Minimum leak size determination, under laboratory and commercial conditions, for bacterial entry into polymeric trays used for shelf-stable food packaging

This study sought to determine the minimum leak size for entry of Enterobacter aerogenes under laboratory conditions, and normal flora under commercial conditions, into tryptic soy broth with yeast extract (TSBYE), homestyle chicken, and beef enchilada packaged in 355-ml polyethylene terephthalate/ethylene vinyl alcohol/polypropylene trays. Channel leaks (diameters of 50 to 200 μm) were made across the sealing area of the trays. Pinholes (diameters of 5 to 50 μm) were made by imbedding laser-drilled metal and plastic disks into the tray lids. For the laboratory simulation, all trays were submerged and agitated for 30 min at 25°C in phosphate-buffered saline that contained 10⁷ CFU/ml of E. aerogenes. Under commercial conditions, trays with channel leaks were processed in retorts to achieve commercial sterility. All trays were subsequently incubated at 37°C for 2 weeks, and their contents plated onto eosin-methylene blue agar (for laboratory simulation) to enumerate E. aerogenes and brain heart infusion agar (for commercial conditions) to determine the presence of any bacteria. Under laboratory conditions, minimum pinhole sizes for E. aerogenes entry approximated 5 μm (TSBYE, metal disks; homestyle chicken, plastic disks), 20 μm (beef, plastic disks), and 30 μm (beef, metal disks). The minimum channel leak sizes for entry of E. aerogenes approximated 10 μm (TSBYE), 70 μm (chicken), and 200 μm (beef enchilada). Under commercial conditions, the minimum channel leak size for bacterial entry approximated 40 μm (TSBYE), 50 μm (homestyle chicken), and more than 200 μm (beef). Results showed that E. aerogenes can enter pinholes as small as 5 μm under a worst-case scenario. This information can be used to set pass and fail parameters for leak detection devices.