Peppermint 469
time on the composition of the essential oil of peppermint. Harvesting at the end of
flowering afforded an inversion of the menthone/menthol ratio, yielding an oil that
was richer in menthol and therefore more valuable commercially. It was possible to
harvest twice a year, thereby increasing the annual yield per hectare. Batches of
differing qualities were obtained with a range of menthol/menthone ratios, according
to harvest time. Pre-drying of M. piperita herbage before distillation did not affect
chemical composition, but allowed steam distillation of greater amounts of plant
material. Composite oil samples from a field trial having six harvest dates were
analyzed to determine the effect of age on their physicochemical properties (Duhan
et al., 1976). Oil and free menthol content increased with time to a maximum in those
plants harvested between 163–178 days. Thereafter, oil content and stem to leaf ratio
decreased.
The results of another field trial on the effect of crop age on the yield of herb oil
and quality of essential oil of M. piperita are reported by Gulati et al. (1978). It is
indicated that in India, proper harvesting occurs between 145 to 160 days for first
harvest and 97 to 111 days for second harvest of the crop. The content of oil and its
chemical constituents vary with the growth and developmental stage of the plant.
Va v e r kova et al. (1997) found that the beginning of bloom may be regarded as a
vegetation period giving the highest content of the essential oil in herb and leaves of
peppermint, and its greatest amount was found in the youngest leaves. Content of
menthol gradually increased to its maximum in the blooming phase while that of
menthone was decreasing.
On suitable soil and with proper cultivation, yields of 15 to 17 tons of peppermint
herb per hectare may be expected. In many places, the custom is to let the herb lie on
the ground for a time in small bundles or cocks. In other countries the herb is distilled
as soon as it is cut. Again, certain distillers prefer the plants to be previously dried or
steamed. The subject is much debated, but the general opinion is that it is best to distil
as soon as cut, and the British Pharmacopoeia directs that the oil be distilled from the
fresh flowering plant. Even under the best conditions of drying, there is a certain loss
of essential oil. If the herbs lie in heaps for any time, fermentation is bound to occur,
reducing the quality and quantity of the oil, as laboratory experiments have proved.
Exposure to frost must be avoided, as frozen mint yields scarcely half the quantity of
oil, which could otherwise be secured. A part of the exhausted herb is dried and used
for cattle food, for which it possesses considerable value. The rest is cut and composted
and eventually ploughed into the ground as fertilizer. There is also a market, chiefly
for herbalists, for the dried herb, which is gathered at the same time of year. It should
be cut shortly above the base, leaving some leafbuds, not including the lowest shrivelled
or discoloured leaves, and dried.
Professor Robert Menary of the University of Tasmania examined the current
cultivation practices for peppermint in Australia where growers were struggling to
obtain yields comparable to the north-eastern states of the U.S.A., where most of the
world’s mint oil is produced. He summarized that planting material was often of poor
quality, no practical benchmarks were being used to regulate irrigation and nitrogen
fertilizer, the two most important inputs during the growing season, in many fields
and for many growers, a lack of uniformity of inputs, and resulting variation in yield
of herb and oil were the main contributors to the poor overall yields and that many
plantings lapsed after relatively few years of production, usually due to poor weed
control, general loss of vigour or change of enterprise (http://www.rirdc.gov.au/99comp/
eoi1.htm#_Ref460804616, Project Title: Best Practice in Peppermint, RIRDC Project
No.: UT-16A).