| Lemon myrtle | |
|---|---|
 | |
| Garden specimen, in flower | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Rosids |
| Order: | Myrtales |
| Family: | Myrtaceae |
| Genus: | Backhousia |
| Species: | B. citriodora |
| Binomial name | |
| Backhousia citriodora | |
Backhousia citriodora, commonly known aslemon myrtle,lemon scented myrtle orlemon scented ironwood, is aflowering plant in the familyMyrtaceae. It is native to the subtropical rainforests of central and south-easternQueensland,Australia, with a natural distribution from Mackay to Brisbane.[1]
The species can reach 6 m (20 ft) in height, but is often smaller. Theleaves areevergreen, opposite,lanceolate, 5–12 cm (2.0–4.7 in) long and 1.5–2.5 cm (0.6–1.0 in) broad, glossy green, with an entire margin. Theflowers are creamy-white, 5–7 mm (0.20–0.28 in) in diameter, produced in clusters at the ends of the branches from summer through to autumn. After petal fall, the calyx is persistent.
A significant fungal pathogen, myrtle rust (Uredo rangelii) was detected in lemon myrtle plantations in January 2011.[2][3] Myrtle rust severely damages new growth and threatens lemon myrtle production.
Lemon myrtle was given the botanical nameBackhousia citriodora by Ferdinand von Mueller in 1853 after his friend, the English botanist,James Backhouse.[4]
The common name reflects the stronglemon smell of the crushed leaves. 'Lemon scented myrtle' was the primary common name until the shortened trade name, 'lemon myrtle', was created by the native foods industry to market the leaf for culinary use. Lemon myrtle is now the more common name for the plant and its products.
Lemon myrtle is sometimes confused with 'lemon ironbark', which isEucalyptus staigeriana. Other common names are sweet verbena tree, lemon scented verbena (not to be confused withlemon verbena), and sweet verbena myrtle.
Aboriginal Australians have long used lemon myrtle, both in cuisine and as a healing plant. The oil has the highestcitral purity; typically higher thanlemongrass. It is also considered to have a "cleaner and sweeter" aroma than comparable sources of citral–lemongrass andLitsea cubeba.[5] In 1888, Bertram first isolated the essential oil fromB. citriodora.[6] In 1925, it was found to be significantlygermicidal, and it was later shown to be antimicrobial.[7]
In the 1940s,Tarax was the first company to useB. citriodora oil as a lemonflavouring duringWorld War II.[8] In 1989,B. citriodora was investigated as a potential leaf spice and commercial crop byPeter Hardwick, who commissioned theWollongbar Agricultural Institute to analyseB. citriodora selections usinggas chromatography.[9] In 2001, a Standards for Oil ofB. citriodora was established by The Essential Oils Unit, Wollongbar, andStandards Australia.[10]
Lemon myrtle is one of the well knownbushfood flavours and is sometimes referred to as the "Queen of the lemon herbs".[11] The leaf is often used as dried flakes, or in the form of an encapsulated flavour essence for enhanced shelf-life. It has a range of uses, such as lemon myrtle flakes inshortbread; flavouring inpasta; whole leaf with bakedfish; infused inmacadamia or vegetable oils; and made intotea, including tea blends. It can also be used as a lemon flavour replacement inmilk-based foods, such ascheesecake, lemon flavouredice-cream andsorbet without the curdling problem associated withlemon fruitacidity.[citation needed]
Backhousia citriodora has two essential oilchemotypes. The citral chemotype is more prevalent and is cultivated in Australia forflavouring andessential oil. Citral as anisolate in steam distilled lemon myrtle oil is typically 90–98%, and oil yield 1–3% from fresh leaf. Thecitronellal chemotype is uncommon, and can be used as an insect repellent.[12][13] The dried leaf hasfree radical scavenging ability.[14]
Lemon myrtleessential oil possessesantimicrobial properties; however, the undiluted essential oil is toxic to human cellsin vitro.[15] When diluted to approximately 1%, absorption through the skin and subsequent damage is thought to be minimal.[16] Lemon myrtle oil has a highRideal–Walker coefficient, a measure of antimicrobial potency.[17] Use of lemon myrtle oil as a treatment for skin lesions caused bymolluscum contagiosum virus (MCV), a disease typically affecting children and immuno-compromised patients, has been investigated. Nine of sixteen patients who were treated with 10% strength lemon myrtle oil showed a significant improvement, compared to none in the control group.[18] A study in 2003 which investigated the effectiveness of different preparations of lemon myrtle against bacteria and fungi concluded that the plant had potential as anantiseptic or as a surface disinfectant, or as an anti-microbial food additive.[19] The oil is a popular ingredient in health care and cleaning products, especially soaps, lotions, skin-whitening preparations and shampoos.[20]
Lemon myrtle is a cultivated ornamental plant. It can be grown from tropical to warm temperate climates, and may handle cooler districts provided it can be protected from frost when young.[1] In cultivation it rarely exceeds about 5 metres (16 ft) and usually has a dense canopy. The principal attraction to gardeners is the lemon smell, which perfumes both the leaves and flowers of the tree. Lemon myrtle is a hardy plant, which tolerates all but the poorest drained soils.[1] It can be slow growing but responds well to slow-release fertilisers.
Seedling lemon myrtle go through a shrubby, slow juvenile growth stage, before developing a dominant trunk. Lemon myrtle can also be propagated from cutting, but is slow to strike.[1] A study into the plant growingadventitious roots found that "actively growing axillary buds, wide stems and mature leaves" are good indicators that a cutting will take root successfully and survive.[21] A further study on temperature recommendedglasshouses for growing cuttings throughout the year.[22] Growing cuttings from mature trees bypasses the shrubby juvenile stage. Cutting propagation is also used to provide a consistent product in commercial production.
In plantation cultivation the tree is typically maintained as a shrub by regular harvesting from the top and sides. Mechanical harvesting is used in commercial plantations. It is important to retain some lower branches when pruning for plant health. The harvested leaves are dried for leafspice, or distilled for the essential oil.
The majority of commercial lemon myrtle is grown inQueensland and the north coast ofNew South Wales, Australia.
A 2009 study has suggested that drying lemon myrtle leaves at higher temperatures improves the citral content of the dried leaves, but discolours the leaves more.[23]