Scientific Name(s): Backhousia citriodora F. Muell.
Common Name(s): Lemon myrtle, Sweet verbena myrtle
B. citriodora (lemon myrtle) is a small genus with 6 species located in tropical regions of eastern Queensland and northeastern New South Wales, Australia.
Backhousia is a tree that grows up to 20 m, but in cultivation rarely exceeds 5 m. Leaves are opposite, glabrous, lanceolate, and dark green, and measure 100 mm. The fruit is a dry indehiscent that splits into 2 chambers.Hegarty 2001, Konczak 2010, Pengelly 1991 Lemon myrtle can be cultivated successfully in cooler areas when young plants are protected from frost.Ryder 2005 This hardy plant tolerates all soils except those that do not drain well. It may grow slowly, but responds well to slow-release fertilizers. It has also been successfully cultivated indoors. B. citriodora is best propagated from cuttings, because the seeds are difficult to germinate. When cultivated, moist, rich soils are preferred.Hess-Buschmann 2004
B. citriodora is grown for the strong lemon fragrance of the foliage. The prolific apical clusters of white 4-petaled flowers are attractive on branches in the summer and autumn.Konczak 2010, Hayes 2003 The species is known to have at least 2 chemical variants; chemovars and their respective aromatic essential oils in the leaves are rich in citral or its close chemical relative citronellal.Hegarty 2001 Citral is more commonly used for its sweet, fresh, lemon-type perfume and flavor.
B. citriodora used to be incorrectly known in forestry literature as lemon ironwood, but in modern use in foods or drinks it is called lemon myrtle. Lemon ironbark is Eucalyptus staigeriana and it also contains citral, but in lower concentrations.Australian 2012
The genus is named after the British botanist James Backhouse (1794 to 1869), and the specific epithet, citriodora, comes from the richly scented lemon leaves. The traditional use is not well documented due to limited density, but it is likely that Australian Aboriginal people used the leaves, which are prominent in bushfoods, as a seasoning.Hess-Buschmann 2004
The plant was named by the German-Australian botanist Baron Ferdinand von Müller in 1853, but its use was not expanded until the 1990s when it was evaluated as a crop plant and cultivated.Hess-Buschmann 2004
In 1889, botanist Joseph H. Maiden, working for the innovative flavor and fragrance German company Schimmel & Co, reported on the potential use of lemon myrtle for commercial production. He was supposedly the first to identify the ingredient citral, which is 90% of the essential oil found in B. citriodora. Other lemon-flavored oils have less citral, such as citrus (3% to 10%), lemongrass (75%), and tropical verbena (74%), which are more common due to their lower cost. With the addition of many acres of trees and more research, the potential for greater commercial opportunity has expanded.Hegarty 2001
B. citriodora citral levels can be higher than 80% in the essential oil from the leaves. The approximate essential oil components are 40% neral (alpha citral) and 50% geranial (beta citral). Identification of the essential oils was authenticated by enantioselective capillary gas chromatographic and isotope-ratio mass spectrometry coupled online with capillary gas chromatography.Nhu-Trang 2006 The Australian TGA and the CMEC indicated that B. citriodora leaf oil was about 98% citral.Complementary Medicine 2012
The leaves of B. citriodora contain 0.33% to 0.86% essential oil.Hegarty 2001 Antioxidant properties were found in Backhousia by testing crude extracts with the Trolox-equivalent antioxidant capacity assay and the Australian Quarantine and Exports Advisory Council (now known as the Biosecurity Advisory Council) methods. Total phenolics were 88.1%, radical scavenging was 56.6%, and antioxidant activity was 46.7%.Hegarty 2001 Antioxidants were found at 102, 60, and 31 mg gallic acid equivalents per gram of dry weight, respectively.Konczak 2010
Uses and Pharmacology
Citral has sedative, antiviral, and antifungal properties. Although not clinically proven, citral is also considered to have antitumor properties.Pengelly 1991
The leaf paste, essential oil, and hydrosols were found to have antibacterial and antifungal activity in S. aureus, E. coli, P. aeruginosa, C. albicans, MRSA, A. niger, K. pneumoniae, and P. acnes.Wilkinson 2003, Zuas 2007
Water, alcohol, and hexane extracts of leaves were tested against food-borne bacteria (Enterococcus faecalis, E. coli, Listeria monocytogenes, P. aeruginosa, Salmonella enteritidis, Salmonella typhimurium, and S. aureus).Hayes 2003, Burke 2004, Dupont 2006, Hayes 2002 Gram-positive and gram-negative bacteria were treated with 0.125% and 0.5% concentrations of B. citriodora essential oil. The bacteria were inhibited at 0.0625% v/v.Zuas 2007
Oil of lemon myrtle was toxic to the following human cell lines: HepG2, F1-73, and primary cell cultures of human skin fibroblasts. Cytotoxicity values ranged from 0.008% to 0.014% (w/v) at 4 hours to 0.003% to 0.012% (w/v) at 24 hours of exposure.Hayes 2003
One gram of Backhousia leaves was extracted in 50 mL of methanol, and the 15.7 mg/mL extract was found to be effective against 2 gram-positive bacteria (Bacillus cereus, Bacillus subtilis) and 2 gram-negative bacteria (Aeromonas hydrophilia, Pseudomonas) in a disk-diffusion method.Complementary Medicines 2012
Thirty-one children were treated with a 10% solution of B. citriodra for the viral infection M. contagiosum. Of the treated group, 81% demonstrated a 90% reduction of lesions in 30 days compared with the vehicle, olive oil.Burke 2004 (van der wouden 2009)
The leaves and flowers are used in tea blends and beverages, dairy biscuits, breads, confectionery, pasta, syrups, liqueurs, flavoured oils, packaged fish (salmon), and dipping and simmer sauces.Konczak 2010
For beverages, 2 leaves per liter have been used; dry lemon myrtle leaves have also been sprinkled on foods.Hegarty 2001
Pregnancy / Lactation
Information regarding safety and efficacy in pregnancy and lactation is lacking.
Information regarding the interaction of Backhousia with drugs, foods, or herbs is lacking.
When applied to the skin at exposure durations of 1 to 12 hours, 18.29 mg/cm2 of the essential oil resulted in reduction of cellular functioning, loss in integrity, loss of cellular vacuolation, cellular necrosis, and lower solubility of the stratum corneum. By comparison, when 0.18 mg/cm2 of the essential oil was applied to skin for 8 hours' duration, the damage due to citral was limited to the epidermal cells.Hayes 2003, Hayes 2002
Cytotoxicity was found in vitro from human cell lines HepG2, F1-73, and primary cell cultures of human skin fibroblasts. Cytotoxicity IC50 values ranged from 0.008% to 0.014% (w/v) at 4 hours to 0.003% to 0.012% (w/v) at 24 hours of exposure. The NOAEL for lemon myrtle oil was calculated as 0.5 mg/L at 24 hours' exposure, and the reference dose was determined as 0.01 mg/L. Therefore, a product containing 1% lemon myrtle oil was found to be low in toxicity and could be used in the formulation of topical antimicrobial products.Hayes 2002
As a topical product, the Australian CMEC recognizes the leaf oil of B. citriodora (lemon myrtle) as safe and suitable for use as an active ingredient at a concentration not exceeding 1% w/w. Safety considerations are based on citral. The following issues regarding safety of B. citriodora should be considered: the existing exposure of the population to the active ingredient citral, given that it is widely used in the household, cosmetic, and food industries and the apparent lack of known adverse reactions associated with such exposure; the paucity of other important data on its metabolism that might inform an assessment of the importance to humans of the physiological alterations induced by topical citral in rats; and that topical administration of B. citriodora presented a potential risk, but that this risk was probably small.
The committee recognized that additional data was needed to determine any safety risk posed by B. citriodora and approved the oil derived from the leaf for topical use only. Concentration should not exceed 10 g/kg, 10 g/L, or 1%. The committee also requires label statements to include warnings that B. citriodora is an irritant that should be used with caution in children and pregnant women.Complementary Medicine 2012
The majority of research has been conducted on citral as a common ingredient. Citral and citral oil are considered safe as 1% dilutions and have GRAS status by the US Food and Drug Administration. When concentrated, citral-rich essential oils can be irritating to the skin.Hegarty 2001
The Australian TGA approved oil derived from the leaf as safe for topical use only. Concentration must not exceed 10 g/kg, 10 g/L, or 1%.Therapeutic Goods 2007
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