Microbial Ecology of Spoilage 397
possibility of surface evaporation or condensation of moisture on a surface should
be considered.
Generally, foods that depend on a certain aw for safety or shelf life considerations
will need to be stored such that the environment does not markedly change this
characteristic. Foods will eventually come to moisture equilibrium with their sur-
roundings. Thus, processors and distributors need to provide for appropriate storage
conditions to account for this fact.
Packaging plays a major role in the vulnerability of the food to the influence of
relative humidity. But even within a sealed container, moisture migration and the
phenomenon of environmental temperature fluctuation may play a role. It has been
observed that certain foods with low aw can be subject to moisture condensing on
the surface due to wide environmental temperature shifts. This surface water will
result in microenvironments favorable to growth of spoilage, and possibly patho-
genic, microorganisms. As a general guideline, the product should be held such that
environmental moisture, including that within the package, does not have an oppor-
tunity to alter the aw of the product in an unfavorable way.
The ability of bacteria to multiply on vegetables is influenced strongly by
environmental factors including the storage temperature, the presence of free water,
the relative humidity, and the gaseous environment. There has been considerable
interest in the possible use of modified atmospheres to extend the storage life of
vegetables [103–106]. Controlled atmospheres usually contain a lower concentration
of oxygen and a higher concentration of carbon dioxide than is found in air. This
tends to slow the respiration of the vegetable, retard ripening, and maintain quality
for a longer time than during storage in air. The minimum oxygen concentration
that will avoid injury to produce is approximately 2% in most cases. Below this
level anaerobic respiration may occur [106]. Controlled or modified atmospheres
are combined with refrigeration for greatest effectiveness.
A study of the effect of controlled atmospheres on the growth of L. monocyto-
genes on asparagus, broccoli, and cauliflower and on the shelf life of these vegetables
was conducted by Berrang et al. [107]. A high relative humidity was maintained
during the experiments. The controlled atmospheres extended the storage time at
4 or 15°C for which the vegetables remained acceptable for consumption but did
not affect the growth of L. monocytogenes. Similar results were obtained with A.
hydrophila [108]. Controlled atmosphere has similar effects on the sensory qualities
and the natural microflora of broccoli [109] and bell peppers [110].
Modified, but not controlled, atmospheres may be used for packaging vegetables
but respiration of the produce can result in high concentrations of carbon dioxide
and depletion of oxygen to concentrations below 1 to 2%. Spoilage of such products
is associated with high counts of facultatively anaerobic bacteria and usually occurs
before any significant multiplication of Clostridium spp. If severe depletion of
oxygen occurred in packs, then it would be expected that a decrease in quality of
the product would make it unsaleable before growth of Clostridium spp. had
occurred. In some cases, however, microenvironments have become established in
vegetable products other than canned foods and have resulted in growth of
C. botulinum and outbreaks of botulism.