Natural antimicrobials as additives and ingredients for the preservation of foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 143

The mode of action of phenolic compounds is generally related to other phenolic
compounds and is thought to involve interference with functions of the cytoplasmic
membrane (Eklund 1985; Davidson 1993; Juven et al. 1994). In addition terpenes
may have other antimicrobial mechanisms. It is suggested that essential oils may
inhibit yeast enzymes resulting in energy depletion and interference with synthesis
of structural components (Conner and Beuchat 1984; Connor et al. 1984).

6.5.1 Oleuropeins
Oleuropein is the bitter component of olives. The storage of yoghurt under olive
oil has been carried out since Biblical times, where it is assumed that the oil has a
preservative effect (Davidson and Naidu 2000). The hydrolysis products of olive
oil possess a wide range of antimicrobial activity (Juven et al. 1972; Fleming
et al. 1973). Oleuropein extracted from olives with ethyl acetate inhibits the
germination and outgrowth of Bacillus cereus spores (Tassou et al. 1991). Tranter
et al. (1993) demonstrated that a low concentration (0.1%) of oleuropein delayed
growth of S. aureus and that higher concentrations (0.4-0.6%) inhibited growth
completely. An intermediate concentration of 0.2% inhibited production of
enterotoxin. The inhibitory effect of commercial oleuropein against Salmonella
has been demonstrated in laboratory medium and reconstituted milk (Tassou and
Nychas 1995). Oleuropein is also a powerful antioxidant. Its high presence in
Mediterranean diets suggests a link with the low incidence of heart disease in that
region (Davidson and Naidu 2000).

6.5.2 Oleoresins
Conner and Beuchat (1984) reported that an oleoresin of cinnamon is inhibitory
against yeasts. Briozzo et al. (1989) reported that essential oil of clove (0.4%) in
a concentrated sugar solution was cidal against various bacteria and the yeast,
Candida albicans . Decimal reductions of greater than three were reported and the
presence of organic matter did not negate activity.

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6.5.3 Thymol and carvacrol
The antimicrobial activity of oregano and thyme have been attributed to their
essential oils, which contain the terpenes carvacrol [2-methyl-5-(1-methlethyl)
phenol] and thymol [5-methyl-2-(1-methylethyl) phenol] respectively (Davidson
and Naidu 2000). Katayami and Nagai (1960) tested thymol and carvacrol against
a range of Gram-positive and Gram-negative bacteria and found inhibition against
all organisms at 0.005%. Essential oil from oregano had the highest activity of a
number of essential oils tested against both fungi and bacteria (Maruzella and
Henry 1958; Maruzella and Ligouri 1958). Ting and Deibel (1992) determined the
minimum inhibitory concentration (MIC) of oregano against L. monocytogenes at
24°C and found it to be 0.5-0.7%. Oregano at 0.5% or 1.0% was bacteriostatic to
L. monocytogenes in laboratory media but not at 1% in beef. Pol and Smid (1999)