Consumption of fecally contaminated green onions has been implicated in several major outbreaks of foodborne illness. The objectives of this study were to investigate the survival and growth of Salmonella and Escherichia coli O157:H7 in green onions during storage and to assess the application of high hydrostatic pressure (HHP) to decontaminate green onions from both pathogens. Bacterial strains resistant to nalidixic acid and streptomycin were used to inoculate green onions at low (∼1logcfu/g) and high (∼2logcfu/g) inoculum levels which were then kept at 4 or 22°C for up to 14 days. Both pathogens grew to an average of 5-6logcfu/g during storage at 22°C and the bacterial populations were fairly stable during storage at 4°C. High-pressure processing of inoculated green onions in the un-wetted, wetted (briefly dipped in water) or soaked (immersed in water for 30min) conditions at 250-500MPa for 2min at 20°C reduced the population of Salmonella and E. coli O157:H7 by 0.6 to >5logcfu/g, depending on the pressure level and sample wetness state. The extent of pressure inactivation increased in the order of soaked>wetted>un-wetted state. The pressure sensitivity of the pathogens was also higher at elevated treatment temperatures. Overall, after pressure treatment at 400-450MPa (soaked) or 450-500MPa (wetted) for a retention time of 2min at 20-40°C, wild-type and antibiotic-resistant mutant strains of Salmonella and E. coli O157:H7 inoculated on green onions were undetectable immediately after treatment and throughout the 15-day storage at 4°C. The pressure treatments also had minimal adverse impact on most sensorial characteristics as well as on the instrumental color of chopped green onions. This study highlights the promising applications of HHP to minimally process green onions in order to alleviate the risks of Salmonella and E. coli O157:H7 infections associated with the consumption of this commodity.
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