Fate of inoculated Listeria monocytogenes on yellow onions (Allium cepa) under conditions simulating food service and consumer handling and storage

Recalls and cases of listeriosis have been associated with the presence of Listeria monocytogenes in fresh-cut refrigerated or frozen onions or associated processing environments. The survival or growth of L. monocytogenes on the outer surface of whole onions and in diced onion was evaluated during simulated retail or consumer storage. Whole and diced yellow onions (Allium cepa) were inoculated with a 6-strain cocktail of L. monocytogenes collected either from agar plates or from broth culture. Marked circles (3.3 cm in diameter) on the outer papery skin of whole onions were spot inoculated (10 μl) at 7 log CFU per circle, dried for 30 min and then stored at 4 or 23 °C for up to 8 weeks. The marked circles or “disks” of the outermost skin layer were excised for sampling. Diced onions were inoculated at 3 log CFU/g and then stored in closed containers at 4 or 10 °C for 28 or 21 days, respectively, or at 23 °C for 38 h. Populations of L. monocytogenes were determined by plating each sample onto both tryptic soy agar and modified Oxford or CHROMagar Listeria agars. At 4 °C, populations of L. monocytogenes on whole onion declined from initial pre-drying levels of 6-7 log CFU/disk to mean levels of 2.39 ± 1.14 log CFU/disk at week 8; at 23 °C, populations declined to below the limit of detection by plating (<0.40 log CFU/disk) and by enrichment in 3, 9, and 12 of 12 samples at weeks 2, 3, and 4, respectively. No significant change in L. monocytogenes populations was observed in diced onion during 28 days of storage at 4 °C. A maximum rate of change of 0.0081 log CFU/g/day; a mean L. monocytogenes population of 6.86 ± 0.44 log CFU/g was observed at 17 days of storage at 10 °C. At 23 °C calculated lag times of 4.7 and 8.6 h, maximum rates of change of 0.15 and 0.16 log CFU/g/h, and maximum L. monocytogenes populations of 6.24 ± 0.07 and 6.10 ± 0.03 log CFU/g after 38 h of storage were observed in diced onion inoculated with agar and broth cultures, respectively. Diced onions support the growth of L. monocytogenes at 10 and 23 °C but not at 4 °C.