Since ancient times, spices and herbs have not been consciously added to foods as preservatives but mainly as seasoning additives due to their aromatic properties. Although the majority of essential oils from herbs and spices are classified as Generally Recognized As Safe (GRAS) (Kabara, 1991), their use in foods as preservatives is limited because of flavour considerations, since effective antimicrobial doses may exceed organoleptically acceptable levels. This problem could possibly be overcome if answers could be given to the following questions:
• Can the inhibitory effect of an essential oil (a mixture of many compounds) be attributed to one or several key constituents?
• Does the essential oil provide a synergy of activity, which simple mixtures of components cannot deliver?
• What is the minimum inhibitory concentration (MIC) of the active compound(s) of the     essential oil?
• How is the behaviour of the antimicrobial substance(s) affected by the homogeneous     (liquid, semisolid) or heterogeneous (emulsions, mixtures of solids and semisolids)
structure of foodstuffs?
• Could efficacy be enhanced by combinations with traditional (salting, heating, acidifica-tion) and modern (vacuum packing, VP, modified atmosphere packing, MAP) methods    of food preservation?
An in-depth understanding of the antimicrobial properties of these compounds is needed to answer these questions but such understanding has been lacking, despite the burgeoning literature on the subject. Methodological limitations (discussed in more detail below) in the evaluation of antimicrobial activity in vitro have led to many contradictory results. More-over, there have been too few studies in real foods (these are considered laborious and often lead to negative outcomes). There is also a need to investigate the appropriate mode of application of an essential oil in a foodstuff. For instance, immersion, mixing, encapsula-tion, surface-spraying, and evaporating onto active packaging are some promising methods of adding these compounds to foods that have not been extensively investigated.

Measuring antimicrobial activity
The antimicrobial activity of plant-derived compounds against many different microorgan-isms, tested individually and in vitro, is well documented in the literature . However, the results reported in different studies are difficult to compare directly. Indeed, contradictory data have been reported by different authors for the same antimicrobial compound (Mann and Markham, 1998; Manou et al., 1998; Skandamis, 2001; Skandamis et al., 2001b). Also, it is not always apparent whether the methods cited measure bacteriostatic or bactericidal activities, or a combination of both. Antimicrobial assays described in the literature include measurement of:
• the radius or diameter of the zone of inhibition of bacterial growth around paper discs impregnated with (or wells containing) an antimicrobial compound on agar media;
• the inhibition of bacterial growth on an agar medium with the antimicrobial compound diffused in the agar;
• the minimum inhibitory concentration (MIC) of the antimicrobial compound in liquid media;
• the changes in optical density or impedance in a liquid growth medium containing the antimicrobial compound.Three main factors may influence the outcome of the above methods when used with essential oils of plants: (i) the composition of the sample tested (type of plant, geographical location and time of the year), (ii) the microorganism (strain, conditions of growth, inoculum size, etc.), and (iii) the method used for growing and enumerating the surviving bacteria.Many studies have been based on subjective assessment of growth inhibition, as in the disc diffusion method, or on rapid techniques such as optical density (turbidimetry) without accounting for the limitations inherent in such methods. In the disc method, the inhibition area depends on the ability of the essential oil to diffuse uniformly through the agar as well as on the released oil vapours. Other factors that may influence results involve the presence