Purpose: To define the distribution of Escherichia coli O157:H7 and Listeria monocytogenes transfer rates between cutting-knives and lettuce leaves and model the bacterial transfer during consecutive cuts of leafy vegetables.
Methods: Lettuce leaves were inoculated with 5 log (HI) or 3 log (LI) CFU/g of E. coli O157:H7 or L. monocytogenes (serovars 1/2a, 4b). 100 sterile knives created single cuts to HI leaves (1 knife/cut). Subsequently, 100 knives, contaminated by cutting HI leaves, were used to cut uncontaminated lettuce leaves (UL), once/knife. Pathogens were enumerated on knives or lettuce and the % transfer-rate (TR%) from lettuce to knife or knife to lettuce, was determined. Next, a contaminated knife, performed consecutive cuts to UL and bacterial transfer from knife to lettuce was evaluated. A semi-mechanistic model, described the transfer of pathogens to knives and cut-vegetables during consecutive cuts. The model-performance was evaluated through sporadic introduction of inoculated lettuce leaves during the cutting process. Extrapolation experiments were also conducted (electric shredding of cabbage or knife-cutting of spinach and cabbage).
Results: Lettuce to knife TR-distributions were left-skewed, suggesting low transfer, for both pathogens. Knife to lettuce log TRs ranged from -2 to -0.4 for E. coli O157:H7 and -2 to 0 for L. monocytogenes, suggesting variable transfer potential. Regarding consecutive cuts, a rapid initial transfer was followed by an asymptotic tail at low cell numbers moving to lettuce or residing on knife. E. coli O157:H7 was transferred at slower rates than L. monocytogenes. These trends were sufficiently described by the transfer-model, with RMSE values of 0.421-0.675 and 0.337-0.466 for E. coli O157:H7 and L. monocytogenes, respectively. The model showed good performance in validation trials. However, during extrapolation experiments the model tended to underestimate bacterial transfer.
Significance: The present model could be a useful tool for quantitative risk assessment during preparation of leafy-green salads.