Purpose: Our objective was to assess the relationship between produce density, as determined by a CT scanner, and transfer rates for Listeria monocytogenes during mechanical slicing of cucumbers, zucchini squash and onions.
Methods: CT scans were performed using a GE BrightSpeed™ Elite CT Scanner (General Electrics Healthcare, Buckinghamshire, UK). Two-dimensional CT images were acquired every 0.625 mm, at a voltage and current of 120keV and 240mA, respectively. Mean densities for each product were calculated from the CT images using MATLAB V2012a. In the slicing experiments, the products were dip-inoculated in a 3-strain avirulent L. monocytogenes cocktail (M3, J22F and J29H) to contain ~ 5 log CFU/g, air-dried and sliced using a NEMCO hand slicer to contaminate the slicer. Thereafter, Listeria transfer from the slicer to an uninoculated product of the same type was assessed by generating 15 additional slices. The 1st and 15th slices were collected and analyzed for Listeria by surface-plating on Modified Oxford Agar.
Results: The 1st and 15th slices of zucchini yielded average L. monocytogenes populations of 3.9 and 3.3 log CFU/g, respectively, which were significantly higher (P > 0.05) than onions (3.7 and 1.6 log CFU/g) and cucumbers (3.5 and 2.2 log CFU/g). Based on the CT scans, the mean density of zucchini (0.491 g/cm3) was significantly lower (P < 0.05) than onions (0.656 g/cm3) and cucumbers (0.604 g/cm3), indicating that bacterial transfer is inversely related to product density.
Significance: The extent of cross-contamination of fresh produce during slicing correlated with product composition. These findings should prove useful in developing improved predictive models for bacterial transfer and expanding current risk assessments across a wider range of products.