Effects of drying methods on the quality and quantity of the essential oil of Mentha longifolia L. subsp. Capensis
Introduction
Mentha longifolia L., also known as wild mint, is a fast-growing aromatic perennial herb that is widespread throughout the Eastern Cape province of South Africa. It is widely used in herbal medicine and believed to be particularly beneficial in building the immune system and fighting secondary infections. The plant is used for the treatment of coughs, colds and influenza. Externally, wild mint is used to treat wounds and swollen glands (Van Wyk, Outdtshoorn, & Gericke, 1997). The essential oil of this plant is partly responsible for the decongestant, antispasmodic and antibiotic effects reported by some workers (Van Wyk & Gericke, 2000).
The leaves of M. longifolia have a wide range of culinary usage in South Africa and, because of their colour, aroma and flavour, they are used in food preparation to enhance taste and appearance. When the leaves are rubbed onto the body and beddings, the strong smell keeps mosquitoes away (Hutchings & Van Staden, 1994). It has also been spread in granaries to keep rodents off the grain (Phillips & Foy, 1990).
The chief component of the essential oil of M. longifolia from South Africa appears to be the monoterpene ketone, menthone (Oyedeji & Afolayan, 2005). This is a different chemotype from to those in other parts of the world, which have carvone (Younis & Beshir, 2004), piperitone (Karousou, Lanaras, & Kokkini, 1998), and piperitenone and its oxide (Venskutonis, 1996) as the major constituents. These menthane monoterpene oxides give M. longifolia its characteristic odour. Some other significant constituents present in almost all the chemotypes are β-pinene, 1,8-cineole, pulegone, limonene, germacrene D and β-caryophyllene.
In vitro antimicrobial studies of the essential oil of this herb showed very strong antibacterial activity (Mimica et al., 2003, Oyedeji and Afolayan, 2005). It was also found to be toxic when administered to mice.
The leaves of aromatic plants are often dried before extraction to reduce moisture content. During this process, many compounds which are dragged to the leaf surface by the evaporating water, are lost (Moyler, 1994). The method of drying usually has a significant effect on the quality and quantity of the essential oils from such plants. In the present study, the effect of different drying methods on the quality and quantity of the leaf oil of M. longifolia were investigated.
Section snippets
Plant material
The shoots of M. longifolia were collected from the wild around Alice, in the Eastern Cape of South Africa in July, 2005. A voucher specimen (ASE1/05) was deposited in the herbarium of the University of Fort Hare. The remaining plant material was divided into three portions. One portion was dried to constant weight in the sun; another portion was left to dry in the laboratory under normal air and at room temperature conditions, while the third part was dried in the oven, which was kept at 40 °C.
Isolation procedure
Results and discussion
The chemical compositions and yields of the oils obtained by the three drying methods differed significantly. Generally, the dried plant material yielded more essential oils than did the fresh leaf materials (Table 1). The total numbers of components of the oil in sun-dried, oven-dried, air-dried and fresh leaves were 26, 25, 22 and 19, respectively.
The monoterpenoids represented 92.6% of the total oil content in the oil from the fresh plant. Pulegone (35.0%), menthone (31.1%) and 1,8-cineole
Acknowledgements
The financial support from the University of Fort Hare, South Africa and University of Lagos, Nigeria is gratefully acknowledged.
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