Purpose: Consequently, this study aimed to quantify the spread of L. monocytogenes during simulated commercial dicing of onions.
Methods: One 2.3 kg batch of Spanish jumbo yellow onions (Allium cepa) was dip-inoculated in a 3-strain avirulent L. monocytogenes cocktail (M3, J22F and J29H) to contain ~ 4 or 2 log CFU/g, air-dried for 90 min, and then diced using an Urschel Model HA dicer, followed by ten 2.3 kg batches of uninoculated onions. Diced onion (50 g/batch) and equipment surface (100 cm2) samples were collected after dicing, added to UVM medium, homogenized by stomaching, appropriately diluted and then plated with/without prior membrane filtration on Modified Oxford Agar to enumerate Listeria. All UVM-diluted samples negative by direct plating were subsequently enriched and plated. Findings from triplicate experiments were analyzed by the Tukey-Kramer HSD test using JMP 10.0.
Results: After dicing one batch of inoculated onions containing L. monocytogenes at 4.1 log CFU/g to contaminate the slicer followed by 10 uninoculated batches, Listeria was quantifiable in all samples with average populations of 3.0, 1.3, and 0.6 log CFU/g in the 1st, 5th and 10th batch, respectively. At the lower inoculation level of 2.7 log CFU/g, L. monocytogenes was detected by enrichment out to the 10th batch in 2 of 3 replicates. At the higher inoculation level, the bottom of the loading drum and the circular dicing blades yielded significantly (P < 0.05) more Listeria compared to other surfaces, after dicing 10 batches of onions.
Significance: These results show the ability for Listeria to cross-contaminate potentially large quantities of onions during mechanical dicing with such findings helping to fill one of the major knowledge gaps in risk assessments for fresh-cut produce.