452 Produce Degradation: Reaction Pathways and their Prevention
14.5.2 CONTROL OF BACTERIAL INFILTRATION
Control of bacterial infiltration in fresh produce is important for ensuring microbial
safety and reducing postharvest losses. From a food safety perspective, surface
sanitizing treatments aimed at enhancing the microbial safety of fruits and vegetables
can be ineffective when human enteric pathogens such as E. coli O157:H7 and
Salmonella infiltrate these products. In fact, bacteria that become internalized in
subsurface areas of fresh produce are more difficult to destroy than those located
on the surface. This is most likely due to the inaccessibility of the internalized
organisms to sanitizing treatments applied to the surface of fresh produce (Adams
et al., 1989; Parish, 1997; Beuchat, 1998; Senkel et al., 1999). Based on the findings
of previous studies (Bartz et al., 1975; Bartz, 1981; Bartz and Showalter, 1981),
inadvertent microbial inoculation of fruit via bacterial infiltration can cause more
extensive postharvest losses than mere contamination of the fruit surface. During
infiltration bacteria may become internalized over a relatively wide area within
certain fruits such as tomatoes, whereas in surface contamination bacteria initiate
proliferation in a limited area (Bartz and Showalter, 1981).
14.5.2.1 Barriers to Effective Control of Infiltration
Certain factors related to the natural environment, irrigation of fields, inherent
properties of fresh produce, or postharvest processing of these products, make it
challenging to control bacterial infiltration. As previously mentioned, environmental
factors such as strong winds, hail, and frost can inflict physical damage (e.g., cuts,
bruises, splits, and punctures) to fresh produce. Damaged fruits with surface lesions
are more prone to bacterial infiltration than are sound fruits (Buchanan et al., 1999;
Merker et al., 1999). Also, preharvest contact of fruits and vegetables with contam-
inated irrigation water or soil makes postharvest control of pathogens more chal-
lenging due to the increased populations of microorganisms on fresh produce enter-
ing packing houses (Beuchat, 1998). Samish et al. (1963) observed that healthy
tomatoes from farms that used overhead irrigation carried more bacteria than toma-
toes from farms that employed furrow irrigation. It is likely that overhead irrigation
systems create a high-humidity microclimate that favors attachment and growth of
certain bacteria on tomatoes.
Another important factor that contributes to bacterial infiltration and further
exacerbates this problem is the inherent variation in porosity in fresh produce.
Porosity and size of natural openings vary widely among types of fruits and vege-
tables or within one cultivar of fruit or vegetable. Variations in porosity have been
observed among stem scars of tomatoes within a given lot (Bartz, 1982) and also
among different cultivars (Bartz, personal communication). Buchanan et al. (1999)
observed that among the Golden Delicious apples immersed in a dye solution, those
apples that were infiltrated to a greater extent by the dye had relatively larger open
channels into the core region.
During postharvest processing of fresh fruits and vegetables, particularly during
washing, certain factors have been shown to contribute to bacterial infiltration. Two
important factors are negative temperature differential (warm product, cold water)