Surface Pasteurization of Post-harvest Raw Whole Onions to Eliminate Listeria Contamination Prior to Further Processing.

Introduction:  The 2011-2012 outbreak of Listeria monocytogenes associated with cantaloupe created a heightened awareness for potential L. monocytogenes contamination of vegetables and produce. Due to easy contamination from various environmental sources and the long term exposure prior to harvest, L. monocytogenes may establish biofilms on produce or post-harvest surfaces. Processing methods to eliminate Listeriaon produce surfaces reduces subsequent contamination of handle/cut/sliced pieces during downstream further processing operations.

Purpose:  In-plant validation of a flame oven for eliminating surface contamination of L. monocytogenesof raw onions destined for further processing.

Methods:  A gas-fired flame oven was used to test for lethality and process effectiveness on native microflora of onions (APC, yeast and mold) and inoculated (nonpathogenic) Listeria innocua (i.e., cocktail of antibiotic resistant ATCC 33090, ATCC 33091, and ATCC 51742). Control onions were sampled without heat treatment while heat-treated onions were placed on a conveyer belt for transit through the gas flames (47 sec) and processed through the oven before being sampled. Process evaluation included microbial enumeration of pre- and post-process levels of organisms. For selective enumeration of L. innocua, plating was done on TSA containing rifamycin and streptomycin (both at 50µg/ml) while tryptic soy agar (TSA) and potato dextrose agar (PDA), were used for indigenous bacteria and yeasts/molds, respectively. The data obtained was analyzed using one way ANOVA (P< 0.05).

Results:  The onions inoculated with L. innocuashowed approximately 5-log reduction. Approximately 5-log reductions were also obtained for indigenous bacterial contamination and 4-log reduction of yeast and molds when compared to unprocessed onions (P< 0.05). The gas flame burnt off the outer paper-like layer of the onions without damaging the onion ‘meaty’ layers and hence reduced yield loss.

Significance:  The gas flame-fired oven heat treatment could be an effective means of reducing transfer of Listeria spp. during subsequent slicing/dicing operations.