Bacterial Infiltration and Internalization in Fruits and Vegetables 449
Golden Delicious, and Rome Beauty) were placed on soil that was artificially
inoculated with E. coli to simulate contamination of drop or windfall apples. E. coli
was detected in samples of the inner core and flesh of all apple varieties. The presence
of Salmonella in the stem scar area of whole tomatoes and in aseptically cut internal
tissues from fruits that were in contact with moist, artificially inoculated soils
indicated that bacteria can infiltrate tomatoes through the stem scar and into the pulp
(Guo et al., 2002). The results of these studies underscore the potential for bacterial
infiltration into fresh fruit outside of laboratory conditions.
Based on the results of dye uptake studies conducted using citrus fruits (Merker
et al., 1999) and apples (Buchanan et al., 1999), it is conceivable that bacteria can
become internalized in deeper tissues of fruits via infiltration with water. The uptake
of dye occurs mainly through natural structures such as the stem scar and blossom
end; however, Merker et al. (1999) reported that lesions such as older puncture
wounds that seemed to be “healed” provide other routes for ingress of dye into citrus
fruits. Buchanan et al. (1999) observed that apples were infiltrated through the
blossom end and those apples that took up a substantial amount of dye had clearly
recognizable open channels into the core region. Dye uptake was also observed
through the skin, especially at sites where the apples sustained bruises or punctures.
In studies involving immersion of apples in bacterial cell suspensions, E. coli
O157:H7 cells have been shown to infiltrate the inner core of sound Red Delicious
apples. The organism spread and attached to the cartilaginous pericarp of the ventral
cavity and seed locules of the fruit. Infiltration started at the blossom end of the
calyx and moved up the floral tube into the core area of the apple (Burnett et al.,
2000). Buchanan et al. (1999) recovered E. coli O157:H7 in greater numbers from
the outer core region (compared to the skin) of intact apples following immersion
in a suspension of the pathogen. These studies support observations that the blossom
end of apples allows infiltration of bacteria into these fruits. Bacterial infiltration
leads to internalization of bacteria in deeper tissues, which can protect the organisms
from contact with sanitizers applied to the surface of the fruit.
14.5.1 FACTORS AFFECTING BACTERIAL INFILTRATION
14.5.1.1 Temperature
During postharvest processing of fruits and vegetables the wash and flume water
used for cleaning these products may permit infiltration of bacteria. This problem
is further exacerbated when there is a negative temperature differential (warm prod-
uct, cold wash water). Previous research involving tomatoes (Bartz and Showalter,
1981; Bartz, 1982, 1999), apples (Buchanan et al., 1999; Burnett et al., 2000), and
citrus fruit (Merker et al., 1999) have demonstrated that immersion of warm fruit
in cold water produces a transient pressure difference that can lead to infiltration of
bacterial cells. Washing of warm, fresh produce with cold water has been shown to
allow uptake of water (Bartz and Showalter, 1981; Bartz, 1982; Zhuang et al., 1995;
Buchanan et al., 1999; Merker et al., 1999). Burnett et al. (2000) reported that
infiltration of E. coli O157:H7 through the floral tube of apples occurred regardless
of the temperature differential during inoculation of the apples. The researchers