title: Survival or growth of inoculated Escherichia coli O157:H7 and Salmonella on yellow onions (Allium cepa) under conditions simulating food service and consumer handling and storage.
abstract: Whole and diced yellow onions (Allium cepa) were inoculated with five-strain cocktails of rifampin-resistant Escherichia coli O157:H7 or Salmonella and 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. coli O157:H7 and Salmonella populations declined more rapidly on onion skins when the inoculum was prepared in broth rather than on agar. Agar-prepared E. coli O157:H7 and Salmonella declined 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. coli O157:H7 and Salmonella, respectively. Populations of E. coli O157:H7 and Salmonella did 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. coli O157:H7 and Salmonella in freshly inoculated onion and 0.2 log CFU/g/h in mixed product. Diced onions support pathogen growth and should be kept refrigerated.
authors.affiliation: Department of Food Science and Technology, Western Center for Food Safety, University of California, Davis, One Shields Avenue, Davis, California 95616-8598, USA.
authors.forename: Vanessa M
authors.initials: VM
authors.lastname: Lieberman
authors.affiliation: Department of Food Science and Technology, University of California, Davis, One Shields Avenue, Davis, California 95616-8598, USA.
authors.forename: Irene Y
authors.initials: IY
authors.lastname: Zhao
authors.affiliation: Department of Food Science, Rutgers University, 65 Dudley Road, New Brunswick, New Jersey 08901-8520, USA.
authors.forename: Donald W
authors.initials: DW
authors.lastname: Schaffner
authors.affiliation: Department of Food Science and Human Nutrition, Citrus Research and Education Center, University of Florida, 700 Experiment Station Road, Lake Alfred, Florida 33850, USA. ljharris@ucdavis.edu.
authors.forename: Michelle D
authors.initials: MD
authors.lastname: Danyluk
authors.affiliation: Department of Food Science and Technology, Western Center for Food Safety, University of California, Davis, One Shields Avenue, Davis, California 95616-8598, USA.
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Harris
mesh_terms: D000362:Agar; D000818:Animals; D015169:Colony Count, Microbial; D019453:Escherichia coli O157 / Q000254:growth & development*; D005511:Food Handling; D005516:Food Microbiology; D005522:Food Services; D061353:Food Storage / Q000379:methods; D019697:Onions / Q000382:microbiology*; D012034:Refrigeration; D012475:Salmonella / Q000254:growth & development*; D013696:Temperature; D013997:Time Factors
chemical_list: D000362:Agar