In spite of modern improvements in slaughter hygiene and food production techniques, food safety is an increasingly important public health issue (WHO, 2002a). It has been estimated that as many as 30% of people in industrialised countries suffer from a food borne disease each year and in 2000 at least two million people died from diarrhoeal disease worldwide (WHO, 2002a). There is therefore still a need for new methods of reducing or eliminating food borne pathogens, possibly in combination with existing methods (the hurdle principle; Leistner, 1978). At the same time, Western society appears to be experiencing a trend of ‘green’ consumerism (Tuley de Silva, 1996; Smid and Gorris, 1999), desiring fewer synthetic food additives and products with a smaller impact on the environment. Furthermore, the World Health Organization has recently called for a worldwide reduction in the consumption of salt in order to reduce the incidence of cardio-vascular disease (WHO, 2002b). If the level of salt in processed foods is reduced, it is possible that other additives will be needed to maintain the safety of foods. There is therefore scope for new methods of making food safe which have a natural or ‘green’ image.
One such possibility is the use of essential oils as antibacterial additives.Essential oils (EOs) (also called volatile or ethereal oils; Guenther, 1948) are aromatic oily liquids obtained from plant material (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits and roots). They can be obtained by expression, fermentation, enfleurage or extraction but the method of steam distillation is most commonly used for commercial production of EOs (Van de Braak and Leijten, 1999). The term ‘essential oil’ is thought to derive from the name coined in the 16th century by the Swiss reformer of medicine, Paracelsus von Hohenheim; he named the effective component of a drug Quinta essentia(Guenther, 1948). An estimated 3000 EOs are known, of which about 300 are commercially important—destined chiefly for the flavours and fragrances market (Van de Braak and Leijten, 1999). It has long been recognised that some EOs have antimicrobial properties (Guenther, 1948; Boyle, 1955) and these have been reviewed in the past (Shelef, 1983; Nychas, 1995) as have the antimicrobial properties of spices (Shelef, 1983) but the relatively recent enhancement of interest in ‘green’ consumerism has lead to a renewal of scientific interest in these substances (Nychas, 1995; Tuley de Silva, 1996).
Besides antibacterial properties (Deans and Ritchie, 1987; Carson et al., 1995a; Mourey and Canillac, 2002), EOs or their components have been shown to exhibit antiviral (Bishop, 1995), antimycotic (Azzouz and Bullerman, 1982; Akgül and Kivanç, 1988; Jayashree and Subramanyam, 1999; Mari et al., 2003), antitoxigenic (Akgül et al., 1991; Ultee and Smid, 2001; Juglal et al., 2002), antiparasitic (Pandey et al.
, 2000; Pessoa et al., 2002), and insecticidal (Konstantopoulou et al., 1992; Karpouhtsis et al., 1998) properties. These characteristics are possibly related to the function of these compounds in plants (Guenther, 1948; Mahmoud and Croteau, 2002).The purpose of this paper is to provide an overview of the published data on the antibacterial activity of those EOs and their components that could be considered suitable for application in or on foods, and to describe their possible modes of action.
The current knowledge on potential antagonists and synergists is presented; legal and safety aspects are discussed and areas for future research are proposed. Although some data are presented on spoilage flora, this paper will focus chiefly on the antibacterial effect of EOs on food borne pathogens and, in particular, those for which food animals are the major reservoir.