control may be just as important to prevent condensation. The presence
of a film of water on the surface will allow access of motile bacteria such
asErwinia Pectobacteriumand pseudomonads to cracks, wounds and
natural openings such as stomata. A combination of constant low
temperature, controlled relative humidity, and a gas phase with reduced
oxygen (ca.2–3%) and enhanced CO 2 (ca.2–5%) has made it possible to
store the large hard cabbages used in coleslaw production for many
months making the continuous production of this commodity virtually
independent of the seasons.
Vegetables should not normally be a cause of public health concern
but the transmission of enteric pathogens such asSalmonella, VTEC and
Shigellais possible by direct contamination from farmworkers and the
faeces of birds and animals, the use of manure or sewage sludge as
fertilizer, or the use of contaminated irrigation water. Celery, watercress,
lettuce, endive, cabbage and beansprouts have all been associated with
Salmonellainfections, including typhoid and paratyphoid fevers, and an
outbreak of shigellosis has been traced to commercial shredded lettuce.
Since salad vegetables are not usually cooked before consumption, it is
important to follow good agricultural practices to avoid their contam-
ination during production. Contamination can be reduced by washing
produce in clean water but even chlorinated water will normally give
only a 2–3 log reduction in microbial numbers as some surface bacteria
are lodged in hydrophobic folds or pores and thus evade treatment.
Not all pathogens are necessarily transmitted to vegetables by direct or
indirect faecal contamination. Organisms such asClostridium botulinum
have a natural reservoir in the soil and any products contaminated with
soil can be assumed to be contaminated with spores of this organism,
possibly in very low numbers. This would not normally present a problem
unless processing or storage conditions were sufficiently selective to allow
subsequent spore germination, growth and production of toxin. In the
past, this has been seen mainly as a problem associated with underproc-
essed canned vegetables, but now it must be taken into consideration in
the context of sealed, vacuum or modified-atmosphere packs of prepared
salads. Those salads containing partly cooked ingredients, where spores
may have been activated and potential competitors reduced in numbers,
could pose particular problems. In 1987 a case of botulism caused by
Clostridium botulinumtype A was associated with a pre-packed rice and
vegetable salad eaten as part of an airline meal. Similar risks may occur in
foil-wrapped or vacuum packed cooked potatoes or film-wrapped mush-
rooms and in all these cases adequate refrigeration appears to be the most
effective safety factor.
Another group of pathogens naturally associated with the environ-
ment includes the psychrotrophic speciesListeria monocytogeneswhich is
commonly associated with plant material, soil, animals, sewage and a
156 Microbiology of Primary Food Commodities