442 Produce Degradation: Reaction Pathways and their Prevention
14.1 INTRODUCTION
The surfaces of fruits and vegetables growing in orchards and fields inevitably
become contaminated with microorganisms from a variety of sources in the natural
environment. In addition to epiphytic bacteria that multiply on leaves and fruits,
human enteric pathogens can be transiently present on the surface of fresh produce.
Bacteria can become internalized in fruits and vegetables by penetrating deeper
tissues through damaged sites on the surface of these products. Internalization may
occur via infiltration with contaminated water in the fields or during postharvest
processing. Internalized bacteria can increase postharvest losses or compromise the
microbial safety of fruits and vegetables.
In response to public health concerns with microbial safety of minimally pro-
cessed fresh fruits and vegetables, various methods for destroying microbial con-
taminants on the surface of these popular food products are actively being studied.
Unfortunately, surface decontamination methods are ineffective in eliminating patho-
genic bacteria that infiltrate fresh produce and become internalized in deeper sub-
surface areas. Internalized bacteria are inaccessible to chemical sanitizers due to
physical protection from subsurface tissue in which they are embedded. Therefore,
prevention of infiltration is crucial for controlling internalization of bacteria and
improving the microbial shelf life and safety of fruits and vegetables.
This chapter provides an overview of the occurrence of bacteria and bacterial
adhesion on fresh produce followed by a review of current published research in the
following areas: (1) bacterial infiltration and internalization in fruits and vegetables;
(2) factors affecting infiltration and internalization; (3) control of bacterial infiltra-
tion; and (4) survival and growth of internalized bacteria. Additionally, recommen-
dations for future research will be provided.
14.2 OCCURRENCE OF BACTERIA ON FRUITS AND VEGETABLES
The surfaces of fruits and vegetables growing in orchards and fields provide a habitat
for a variety of microorganisms including bacteria, yeast, and molds. Among these
microbial groups bacteria constitute a major part of the microflora on leaf surfaces
and the surface of fruits. In fact, epiphytic bacteria are the first to populate newly
formed plant tissues, including buds. Bacterial colonization of newly formed buds
occurs from small populations of resident epiphytic bacteria within the bud tissue
or from a variety of environmental sources or vectors (Andrews and Hirano, 1991;
Beattie and Lindlow, 1994; Beattie, 2002). Sources of bacterial contamination in
fresh produce include soil, water, windblown dust, insects, animals (wild or domes-
ticated), animal manure used as fertilizer, humans, harvesting equipment, transport
vehicles, and processing equipment (Centers for Disease Control and Prevention,
1997; Beuchat, 1998). Of these sources inadequately composted animal manure is
a major contributor to contamination of fresh produce with human pathogens (U.S.
FDA, CFSAN, 1998; De Roever, 1999; Himathongkham et al., 1999). Apart from
epiphytic bacteria that multiply on undamaged fruit and vegetable surfaces, other
bacteria including human enteric pathogens can be transiently present on plant
surfaces (Leben, 1965). Contamination of tree fruit with human enteric pathogens