Purpose: Given recent safety concerns with fresh-cut produce, this study aimed to model Listeria monocytogenes growth in sanitizer-treated diced onions, tomatoes and celery during storage at 4, 7 and 12°C.
Methods: Retail 8-kg batches of onions, tomatoes, and celery were washed, diced (Nemco Slicer Model 55500-2, 3/8 inch blade), and then dip-inoculated in a 3-strain avirulent L. monocytogenes cocktail (M3, J22F, and J29H) to obtain ~4 log CFU/g. After 8 min of draining, the samples were immersed in 80 ppm chlorine (XY-12, Ecolab) adjusted to pH ~6.0 or tap water for 2 min, centrifugally dried, and aerobically packaged. L. monocytogenes was periodically enumerated on Modified Oxford Agar during 18 d of storage at 4, 7,12, 16, or 23oC with the Baranyi and Robert model used to describe growth.
Results: As expected, none of the samples exhibited significant growth at 4°C. At 7°C, Listeria populations increased 1 - 3 logs after 18 d of storage with diced water-treated onions showing the highest populations. However, these growth data were insufficient for modeling. The Baranyi and Robert model predicted a ~5-log increase for all samples stored at 12, 16, or 23oC, with the maximum growth rates ranging from 0.0251 log/day for diced onions at 16oC to 0.33 log/day for samples stored at 12oC. The laboratory growth and model predictions were similar for all samples with the root mean squared error (RMSE) and bias respectively ranging from 0.2762 for onions at 23oC to 0.6321 for celery stored at 16oC, and from -0.00947 for tomatoes at 12 oC to 0.03105 for onions at 23oC.
Significance: These findings will be useful to risk assessors in predicting human exposure to Listeria from contaminated onions, tomatoes and celery.