Quantitative Transfer of Listeria monocytogenes during Mechanical Slicing and Dicing of Cantaloupe and Honeydew Melon

Introduction: The 2011 multistate outbreak of listeriosis associated with whole cantaloupe has heightened concerns regarding best commercial handling and preparation practices for melons.

Purpose: Consequently, this study aimed to quantify Listeria monocytogenes transfer from one inoculated cantaloupe or honeydew melon to subsequent uninoculated melons during mechanical slicing and dicing.

Methods: Retail cantaloupe and honeydew melons were washed, dip-inoculated in a 3-strain avirulent L. monocytogenes cocktail (strains M3, J22F, and J29H) to contain ~5 log CFU/cm², air-dried for 1h and stored at 4°C for 24h.  In triplicate experiments, two cavity-cleaned inoculated melon halves were mechanically sliced with the rind (0.75 inch slices, Vollrath Redco 401N) or diced without the rind (1.0 inch dices, Nemco 55650 dicer) followed by eight uninoculated melon halves.  Alternate slices (~50 to 100 g/slice) and diced melon halves as well as Kimwipe^® samples from the slicer/dicer pusher and blade were homogenized in UVM, serially diluted and surface-plated with/without membrane filtration on trypticase soy agar containing 0.6% yeast extract, 0.1% esculin, and 0.05% ferric ammonium citrate to obtain black Listeria colonies which were counted after 24h at 37°C.  Any samples negative for Listeria by direct plating were subsequently enriched for 48h and then streaked onto Modified Oxford Agar for confirmation.

Results: Cantaloupe and honeydew melons inoculated at 5.3 and 4.4 log CFU/cm², respectively, yielded L. monocytogenes populations of 1.9 and 1.8 CFU/g in the melon flesh after slicing.  Cantaloupe inoculated at 3.1 CFU/cm² after rind removal yielded 2.1 CFU/g after dicing.  Listeria populations in slices from previously uninoculated cantaloupe and honeydew melon were similar (P > 0.05), ranging from -0.23 to 1.24 and -0.23 to 0.34 CFU/g, respectively. The pusher and blade of the mechanical slicer and dicer were the primary contributors to Listeria transfer during slicing and dicing.

Significance: While the rough cantaloupe surface is far more prone to contamination than honeydew, these results demonstrated similar transfer rates to both types of melon during slicing with these findings useful in further refining risk assessment models for fresh-cut produce.