PEPPERMINT OIL.

A certain amount of peppermint oil is distilled in Italy from plants
whose botanical origin is not yet clearly settled. This oil, which is of
excellent quality, has the following characters:—

Specific gravity.

Optical rotation.

Refractive index

Menthol as esters

Total menthol

Menthone.

0-905 to 0-926

— 2° — 27°

1-4615 ',j 1-4685

3 to 11 per cent.

45 „ 65

8 „ 20

It is soluble in 3 volumes of 70 per cent, alcohol.

During the last few years, however, Mitcham plants have been

cultivated in Italy, and the resulting oil approximates, both in odour

and in general characters, much more closely to ordinary English dis-

tilled oil than the oils above described. This so-called Italo-Mitcham

oil is prepared at Messina and at Pancalieri, Piedmont, Northern Italy.

The oil has been examined by Umney and Bennett,

2

and has characters

falling within the following limits :—

1

Pharm. Journ., iii. 11 (1880), 220.

2

Chemist and Druggist, 66 (1905), 945 ; 67 (1905), 970.