Transfer of Listeria monocytogenes during mechanical slicing of onions

• Listeria can be spread to large quantities of onions during mechanical slicing.
• Numbers of Listeria transferred can be predicted using an exponential decay model.
• Proper slicer cleaning/sanitizing and cold chain management is needed for onions.

A series of recalls in 2012 involving diced yellow onions contaminated with Listeria monocytogenes from one manufacturer 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 of onions. Spanish yellow onions (Allium cepa) were dip-inoculated in a 3-strain avirulent L. monocytogenes cocktail (M3, J22F and J29H) to contain ∼8.6, 6.8, or 5.9 log CFU/onion. After air-drying, one inoculated onion was sliced using a mechanical hand slicer, followed by twenty uninoculated onions with the first, fourth, and last slices collected for Listeria analysis. Each sample was added to UVM medium, homogenized by stomaching, appropriately diluted and then plated, with or without prior membrane filtration, on Modified Oxford Agar with the plates incubated at 35 °C for 48 h. All UVM samples were enriched for 48 h and then streaked to MOX if Listeria was not quantifiable by direct plating. After slicing one onion inoculated at 8.6 log CFU/onion followed by 20 uninoculated onions, onions 1, 10 and 20 yielded average Listeria populations of 6.7, 3.8, and 2.7 log compared to 3.2, 1.6, and 1.7 log CFU/onion and 3.6, 1.3, and 0.9 log CFU/onion when the initial onion contained 6.8 and 5.9 CFU/onion, respectively. Listeria transfer during onion slicing was best described using an exponential decay model which should help to further the understanding of Listeria transfer during onion processing and provide the industry with guidelines to improve end product safety.