LABIATJE 219
Eabak draws the following conclusions as the results of his ex-
periments :—
Conditions of soil and climate are influential factors in the forma-
tion of oil and its constituents in the peppermint plant. Light sandy
or loamy soils appear to be most favourable for the production of an oil
of high quality.
The yield of oil distilled from fresh plants apparently decreases as
the plant matures. Drying the plant before distillation results in a
considerable loss of oil. The largest proportion of oil is found in the
leaves and flowering tops.
The percentage of esters in the oils increases as the plants approach
maturity. The menthol-content of the oil bears a close relationship
to the ester-content. The free acidity and ester-content of the oil
distilled from dry plants is considerably higher than in the oil from
fresh plants. The drying of the plants causes changes favourable to
esterification, while the percentage of free and total menthol in oils
distilled from dried plants is also uniformly high.
The formation of esters and menthol takes place most readily in
the leaves and tops of the plant, the metabolic processes showing
increased activity as the plant matures.
The effect of shade upon the peppermint plant is to decrease
esterification and the formation of menthol, and is due possibly to
the lessened activity of the elimination of water by the plant.
The action of frost noticeably increases esterification and the
formation of menthol.
The mint is usually left in the sun in small cocks for a short time
before distillation. With regard to the advisability of drying the plants
before distillation, thus reducing the bulk and so increasing the charge
of a still, Mr. Todd, a well-known Michigan grower, carried out experi-
ments on a very large scale, and came to the following conclusions:—- No loss of essential oil of peppermint through diffusion in the
atmosphere is occasioned by thorough drying of the plants and pro-
longed exposure to atmospheric action prior to distillation. (This con-
clusion, however, does not appear to be quite correct.) - Such exposure does not increase the crystallising tendency of the
essential oil. - A heavy and insoluble resinoid is produced by oxidation, increas-
ing the specific gravity of the oil and affecting the boiling-point and
solubility, by raising the former and decreasing the latter. - To obtain the best results as to the quality of oil produced and the
facility of handling and distillation, the plants should be dried as thor-
oughly as possible without endangering the loss of leaves and blossom;.
distillation should then take place as soon as convenient to prevent the
formation of resin.
The earliest elaborate research on the composition of American
peppermint oil is that of Power and Kleber.
1
The oil examined by these
chemists was distilled from the dried plant and had the following char-
acters :—
Specific gravity 0'9140
Optical rotation - 32°
Menthol as esters 14-12 per cent.
Free menthol 45*5 „. Rundschau, 12 (1894), 157.