Quantification of Bacterial Transfer from Cucumber Skin to Flesh and Peeler during Peeling

Introduction:  Fresh fruit and vegetables have been recognized as a source of infection in many foodborne disease outbreaks. In 2013, a multistate outbreak of Salmonella linked to cucumber imported from Mexico occurred. Microorganisms on the surface of cucumbers can contaminate interior flesh during slicing or peeling.

Purpose:  Little is known about the degree to which bacteria can transfer from the surface of fresh produce items during peeling. This study quantified the transfer of the pathogen surrogate Enterobacter aerogenes from the surface of fresh cucumber to flesh during peeling. 

Methods: A food-grade strain of nalidixic acid resistant E. aerogenes that has similar attachment characteristic to Salmonella was grown in 10 ml of tryptic soy broth containing 50 μg/ml nalidixic acid at 37°C for 24 h and re-suspended in phosphate-buffered saline after centrifugation. The cucumber was surface inoculated with 10 ml of E. aerogenes (~10⁹ CFU/ml) in a zip top plastic bag by massaging for 30 seconds. Percent transfer was determined between the cucumber skin and the peeled flesh and between the cucumber skin and the peeler over 24 h.

Results:  Transfer of E. aerogenes from inoculated cucumber skin to flesh and peeler occurred during peeling, although a majority of Enterobacter remained on the peel. The percent transfer of Enterobacter from cucumber skin to flesh during peeling was between 0.52 to 7.56% and transfer rate decreased as drying time after inoculation was extended to 24 h. The percent transfer to the peeler ranged from 0.11 to 4% but did not change in a consistent pattern. A statistically significant difference was only observed in freshly inoculated cucumber at 0 h (P < 0.0001 and P = 0.0004 for flesh and peeler, respectively).

Significance: This study quantifies the effect of peeling on removal of bacteria from the surface of cucumbers, and subsequent transfer to interior flesh and peeler.