Advances in Food Mycology

264 Aurelio López-Malo et al.
aflatoxin (Bullerman, 1974). The active compounds were reported to
be cinnamic aldehyde and eugenol, which are the major constituents
of essential oils from cinnamon and clove (Bullerman et al., 1977).
Eugenol has been reported as one of the most effective natural antimicrobials
from plant origin acting as a sporostatic agent (Al-Khayat
and Blank, 1985). Thymol at 100 ppm inhibited A. parasiticus growth
for 7 days at 28°C (Buchanan and Shepherd, 1981). A. flavus was
shown to be more sensitive than two other Aspergillus species when
exposed to essential oils of oregano and cloves (Paster et al., 1990).
1.2. Mechanisms of Action
Some essential oils, plant extracts, and oleoresins influence certain
biochemical and/or metabolic functions, such as respiration or
production of toxins or acids, indicating that the active components in
various oils and oleoresins may have different specificities for target
sites on or in microbial cells. Much of the research on spices has
included speculation on the contribution of the terpene fraction to
their antimicrobial activity. Few of the studies, however, have
attempted to isolate and identify the antimicrobial fraction, and no
references are found which relate to the mechanism by which spices
inhibit microorganisms. It seems reasonable that since many of the
components of the essential oils, such as eugenol and thymol, are
similar in structure to active phenolic antimicrobials, their modes of
action could be assumed to be similar (Davidson, 2001; López-Malo
et al., 2005a).
The possible modes of action of phenolic compounds have been
reported in several reviews (Wilkins and Board; 1989; Beuchat, 1994;
Nychas, 1995; Sofos et al., 1998; Davidson, 2001; López-Malo
et al., 2000, 2005a). These mechanisms have not been completely elucidated,
however, the effect of phenolic compounds is concentration
dependent (Prindle and Wright, 1977). At low concentration, phenols
affected enzyme activity, especially of those enzymes associated with
energy production, whereas at higher concentrations, protein denaturation
occurred. The effect of phenolic antioxidants on microbial
growth and toxin production could be the result of the ability of phenolic
compounds to alter microbial cell permeability, permitting the
loss of macromolecules from the interior. They could also interact
with membrane proteins causing a deformation in structure and functionality
(Fung et al., 1977). Conner and Beuchat (1984a, b) suggested
that antimicrobial activity of essential oils on yeasts could be the
result of disturbance in several enzymatic systems involved in energy