presence of oxygen because they have a mechanism for destroying
superoxide based on the accumulation of millimolar concentrations of
manganese.
3.2.4 Antimicrobial Barriers and Constituents
All foods were at some stage part of living organisms and, as such, have
been equipped through the course of evolution with ways in which
potentially damaging microbial infections might be prevented or at least
limited.
The first of these is the integument: a physical barrier to infection such
as the skin, shell, husk or rind of a product. It is usually composed of
macromolecules relatively resistant to degradation and provides an
inhospitable environment for micro-organisms by having a low water
activity, a shortage of readily available nutrients and, often, antimicro-
bial compounds such as short chain fatty acids (on animal skin) or
essential oils (on plant surfaces).
The value of these physical barriers can be clearly seen when they are
breached in some way. Physical damage to the integument allows
microbial invasion of the underlying nutrient-rich tissues and it is a
commonplace observation that damaged fruits and vegetables deterio-
rate more rapidly than entire products, and that this process is initiated
at the site of injury. Consequently it is important to the farmer and food
processor that harvesting and transport maintain these barriers intact as
far as possible.
As a second line of defence, the product tissues may contain antimi-
crobial components, the local concentration of which often increases as a
result of physical damage. In plants, injury can rupture storage cells
containing essential oils or may bring together an enzyme and substrate
which were separated in the intact tissue. The latter occurs in plants such
as mustard, horseradish, watercress, cabbage and other brassicas to
produce antimicrobial isothiocyanates (mustard oils) and in Allium
species (garlic, onions and leeks) to produce thiosulfinates such as allicin
(Figure 3.3). A class of antimicrobials known collectively as phytoalexins
are produced by many plants in response to microbial invasion, for
example the antifungal compound phaseollin produced in green beans.
Many natural constituents of plant tissues such as pigments, alkaloids
and resins have antimicrobial properties, but limited practical use is
made of these. Benzoic and sorbic acids found in cranberries and
mountain ash berries respectively are notable exceptions that are used
in their pure forms as food preservatives. Considerable attention has
been directed to the antimicrobial properties of those plants used as
herbs and spices to flavour food (Table 3.7). Analysis of their volatile
flavour and odour fractions, known as essential oils, has frequently
32 Factors Affecting the Growth and Survival of Micro-organisms in Foods