In this paper, a thermal process for the surface pasteurization of ready-to-eat (RTE) meat products for the reduction of Listeria monocytogenes on such products (turkey bologna, roast beef, corned beef, and ham) is described. The process involves the passage of products through a ‘‘tunnel’’ of heated coils on a stainless steel conveyor belt at various treatment times relevant to the manufacture of processed meat for the surface pasteurization of RTE meat products. Two inoculation procedures, dip and contact inoculation, were examined with the use of a four-strain cocktail of L. monocytogenes prior to heat processing. With the use of radiant heat prepackage surface pasteurization, 1.25 to 3.5-log reductions of L. monocytogenes were achieved with treatment times of 60 to 120 s and air temperatures of 475 to 750°F (246 to 399°C) for these various RTE meats. Reduction levels differed depending on the type of inoculation method used, the type of product used, the treatment temperature, and the treatment time. Prepackage pasteurization (60 s) was also combined with postpackage submerged water pasteurization for formed ham (60 or 90 s), turkey bologna (45 or 60 s), and roast beef (60 or 90 s), resulting in reductions of 3.2 to 3.9, 2.7 to 4.3, and 2.0 to 3.75 log cycles, respectively. These findings demonstrate that prepackage pasteurization, either alone or in combination with postpackage pasteurization, is an effective tool for controlling L. monocytogenes surface contamination that may result from in-house handling.
A mixed cocktail of four strains of Listeria monocytogenes was resuspended in product purge and added to a variety of ready-to-eat (RTE) meat products, including turkey, ham, and roast beef. All products were vacuum sealed in shrink-wrap packaging bags, massaged to ensure inoculum distribution, and processed by submersion eating in a precision-controlled steam-injected water bath. Products were run in pairs at various time-temperature combinations in either duplicate or triplicate replications. On various L. monocytogenes-inoculated RTE deli meats, we were able to achieve 2- to 4-log cycle reductions when processed at 195°F(90.6°C), 20o°F ( 93.3°C), or 205°F (96.1°C) when heated from 2 to 10 min. High-level inoculation with L. monocytogenes (~10⁷ CFU/ml) ensured that cells infiltrated the least processed surface areas, such as surface cuts, folds, grooves, and skin. D- and z-value determinations were made for the Listeria cocktail resuspended in product purge of each of the three meat categories. However, reduction of L. monocytogenes in product challenge studies showed much less reduction than was observed during the decimal reduction assays and was attributed to a combination of surface phenomena, including surface imperfections, that may shield bacteria from the heat and the migration of chilled purge to the product surface. The current data indicate that minimal heating regimens of 2 min at 195 to 205°F can readily provide 2-log reductions in most RTE deli meats we processed and suggest that this process may be an effective microbial intervention against L. monocytogenes in RTE deli-style meats.
Surface pasteurization for inactivation of Listeria monocytogenes was evaluated for radiant heat prepackage pasteurization, submersed water postpackage pasteurization, and combinations of the two techniques on various types of ready-to-eat deli turkey products obtained from at least four different manufacturers. Products were inoculated either by in-package liquid inoculum or surface sponge-contact with approximately 109 CFU of L. monocytogenes. Additional testing of radiant heat pasteurization was performed with low-level inoculation of product undersides with approximately 100 CFU of L. monocytogenes followed by enrichment recovery after pasteurization. Prepackage pasteurization provided 2.0 to 2.8 log reductions when processed for 60 s and 2.8 to 3.8 log reductions when processed for 75 s. An improved radiant oven provided 3.53 (60 s) and 4.76 (75 s) log reductions of L. monocytogenes. No positive samples were detected after enrichment when 40 samples of deli turkey (4 to 4.5 kg) undersides were inoculated at low levels and processed for 75 s. Submersed water postpackage pasteurization provided 1.95 to 3.0 log reductions when processed for 2, 3, 4, or 5 min, and combinations of the two processes gave 3.0 to 4.0 log inactivation of L.monocytogenes using either 60 1 60 s or 60 1 90 s for the prepackage and postpackage pasteurization processes, respectively. These processes, either individually or in combination, can provide postprocess elimination of bacteria for the manufacture of safe ready-to-eat deli meats.
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 Listeria on 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. monocytogenes of 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. innocua showed 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.
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