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Survival or Growth of Inoculated Listeria monocytogenes on Yellow Onions (Allium cepa) under Conditions Simulating Food Service and Consumer Handling and Storage

Vanessa Lieberman (Staff Researcher), Linda J. Harris (Principal Investigator)

Fresh-cut onions were recalled in 2012 after Listeria monocytogenes was isolated from finished product. The purpose of this study was to evaluate the survival or growth of L. monocytogenes in both whole and diced onion. Whole and diced yellow onions (Allium cepa) were inoculated with a six-strain cocktail of rifampin-resistant L. monocytogenes and stored under conditions to simulate food service or consumer handling. The inoculum was prepared on agar plates (for both whole onion experiments) or in broth (for diced onion experiments). 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, and after drying for 30 min onions were stored at 4 or 23 degrees C, 30 to 50% relative humidity, for up to 56 days. Diced onions were inoculated at 3 log CFU/g and then stored in closed containers at ambient conditions. Populations were determined by plating onto tryptic soy agar and Modified Oxford or Chromagar Listeria with added rifampicin (for diced onion). Populations of L. monocytogenes declined to below the limit of detection by day 42 at 23 degrees C; at 4 degrees C populations fell to 4.04 log CFU/disk at day 56. Both agar and broth cultured inoculum increased by 3 log CFU/g over 38 h of storage at 23 degrees C in diced onion; growth rates were not significantly different (P < 0.05; 0.15 and 0.12 log/CFU/g/h, respectively). A longer lag phase was observed for broth-prepared inoculum (9.8 h) compared to agar-prepared inoculum (7.0 h). L. monocytogenes can survive for long periods on onion skin. Multiplication on diced onion is evident at times that exceed food service or consumer ambient hold times (4 and 2 h, respectively).

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