Natural antimicrobials as additives and ingredients for the preservation of foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 142

toxicological data is provided. Roller and Board (2003) draw attention to fact that,
since the majority of antimicrobial components of herbs are hydrophobic, methods
based on agar diffusion to assess their antimicrobial activity are not always
appropriate. Unfortunately much of the published scientifi c literature describing
the antimicrobial activity of plant derived antimicrobials is based on agar
diffusion. Roller and Board (2003) along with Pol (2001) warn that factors such
as temperature, pH, fat and proteins, surfactants, minerals (especially salt) and
other food components can greatly reduce the effect in real foods and make
predictions diffi cult.
The lipophilic properties of many phenolic compounds such as cinnamic acid
and cavracrol play an important role in providing an antimicrobial effect. In fatty
foods, however, the more lipophilic compounds can be expected to partition in the
lipid phase and thereby lose antimicrobial activity (Juven et al. 1994, Roller and
Board, 2003).
As with other naturally occurring preservatives, it is becoming increasingly
apparent that no single plant derived antimicrobial will provide a 'magic bullet'.
This has resulted in increasing focus on the development of combination systems
or 'hurdle technology' that, if successful, can lower the concentration of plant
derived antimicrobials to a low enough level that does not impart an off-fl avour or
aroma to the food. Similarly the use of plant-derived materials in combination
with novel non-thermal pasteurisation methods such as UHP and pulsed electric
fi eld (PEF) is an increasingly studied area of research.

6.5 Essential oils
In recent years there has been considerable interest in the use of essential oils in
food preservation, although their fi rst use dates back to prehistoric times. Reports
as long as 5000 years ago can be traced to 1550 BC , when the ancient Egyptians
used cinnamon, cumin and thyme both for food preservation and mummifi cation
(Davidson and Naidu 2000).
The antimicrobial compounds present in plant materials are commonly found
in the essential oil fractions obtained by steam or supercritical distillation,
pressing, or extraction by liquid or volatile solvents. Screening and isolation of
antimicrobial phyto-phenols requires a multidisciplinary approach (Davidson and
Naidu 2000). When choosing a solvent for extraction, its ability to extract
components of a solute has to be considered. The more effi cient the extraction, the
greater is the range of compounds present in the extract. The need to use pure
solvents is essential, as is the requirement to carry out extraction under mild
conditions utilising, whenever possible, solvents of low reactivity. The possibility
of generating artefacts should never be discounted. Techniques to isolate bioactive
phytochemicals have been reviewed by Ghisalberti (1993).
Studies have been carried out on the mode of action of spices or their essential
oils to inhibit microorganisms. Since it is apparent that the terpenes in essential
oils are the primary antimicrobials, the mechanism most likely involves phenols.

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