ABSTRACTTRANSFER AND SURVIVAL OF LISTERIA MONOCYTOGENES DURING SLICING, DICING, AND STORAGE OF ONIONSByAndrew M. ScollonA series of recalls in 2012 involving diced yellow onions contaminated with Listeria monocytogenes has heightened public health concerns surrounding the preparation, handling, and use of raw onions. In response, this study aimed to quantify the extent of L. monocytogenes transfer during mechanical slicing and dicing of onions, assess sanitizer efficacy during commercial washing practices, and examine Listeria growth in diced onions during storage under different in-package atmospheres. Spanish yellow onions (Allium cepa) were inoculated with a 3-strain avirulent L. monocytogenes cocktail (M3, J22F and J29H), air-dried, and sliced or diced using a mechanical hand slicer or commercial onion dicer. Transfer from inoculated onions to sequentially processed uninoculated onions was determined by slicing or dicing inoculated followed by uninoculated onions. Samples were collected and analyzed for Listeria by direct plating or enrichment. The drum and rolling dicer blades were of greatest concern for the commercial dicer in terms of Listeria transfer, containing greater populations (P < 0.05) than the paddles and cross-cut blades after dicing uninoculated onions. Listeria transfer during onion slicing and dicing was best described using an exponential decay model, with populations on uninoculated product, sliced after an onion inoculated at 108 CFU/onion, decreasing to 104 CFU/onion after 10 onions, and populations on uninoculated product, diced after 2.2 kg of onion inoculated at 104 CFU/g decreasing to ~ 10 CFU/g after 13 kg. All sanitizer treatments effectively reduced cross-contamination from the wash water while an initial packaging atmosphere of 15% CO2, 5% O2, 80% N2, reduced (P < 0.05) growth of Listeria during storage.
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