Produce Degradation Pathways and Prevention

Microbial Ecology of Spoilage 405

generally, lactic acid starter cultures have a protective effect against foodborne
pathogens in milk and in meat products [206,207] and their antagonistic properties
have been studied extensively and reviewed recently [208,209]. Lactic acid bacteria,
among which Leuconostoc spp. predominate, can reach high counts on minimally
processed vegetables after a few days of storage, and more particularly in products that
contain high carbon dioxide concentrations [33,210,211]. It is worth noting that a strain
of Leuconostoc spp. produces bacteriocins active against L. monocytogenes [212].
It has been observed that fluorescent pseudomonads can activate growth of
L. monocytogenes in pasteurized milk as well as on meat, although other workers
found that pseudomonads slightly inactivated L. monocytogenes in pasteurized milk
after the exponential growth phase [213]. Presumably this occurred because
pseudomonads hydrolyzed milk proteins, whereas L. monocytogenes alone did not
[214,215]. There are no similar results for vegetable products, but fluorescent
pseudomonads are the main component of their microflora and can actively partic-
ipate in the breakdown of plant tissues. Their role in releasing potential nutrients
for pathogenic microorganisms is worth investigating further.

12.5.4 COHABITATION WITH OTHER MICROORGANISMS MAY

AFFECT SURVIVAL AND GROWTH OF PATHOGENS

While the pH of many vegetables is in a range suitable for growth of pathogenic
bacteria, some, for example, fully ripe tomatoes, are in a pH range (3.9 to 4.4) that
prevents or retards growth. Yeasts and molds, on the other hand, have a competitive
advantage over bacteria that may access bruised tissues of acidic vegetables and
many fruits, because they are able to grow at a lower pH range (2.2 to 5.0) charac-
teristic of much of this produce. Spoilage of fruits is often caused by specific molds

FIGURE 12.2Cycle of infection and contamination of fresh produce. (Modified from
Beuchat, L.R., J. Food Prot., 59, 204, 1996.)

HUMAN

ANIMAL

Water

PRODUCE

Feces

Sources in

kitchen

Insects/Flies

Dust, soil

Manure

Cross

contamination

Harvesting /processing

equipments

Transport

container/vehicle

Soil

Cross

Meat, milk, eggs contamination

Silage, feed

Plant