Antifungal activities of essential oils applied by dip-treatment on areca palm (Areca catechu) leaf sheath and persistence of their potency upon storage
Introduction
Over the past 20 years, the use of bio-based packaging materials to prolong the shelf-life and improve the quality of fresh food products has been receiving increased attention (Del Nobile et al. 2009). In southeast Asia and India, areca palm (Areca catechu) leaf sheath has been traditionally used as an environmentally friendly food packaging material for a number of years (Kalita et al. 2008). In Thailand, in particular, the smoked areca palm leaf sheath has been used to wrap various intermediate moisture foods (IMF) such as fruit pastes of durian and dried fish. Within about a month, however, the quality of IMF wrapped with the smoked areca palm leaf sheath normally deteriorates due to the presence of molds that develop on the surface of the food as a result of the relatively high water activity of the IMF product. Laboratory testing under a severe storage condition at relative humidity of 80% to investigate shelf-life of IMF products wrapped with the raw areca palm leaf sheath also revealed that molds could be observed on the IMF and the leaf sheath within 7 days (Nitimongkonchai 1999).
To extend the shelf-life of IMF products packed inside the areca palm leaf sheath even further, application of essential oils extracted from herbs or plants used in cooking to areca palm leaf sheath is worth exploring, and this is the main objective of this research. There have been reports of successful use of various essential oils, such as cinnamon oil (Wang et al., 2005, Kyu Kyu Win et al., 2007), clove oil (Rakotonirainy and Lavedrine 2005), anise oil (Matan and Matan 2008), and peppermint oil (Matan et al. 2009), to protect against molds and fungi on various cellulose-based materials such as wood, fruit, and paper.
The present work deals with the inhibitory effects of those essential oils and their main components on the growth of molds commonly found on the leaf sheath surface. Dip-treatment was employed for molds testing on the areca palm leaf sheath since only essential-oil-coating on the surface is required to effectively prevent molds on the surface (Matan and Matan 2008). Major components in essential oils on the areca palm leaf sheath surface during storage were also examined.
Section snippets
Chemicals, essential oils, and areca palm leaf sheath
Food-grade essential oils (cinnamon oil containing 74% cinnamaldehyde, clove oil containing 75% eugenol, anise oil containing 98% trans-anethole, and peppermint oil containing 61% menthol) derived by steam distillation were provided by Thai China Flavours and Fragrances Industry Co., Ltd., Bangkok, Thailand. Pure substances of cinnamaldehyde, eugenol, trans-anethole, and menthol were purchased from Sigma–Aldrich, Singapore.
Areca palm leaf sheath was obtained from Khiriwong Village, Lansaka
Inhibition of molds by essential oils and their main components
In the agar dilution method (Table 1) all four essential oils had a fungistatic effect on the test molds. For any individual essential oil, there was no difference in MIC between the four test molds. Vegetable oil used at 10–500 μg ml−1 as a control did not inhibit the test molds. With an MIC of 50 μg ml−1, cinnamon oil was the most potent inhibitor. Strong antifungal activity of this oil has been reported by many authors, e.g., Velluti et al., 2003, Wang et al., 2005, and Tzortzakis (2009). At
Conclusions
Cinnamon, clove, anise, and peppermint oils were inhibitory to four molds previously isolated from areca palm leaf sheaths. These essential oils at their MICs were capable of inhibiting spore germination and growth of these molds on leaf sheath discs for at least 12 weeks in storage at 25 °C and 100% RH. The principal component of clove and anise oil, and especially cinnamon oil, cinnamaldehyde, was the most active and stable component. Eugenol and trans-anethole, main components in,
Acknowledgements
This study was supported by the Institute of Research and Development, the Wood Science and Engineering Research Unit, and the Thailand Center of Excellence in Physics (ThEP) through the Plasma Agricultural Application Laboratory, Walailak University, Thailand. The authors gratefully acknowledge an anonymous reviewer for the English editing of the manuscript.
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