Purpose: Our objective was to evaluate the behavior of Salmonella and Escherichia coli O157:H7 on onion skins and in chopped onions during storage.
Methods: The outer skin of whole yellow onions was inoculated with cocktails of rifampicin-resistant Salmonella or E. coli O157:H7 (ECO157) at 7 log CFU/9 cm2 and, after drying for 30 min, stored at 4 or 23°C for 28 days. Commercial fresh-cut and freshly-chopped onions were inoculated at 3 log CFU/g and stored at 4 or 23°C for 6 or 1.5 days, respectively. Salmonella and ECO157 populations were determined by plating prepared samples onto non-selective and selective agar supplemented with rifampicin. When necessary, samples were enriched using standard methods.
Results: Populations of Salmonella and ECO157 declined by 3 log CFU on onion skin within the 30 min drying time. Salmonella and ECO157 were not detected by enrichment when stored at 23°C for 7 days. At 4°C, Salmonella was detected by plating in two of six samples after 28 days; ECO157 was not detected by enrichment of samples stored for 21 days. Less than 0.5 log CFU/g reductions in populations of Salmonella or ECO157 were observed in chopped onions over 6 days of storage at 4°C. Less than 0.5 log CFU/g increases in Salmonella or ECO157 were observed in the first 8 h of storage at 23°C; after 12 and 24 h populations increased by 2 and 3 log CFU/g. Growth rates and maximum population densities were higher in closed containers and for freshly-chopped onions.
Significance: Salmonella and E. coli O157:H7 survive poorly on onion skin, but refrigeration is important to prevent growth of these organisms in chopped onions.