identified compounds such as allicin in garlic, eugenol from allspice
(pimento), cloves and cinnamon, thymol from thyme and oregano, and
cinnamic aldehyde from cinnamon and cassia which have significant
antimicrobial activity (Figure 3.4). As a consequence, herbs and spices
may contribute to the microbiological stability of foods in which they are
used. It has, for example, been claimed that inclusion of cinnamon in
raisin bread retards mould spoilage. Usually, however, their role in
preservation is likely to be minor and, in some cases, they can be a source
of microbial contamination leading to spoilage or public health prob-
lems. Outbreaks of botulism associated with crushed garlic in oil and
home canned peppers demonstrate that even in relatively high concen-
trations plant antimicrobials are not a complete guarantee of safety.
Antimicrobial components differ in their spectrum of activity and
potency, they are present at varying concentrations in the natural prod-
uct, and are frequently at levels too low to have any effect. Hops and
their extracts are ubiquitous ingredients in beer. Humulones contained in
the hop resin and isomers produced during processing, impart the
characteristic bitterness of the product but have also been shown to
possess activity against the common beer spoilage organisms, lactic acid
bacteria. When first introduced into brewing, hops probably contributed
to microbiological stability, but this is less likely nowadays with the
relatively low hopping rates used. In fact the ability of lactic acid bacteria
to acquire resistance to hop resins means that the brewery environment
probably acts as a very efficient natural enrichment culture for humu-
lone-tolerant bacteria, thus negating any beneficial effects.
A rather different example of the importance of plant antimicrobials is
provided by oleuropein, the bitter principle of green olives. In the
Figure 3.4 Essential oil components with antimicrobial activity
34 Factors Affecting the Growth and Survival of Micro-organisms in Foods