470 Handbook of herbs and spices
28.4 Chemical composition....................................................................
Peppermint oil is a colourless, yellowish or greenish liquid, with a peculiar, highly
penetrating odour and a burning, camphorescent taste. It thickens and becomes reddish
with age, but improves in mellowness, even if kept as long as ten or fourteen years.
The essential oil contains alpha- and beta-pinene, Cineole, Jasmone, Isomenthol,
Isomenthone, Ledol, Limonene, Menthofuran, Menthol, Menthone, Menthyl acetate,
Neomenthol, Piperitone, Pulegone and Viridiflorol. The chief constituent of peppermint
oil is Menthol, but it also contains menthyl acetate and isovalerate, together with
menthone, cineol, inactive pinene, limonene and other less important bodies. On
cooling to a low temperature, separation of menthol occurs, especially if a few
crystals of that substance be added to start crystallization. The value of the oil
depends much upon the composition. The principal ester constituent, menthyl acetate,
possesses a very fragrant minty odour, to which the agreeable aroma of the oil is
largely due. The alcoholic constituent, menthol, possesses the well known penetrating
minty odour and characteristic cooling taste.
Peppermint leaf contains luteolin, hesperidin, and rutin; caffeic, chlorogenic, and
rosmarinic acids, and related tannins; choline; a- and b-carotenes; gum; minerals;
resin; a and g tocopherols; a-amyrin and squalene triterpenes; volatile oil (1.2–3%)
composed mostly of monoterpenes – 29–55% menthol, 10–40% menthone, 2–13%
cineole, 1–11% pulegone, 1–10% menthyl acetate, 0–10% menthofuran, and 0.2–6%
limonene (Bradley, 1992; Bruneton, 1995; Leung and Foster, 1996; Wichtl and Bisset,
1994). Gherman et al. (2000) analyzed seven different peppermint samples by gas
chromatography and gas chromatography–mass spectrometry (GC:MS). The main
volatile compounds identified by the gas chromatography–mass spectrometric analysis
of M. piperita were menthol, menthone, isomenthone, 1,8-cineole, menthyl acetate,
limonene, b-myrcene and carvone. The active principles of the oil are menthol,
menthone, isomenthone, menthyl acetate, a-pinene, b-pinene, champhor, limonene,
linalool and piperitone. The qualitative fatty acid composition is dominated by palmitate
(16:0), linoleate (18:2) and linolenate (18:3) (Maffei and Scannerini, 1992).
From M. piperita leaves, 16 free lipophilic flavonoid aglycones were isolated and
identified by Voirin and Bayet (1992). The variation of this flavonoid composition
studied by the means of HPLC techniques from the youngest to the oldest leaves
showed that the A- and B-ring O-methylation patterns of leaf pairs differ according
to leaf age and would indicate the sequential activity of 4¢-O- and 6-O-methyl-
transferases. Different steps of monoterpene metabolism – disappearance of limonene,
accumulation of 1,8-cineole, reduction of menthone to menthol and acetylation of
menthol – have been studied in different parts of M. piperita leaves of different ages.
The analyses of different samples (leaf strips, disks and individual peltate trichomes,
translucent or containing crystals from either epidermis) show that all these dynamic
changes start at the distal extremity of the leaf and shift progressively towards the
base. Except for the peltate trichomes localized within the leaf area, in which a
metabolic step is being realized, the trichomes of other parts present a homogeneous
monoterpene composition. The measurements of amounts of chlorophyll in two parts,
distal and basal, of youngest leaves of terminal buds show that chlorophyll biosynthesis
starts also at the distal extremity of the leaf (Voirin and Bayet, 1996).
In M. piperita leaves, the peltate glands may be divided into two types according
to the presence or the absence of crystals in the head of the trichomes. In both types,
the free lipophilic flavonoid aglycones, characterized by UV and chromatographic
data, are located in the head of the peltate glands, together with monoterpenes (Voirin