Introduction

The site of contamination of a food can have significant impact on the techniques that will be effective for inactivating pathogenic microorganisms. If contamination is limited to the surface of a food, then surface treatments would be sufficient to eliminate the pathogens. However, if there are conditions under which microorganisms can become internalized, then surface treatments would be insufficient to achieve inactivation since the treatment would not come in intimate contact with the microorganisms in the interior of the food. Previous research with tomatoes (Bartz and Showalter, 1981) and apples (Buchanan et al., 1999) have indicated that immersion of warm fruit in cold water can lead to a transitory pressure differential that can result in the infiltration of bacteria. However, there appeared to be no data available to demonstrate whether this could occur in citrus fruits. As a means of providing an initial assessment of the potential for infiltration to occur in citrus fruits, a preliminary study was performed that examined the uptake of a dye by intact oranges and grapefruits.

Material and Methods

Fruit.

Fruit were obtained from commercial sources and included oranges and grapefruit from both Florida and California (Tables 1 and 2). All fruit were held at 4 °C until used. On the day before a trial was to be performed the appropriate number of fruit were transferred to warm to room temperature (21 °C). Approximately 40 pieces of each type of fruit were retained within the cold room. All fruit were individually inspected for defects (e.g., breaks in the peel, areas of decay), and any defective fruit were discarded. Thus, only intact, visually sound fruit were used in the experiment. Individual trials were performed using sets of 100 fruits (Tables 1 and 2).

Preparation of Dye Solutions.

On the day prior to the initiation of a trial, two large fiberglass pans each containing 1500 ml of water were placed in the 4 °C cold room and allowed to equilibrate to refrigeration temperature. On the day of the trial, an additional two pans of water were allowed to equilibrate to room temperature. Sufficient dye, FD&C Blue No. 1 brilliant blue Cl 42090, was added to each of the four water pans to achieve a final concentration of 200 mg/liter of water.

Immersion of Fruit.

An appropriate number of fruit per trial (Tables 1 and 2 ) were immersed in the designated pan so that the fruit was completely submerged for 10 minutes. Three combinations of water and fruit temperatures were used: warm fruit (21 °C) in cold water (4 °C), cold fruit (4 °C) in warm water (21 °C), and warm fruit (21 °C) in warm water (21 °C). After being immersed for 10 minutes the fruit were removed and rinsed individually with tap water to remove any dye on the exterior surface of the fruit.

Examination of the Fruit.

Using a sharp knife, the flower end (cut #1) and the stem end (cut #2) of the fruit were cut off, wiping the knife between cuts to prevent any carry over of any dye (Fig 1). The remaining fruit was then cut in half (cut #3) from the flower end to the stem end. The dissected fruit was then examined visually for dye penetration and classified as either not taking up the dye, having slight dye penetration, or having heavy penetration. The fruit were also examined for the uptake of dye through puncture wounds and for the presence of internal fungal contamination. Photographs of representative fruit were taken.

Results and Discussion

Three different temperature scenarios were analyzed, with the majority of samples examined being warm fruit being immersed into cold water. Those conditions are the that would maximize uptake from the inward pressure differential generated due to cooling of the airspaces within the fruit. The results of the study are summarized in Table 3. The study indicated that infiltration of water can occur into an intact fruit. It occurred most often when the temperature differential between fruit and water favored uptake, though there was some evidence of low levels of dye uptake in grapefruit when there was no temperature differential. Dye uptake occurred at a reduced rate when warm fruit were immersed in a warm dye solution, and did not occur when cold fruit were placed in the warm water.

In general, grapefruit were much more susceptible to infiltration than oranges, possibly because of its larger "air space." This would result in a greater pressure differential as the air space contracted as the fruit cooled. Dye uptake appeared to occur primarily through two routes of entry (Fig 3 - 6). A major route of entry was through the stem scar. In some instances (particularly with grapefruit), the amount of dye taken up was substantial. The second route of entry was through "healed" puncture wounds, resulting in the pooling of the dye below the peel surface. The size of these punctures were small enough that the fruit were not eliminated when individually inspected prior to the initiation of the experimental trial. It is not likely that these fruit would have been culled under even the most stringent of pre-sorting programs.

The results of the current dye uptake study are consistent with infiltration studies that have evaluated the uptake of water by produce (e.g., apples, tomatoes) and other foods (e.g., eggs) when there is a transitory pressure differential between the interior and exterior of the food. Additional studies will be needed to demonstrate that routes of entry in citrus fruit are sufficiently large to allow pathogenic microorganisms to be carried along. The potential for uptake of pathogenic and non-pathogenic bacteria in this manner has been demonstrated with other produce such as tomatoes (Bartz and Showalter, 1981; Zhuang et al., 1995) and apples (Buchanan et al., 1999).

References

J. A. Bartz and R. K. Showalter. 1981. Infiltration of tomatoes by aqueous bacterial suspensions. Phytopathology 71:515-518.

R. L. Buchanan, S. G. Edelson, R. L. Miller, and G. M. Sapers. 1999. Contamination of intact apples after immersion in an aqueous environment containing Escherichia coli O157:H7. J. Food Protection 62:444-450.

R. Y. Zhuang, L. R. Beuchat, and F. J. Angulo. 1995. Fate of Salmonella montevideo on and in raw tomatoes as affected by temperature and treatment with chlorine. Appl. Environ. Microbial. 61:2127-2131.

Figures

Figure 1.Location of cuts used to dissect fruit to examine them for dye uptake.

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Figure 2. Example of fruit in which no dye penetrated. Fruit were from groups in which no temperature differential was present (warm fruit/warm dye).

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Figure 3. Examples of the uptake of dye by Florida grapefruit. Panel A. Grapefruit in which dye uptake was designated as "heavy". Dye appears to have entered through the stem scar and spread throughout center of fruit. Panel B. Grapefruit in which dye uptake was designated "slight-to-moderate". All specimens shown were from positive differential group.

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Figure 4. Examples of the uptake of dye by Florida grapefruit. Panel A. Arrow #1. Grapefruit in which dye uptake was "heavy". Dye seems to have penetrated both ends of the grapefruit. Arrow #2. Grapefruit - dye uptake designated moderate. Panel B. Grapefruit in which dye uptake was described as "heavy". All specimens shown were from positive differential group.

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Figure 5. Examples of the uptake of dye by oranges. Panel A. Florida orange in which dye uptake was designated "slight-to-moderate". Panels B and C. California oranges in which dye uptake was described as "slight-to-moderate". All fruit shown were from positive differential group.

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Figure 6. Examples of dye uptake by California grapefruit. Panel A. Grapefruit with "heavy" dye uptake. Panel B. Grapefruit with "slight-to-moderate" dye uptake. All specimens shown were from positive differential group.

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Figure 7. Florida Grapefruit in which no dye penetrated, but in which internal mold contamination occurred [black material surrounding seeds].

R. Merker, S. Edelson-Mammel, V. Davis, and R. L. Buchanan
November 4, 1999