P3-98 Transfer of Listeria monocytogenes during Mechanical Slicing of Onions

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Andrew Scollon, Michigan State University, East Lansing, MI
Haiqiang Wang, Michigan State University, East Lansing, MI
Elliot Ryser, Michigan State University, East Lansing, MI
Introduction: A series of recalls in 2012 involving diced onions contaminated with Listeria monocytogenes from one manufacturer has heightened public health concerns surrounding the preparation, handling and use of raw onions in salads, pre-made sandwiches, and other ready-to-eat foods.

Purpose: In order to assess current commercial slicing practices, this study aimed to quantify the extent of L. monocytogenes transfer from one artificially contaminated onion to subsequent onions during mechanical slicing.

Methods: Locally purchased Spanish jumbo yellow onions (Allium fistulosum) were dip-inoculated in a 3-strain avirulent L. monocytogenes cocktail (M3, J22F and J29H) to contain ~ 6, 4, or 2 log CFU/g. After air-drying for 90 min, one inoculated onion was sliced using a NEMCO model 56750-2 onion slicer, followed by 20 uninoculated onions with the top, middle, and bottom slices from each onion collected for Listeria analysis. Each slice was added to 50 ml of UVM medium, homogenized by stomaching for 1 min, appropriately diluted and then plated with or without prior membrane filtration on Modified Oxford Agar with the plates incubated at 35oC for 48 h. All UVM samples were incubated for 48 h and then streaked to MOX if Listeriawas not quantifiable by direct plating.

Results: After slicing one inoculated onion containing L. monocytogenes at 6.1 log CFU/g to contaminate the slicer followed by 20 uninoculated onions, Listeria was quantifiable in all samples with average populations of 4.3, 1.3, and 0.3 log CFU/g in 1st, 10th and 20th onion, respectively. At the lower inoculation level of 4.1 log CFU/g, L. monocytogenes was sporadically detected out to the 20th onion at a level of -0.054 log CFU/g.  At the lowest inoculation level of 2.8 log CFU/g, L. monocytogenes was sporadically detected out to the 20thonion at a level of -0.001 log CFU/g.

Significance:   These results show the ability for Listeria to cross-contaminate large numbers of onions during mechanical slicing with such findings needed to help fill one of the major knowledge gaps in risk assessments for fresh-cut produce.