Survival or Growth of Inoculated Escherichia coliO157:H7 and Salmonellaon Yellow Onions (Allium cepa)under Conditions Simulating Food Service and Consumer Handling and Storage

Whole and diced yellow onions (Allium cepa)were inoculated with five-strain cocktails of rifampin-resistant Escherichia coliO157:H7 or Salmonellaand stored under conditions to simulate food service or consumer handling. The inoculum was grown in broth (for both whole and diced onion experiments) or on agar plates (for whole onion experiments). Marked circles (3.3 cm in diameter) on the outer papery skin of whole onions were spot inoculated (10 µl in 10 drops) at 7 log CFU per circle, and onions were stored at 4°C, 30 to 50% relative humidity, or at ambient conditions (23°C, 30 to 50% relative humidity). Diced onions were inoculated at 3 log CFU/g and then stored in open or closed containers at 4°C or ambient conditions. Previously inoculated and ambient-stored diced onions were also mixed 1:9 (wt/wt) with refrigerated uninoculated freshly diced onions and stored in closed containers at ambient conditions. Inoculated pathogens were recovered in 0.1% peptone and plated onto selective and nonselective media supplemented with 50 μg/ml rifampin. Both E. coliO157:H7 and Salmonellapopulations declined more rapidly on onion skins when the inoculum was prepared in broth rather than on agar. Agar-prepared E. coliO157:H7 and Salmonelladeclined by 0.4 and 0.3 log CFU per sample per day, respectively, at ambient conditions; at 4°C the rates of reduction were 0.08 and 0.06 log CFU per sample per day for E. coliO157:H7 and Salmonella,respectively. Populations of E. coliO157:H7 and Salmonelladid not change over 6 days of storage at 4°C in diced onions. Lag times of 6 to 9 h were observed with freshly inoculated onion at ambient conditions; no lag was observed when previously inoculated and uninoculated onions were mixed. Growth rates at ambient conditions were 0.2 to 0.3 log CFU/g/h for E. coliO157:H7 and Salmonellain freshly inoculated onion and 0.2 log CFU/g/h in mixed product. Diced onions support pathogen growth and should be kept refrigerated.