Bacterial Infiltration and Internalization in Fruits and Vegetables 455
the chlorine. However, the use of chlorination in combination with conditions to
prevent infiltration into fresh produce can offer a margin of safety against bacterial
infiltration that either intervention is unable to provide separately.
14.6 SURVIVAL AND GROWTH OF INTERNALIZED BACTERIA
Results of previous research indicated the potential for survival and growth of
bacteria internalized in fresh produce. Hill and Faville (1951) inoculated oranges,
still attached to the tree, with Aerobacter, Achromobacter, and Xanthomonas. The
researchers reported a 3-log increase in bacterial numbers during 5 weeks following
inoculation. In a market survey of fresh produce bacteria were frequently isolated
from inside whole tomatoes, cucumbers, beans, and peas and less frequently in
bananas and melons. Bacteria were rarely isolated from inside grapes, olives, peaches,
and citrus fruit (Samish et al., 1963). From a food safety perspective, human patho-
genic bacteria, internalized within fruits and vegetables, have to survive in these
products to pose a health hazard to consumers. For certain pathogens even low
numbers of internalized cells surviving in fresh produce might be important. For
example, under natural conditions, E. coli O157:H7 could pose a significant health
risk because the infective dose is less than 1,000 cells (Ackers et al., 1998). E. coli
O157:H7 can grow within damaged or decayed apples (Riordan et al., 2000) and in
areas of the fruit that are not accessible to washing (Sapers et al., 2000; Annous et
al., 2001).
The survival and growth of bacteria internalized in fruits and vegetables may
be influenced by several factors, including type of bacteria, physical and chemical
characteristics of the fruit or vegetable, and postharvest processing and storage
conditions. Walderhaug et al. (1999) studied the potential for survival and growth
of Salmonella Hartford and E. coli O157:H7 in California Valencia oranges artifi-
cially inoculated with these pathogens. The organisms were introduced in whole
oranges via injection into the core region, the albedo (white part of the orange) and
the orange section portion. The oranges were also inoculated through simulated
wounds produced by the tips of 1-mL plastic pipettes. The pathogens were deposited
in the oranges at two depths: shallow (4 to 5 mm) into the albedo and deep (10 to
11 mm) into the orange section portion. The oranges were held at 4°C or at 21°C
for 5 d.
Results of this study indicated some differences in survival of internalized
pathogens based on type of bacteria tested and in storage temperature for the oranges.
Survival of E. coli O157:H7 decreased after 5 d at 4°C irrespective of the internal
location of the organism. At 21°C an increase (2-log) in growth of E. coli O157:H7
was observed in the section portions that were inoculated via simulated wounds. No
growth of this pathogen occurred when it was injected directly into the alberdo;
however, almost a 1-log increase in numbers occurred when the organism was
injected into the core or introduced to the alberdo through a simulated wound.
Generally, Salmonella survived better than E. coli O157:H7 in oranges at 4°C;
populations of the pathogen remained relatively constant in all injected oranges. At
21°C Salmonella increased in all experimental treatment samples; increases ranged
from 0.2 to 3.9 log 10 CFU/mL. Other researchers have demonstrated that populations