Lemon Myrtle

Scientific Name(s): Backhousia citriodora F. Muell. Family Myrtaceae

Common Name(s): Lemon myrtle , sweet verbena myrtle

Uses

The leaves and flowers of lemon myrtle are used in tea blends and beverages, biscuits, breads, confectionery, pasta, syrups, liqueurs, flavored oils, packaged fish (salmon), and dipping and simmer sauces. Topical 10% solution of lemon myrtle essential oil showed a 90% reduction of symptoms in children treated for Molluscum contagiosum . The leaf paste, essential oil, and hydrosols have antibacterial and antifungal activity against Staphylococcus aureus , Escherichia coli , Pseudomonas aeruginosa , Candida albicans , methicillin-resistant S. aureus (MRSA), Aspergillus niger , Klebsiella pneumonia , and Propionibacterium acnes . The Complementary Medicines Evaluation Committee (CMEC) of the Australian Therapeutic Goods Administration (TGA) reported the proposed use as an antiseptic therapy in the treatment of pimples and acne. B. citriodora leaf oil is about 98% citral.

Dosing

3 cups per day of 2 to 4 mL of a 1% alcoholic solution; 0.18 mg/cm 2 of 1% lemon myrtle oil applied topically at exposure durations of 8 hours has been used as a topical antimicrobial. Two lemon myrtle leaves in 1 L of water have been used as a beverage.

Contraindications

Contraindications have not yet been identified.

Pregnancy/Lactation

Information regarding safety and efficacy in pregnancy and lactation is lacking.

Interactions

None documented.

Adverse Reactions

Research reveals no information regarding adverse reactions.

Toxicology

When 18.29 mg/cm 2 of the essential oil is applied to the skin for 1 to 12 hours, a reduction in cellular functioning, loss in integrity, loss of cellular vacuolation, cellular necrosis, and lower solubility of the stratum corneum were documented. When 0.18 mg/cm 2 of the essential oil was applied to the skin for 8 hours, the damage due to the citral component affected only epidermal cells. Oil of lemon myrtle was toxic to the hepatocarcinoma-derived human cell line (HepG2), F1-73 (a fibroblast cell line derived from healthy skin), and primary cell cultures of human skin fibroblasts. Cytotoxicity 50% inhibitory concentration (IC 50 ) 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 no-observed-adverse-effect level (NOAEL) for lemon myrtle oil was calculated as 0.5 mg/L at 24 hours of exposure, and the reference dose was determined as 0.01 mg/L. Therefore, lemon myrtle 1% oil was low in toxicity and could be safely used in topical antimicrobial products. The majority of research has been conducted on the compound citral as a common ingredient. Citral and citral oil are considered safe at a 1% dilution and has GRAS (generally recognized as safe) status by the US Federal Drug Administration.

Botany

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. 1 , 2 , 3 Lemon myrtle can be cultivated successfully in cooler areas when young plants are protected from frost. 4 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. 5

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. 2 , 6 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. 1 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. 7

History

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. 5

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. 5

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. 1

Chemistry

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. 8 The Australian TGA and the CMEC indicated that B. citriodora leaf oil was about 98% citral. 9

The other main chemovar, citronellal, can contribute up to 80%, which makes the leaves unsuitable for commercial use. The oil provides most of the aroma and flavor. 1 , 5

The leaves of B. citriodora contain 0.33% to 0.86% essential oil. 1 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%. 1 Antioxidants were found at 102, 60, and 31 mg gallic acid equivalents per gram of dry weight, respectively. 2

Uses and Pharmacology

Citral has sedative, antiviral, and antifungal properties. Although not clinically proven, citral is also considered to have antitumor properties. 3

Antibacterial/Antifungal

The CMEC reported the proposed use of B. citriodora as an antiseptic for use in pimples and acne. 10

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 . 11 , 12

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 ). 6 , 13 , 14 , 15 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. 12

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. 6

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. 9

Clinical data

Thirty-one children were treated with a 10% solution of B. citriodra for M. contagiosum . Of the treated group, 81% demonstrated a 90% reduction of lesions in 30 days compared with placebo. 13

Other uses

Flowers and fruits are used as flavorings, antiseptics, and surface disinfectants, and in foods as a natural antimicrobial agent. 1 , 5

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. 2

Dosage

Dosages include 15.7 mg/mL of leaf extract in methanol as an antibacterial agent 9 and 3 cups per day of 2 to 4 mL of a 1% alcoholic solution. 3

0.18 mg/cm 2 of 1% lemon myrtle oil applied topically at exposure durations of 8 hours has been used as a topical antimicrobial. 6 , 15

For beverages, 2 leaves per liter have been used; dry lemon myrtle leaves have also been sprinkled on foods. 1

Pregnancy/Lactation

Information regarding safety and efficacy in pregnancy and lactation is lacking.

Interactions

Information regarding the interaction of Backhousia with drugs, foods, or herbs is lacking.

Toxicology

When applied to the skin at exposure durations of 1 to 12 hours, 18.29 mg/cm 2 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/cm 2 of the essential oil was applied to skin for 8 hours' duration, the damage due to citral was limited to the epidermal cells. 6 , 15

Cytotoxicity was found in vitro from human cell lines HepG2, F1-73, and primary cell cultures of human skin fibroblasts. Cytotoxicity IC 50 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. 15

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. 9

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. 1

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%. 16

Bibliography

1. Hegarty MP, Hegarty EE, Wills RB. Food Safety of Australian Plant Bushfoods . Rural Industries Research and Development Corporation. RIRDC Project No AGP-1A. March 2001.
2. Konczak I, Zabaras D, Dunstan M, Aguas P. Antioxidant capacity and phenolic compounds in commercially grown native Australian herbs and spices. Food Chem . 2010;122(1):260-266.
3. Pengelly A. Backhousia citriodora , medical herb. Aust J Med Herbalism . 1991; 3.
4. Ryder M, Latham Y. Cultivation of Native Foods in South Eastern Rural Industries Research and Development Corporation. 04/178, 2005.
5. Hess-Buschmann S. New Crop Industries Handbook . Rural Industries Research and Development Corporation; 2004: 353-3578.
6. Hayes AJ, Markovic B. Toxicity of Australian essential oil, ( Backhousia citriodora ), (lemon myrtle). Part 2. Absorption and histopathology following application to human skin. Food Chem Toxicol . 2003;41(10):1409-1416.
7. Backhousia citriodora . Australian Native Plants Society (Australia). http://www.anpsa.org.au/b-cit.html . Accessed May 1, 2012.
8. Nhu-Trang TT, Casabianca H, Grenier-Loustalot MF. Authenticity control of essential oils containing citronellal and citral by chiral and stable-isotope gas-chromatographic analysis. Anal Bioanal Chem . 2006;386(7-8):2141-2152.
9. Complementary Medicines Evaluations Committee, 22nd Meeting; Extracted Ratified Minutes. August 25, 2000. http://www.tga.gov.au/pdf/archive/cmec-minutes-22.pdf . Accessed May 11, 2012.
10. Cock I. Antibacterial activity of selected Australian native plant extracts. Internet J Microbiol . http: / / www.ispub.com / journal / the - internet - journal - of - microbiology / volume - 4 - number - 2 / antibacterial - activity - of - selected - australian - native - plant - extracts.html .
11. Wilkinson JM, Hipwell M, Ryan T, Cavanaugh HM. Bioactivity of Backhousia citriodora : antibacterial and antifungal activity. J Agric Food Chem . 2003;51(1):76-81.
12. Zuas O, Dykes GA. In vitro antimicrobial activity of lemon myrtle ( Backhousia citriodora ) oil against food pathogenic bacteria. Artocarpus . 2007;7(1):34-38.
13. Burke BE, Baillie JE, Olson RD. Essential oil of Australian lemon myrtle ( Backhousia citriodora ) in the treatment of molluscum contagiosum in children. Biomed Pharmacother . 2004;58(4):245–247.
14. Dupont S, Caffin N, Bhandari B, Dykes G. In vitro antibacterial activity of Australia native herb extracts against food-related bacteria. Food Control . 2006;17(11): 929-932.
15. Hayes AJ, Markovic B. Toxicity of Australian essential oil Backhousia citriodora (Lemon myrtle). Part 1, Antimicrobial activity and in vitro cytotoxicity. Food Chem Toxicol . 2002;40(4):535-543.
16. Therapeutic Goods Administration, Australian Government. Substances that may be used in Listed Medicines in Australia. Department of Health and Ageing. December 2007.

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