Malagasy aromatic plants: essential oils, antioxidant and antimicrobial activities

This work examined the chemical profile of nine volatile oils from Madagascar and evaluated their antimicrobial and antioxidant activity. Essential oils with the highest levels of eugenol exhibited the highest antioxidant activity. Clove bud (Syzygium aromaticum) oil appeared the most promising for potential applications as a food preservative because showed the highest antioxidant and antimicrobial against bacteria (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa) and fungi (Aspergillus niger, Saccharomyces cerevisiae). Cinnamon leaf (Cinnamomum zeylanicum) oils with a very high antioxidant activities were also effective though less than the clove bud oil against the growth of the microorganisms tested. Cinnamon bark (C. zeylanicum) oil exhibited a low antioxidant activity yet had a high antimicrobial activity against the five tested microorganisms. Linalool basil oil (Ocimum basilicum) showed an intermediate antioxidant activity and was most active against A. niger and E. coli. P. nigrum essential oil was most active against fungi (A. niger, S. cerevisiae), while Tagetes minuta was most active against S. aureus. The bark essential oil of Ravensara aromatica showed low antioxidant and antimicrobial activity. INTRODUCTION As part of the international development program Agribusiness in Sustainable Natural African Plant Products (ASNAPP, www.asnapp.org) we are seeking to identify new uses and applications of African natural products including essential oils and botanicals in order to both enhance their industrial and commercial uses and to ascertain potentially new human health applications for these products. Essential oils from aromatic plants have been known since antiquity to possess biological activity, mainly antimicrobial, antifungal and antioxidant properties (Barata, et al., 1998; Griffin, et al., 1999; Zygadlo and Juliani, et al., 2000). With growing interest in the use of essential oils in both the food and the pharmaceutical industries research has shifted into examining new applications for essential oils (Juliani and Simon, 2002). Epidemiological studies have suggested positive associations between the consumption of phenolic-rich foods or beverages and the prevention of diseases (Scalbert and Williamson, 2000). The essential oils are complex mixtures of different components, some of which are rich in phenolic constituents (Williams, 1997). This current work sought to analyze the chemical profile of commercially derived essential oils from selected Malagasy plants and to evaluate their antimicrobial and antioxidant activity. Proc. XXVI IHC – Future for Medicinal and Aromatic Plants Eds. L.E. Craker et al. Acta Hort. 629, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 78 MATERIAL AND METHODS The oils were obtained from Biosave Madagascar SARL (BS, Ocimum basilicum, Tagetes minuta, Pelargonium graveolans, Piper nigrum, Ravensara aromatica, bark), EPAM Tombohobe Sud (EPAM, Cinnamomum zeylanicum leaf oil), Phaels Flor (PF, C. zeylanicum leaf and bark oils) and Parapharma (PP, Syzygium aromaticum). The diluted essential oils (2-500 μl oil / 1 ml of dimethyl sulfoxide) were screened against fungi (Aspergillus niger, Saccharomyces cerevisiae), bacteria (Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa) and the standard method used to determine in vitro antibacterial and antifungal activity of essential oils consisted of utilizing suspensions of microorganisms and monitoring for growth inhibition. The results were expressed as the minimal concentration of essential oil (μl essential oil/ml) that caused visual inhibition of growth. The same essential oils were screened for in vitro antioxidant activity using the ABTS screen. The antioxidant activity was related to Trolox (a water soluble analogue of Vitamin E) and expressed as mg of Trolox per ml of oil (TEAC) (Re et al., 1999). The volatile oils were analyzed by a gas chromatograph (GC) coupled to a mass spectrometer (MS) and Flame ionization detectors (FID) (Agilent GC System 6890 Series, Mass Selective Detector, Agilent 5973 Network, FID detector). Samples were injected with an autosampler (Agilent 7683 Series), the Inlet temperature was 220 °C. , in a HP5MS (30 m, 0.25 ID, 0.25 μm) column, temperature program, 60 °C 1 min, 4 °C/min, 200 °C 15 min. Helium constant flow was set at 1 ml/min. Individual identifications were made by matching their spectra with those from mass spectral libraries (Wiley275.L). Components were listed in order of elution in HP5 (DB5 equivalent) column. RESULTS AND DISCUSSION Essential oils with the highest levels of eugenol (Table 1) exhibited also the highest antioxidant activity (Table 2). Accordingly, the highest antioxidant activity was found in Syzygium aromaticum (clove bud) oil (eugenol 75 %, TEAC 600), follow by the C. zeylanicum (cinnamon) leaf oils (eugenol 59-64 %, TEAC 560-580). A lower activity was found in O. basilicum oil (linalool basil) (linalool 60 %, eugenol 9 %, TEAC 192), while the C. zeylanicum bark was even lower (traces of eugenol, cinnamaldehyde 38 %, cinnamylacetate 14 %, TEAC 22.9). The lowest antioxidant activity was observed in R. aromatica, richest in methychavicol (1.7 TEAC) (Table 1, 2). All essential oils exhibited a relatively high antifungal activity (Table 2), with most oils being effective inhibiting A. niger more than S. cerevisiae. Cinnamon bark and leaf, and P. nigrum oils were the most active against A. niger and S. cerevisiae. Other authors, observed that cinnamaldehyde, the main component of cinnamon bark, inhibited the growth of bacteria and yeast (Barata, et al., 1998; Zygadlo and Juliani, 2000). The essential oils showed a large variation in the antibacterial activity (Table 2). The most active oils against E. coli were geranium (P. graveolans) and basil (O. basilicum). The eugenol containing oils (cinnamon leaf and clove bud) inhibited the growth of S. aureus, as did the tagetes (T. minuta) oil. In the eugenol-rich containing essential oils, there are a close relationship between eugenol content and antimicrobial activity against P. aeruginosa, with clove bud oil the most active. Eugenol has been shown to be more active against microorganisms than methyleugenol and methylchavicol (Griffin, et al, 1999; Zygadlo and Juliani, 2000). This is in accordance with the relative inactivity of R. aromatica, composed mainly of methylchavicol. In conclusion, there was a close relationship between antioxidant activity and eugenol levels. Of all the organisms tested, only in P. aeruginosa appeared to be an association between antioxidant and antimicrobial activity. Within this range of nine essential oils, the clove bud oil appeared the most promising for potential applications as a food preservative because showed the highest antioxidant and antibacterial activities. Cinnamon leaf oils with very high antioxidant activities were also effective, though less than the clove bud oil, against the growth of the microorganisms tested. The cinnamon