Reduction of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella on Whole Yellow Onions (Allium cepa) Exposed to Hot Water

Abstract

In-home or food service antimicrobial treatment options for fresh produce are limited. Hot water treatments for whole (unpeeled) produce have been proposed but data to support this practice for onions are not available. Separate cocktails of rifampin-resistant Escherichia coli O157:H7, Listeria monocytogenes , or Salmonella were cultured on agar and suspended in sterile water. The outer papery skin at the equator or root or stem ends of the whole yellow onions was spot inoculated at 6 log CFU/onion. After drying for 30 min and, in some cases, storage at 4°C for 6 days, onions were immersed in water at ~100°C for 5 s or 85°C for 10 to 180 s. There was no significant difference ( P > 0.05) in the mean decline of Salmonella on onions that were exposed to hot water after drying the inoculum for 30 min or after storage at 4°C for 6 days. Exposure of whole onions at 100°C for 5 s reduced E. coli O157:H7 and L. monocytogenes populations by >5 log CFU/onion at all inoculum sites and Salmonella populations by >5 log CFU/onion at the stem end and equator but not consistently at the root end. Mean root-end reductions of ≥5 log CFU/onion of E. coli O157:H7, L. monocytogenes , or Salmonella were achieved consistently when the root end was fully immersed in 85°C hot water for 45 or 60 s, except in a small number of cases (4/57; 7%) when the root end oriented above the water line during treatment. When onions were held at 85°C for 180 s with the root end above the water line in an uncovered water bath, no significant declines in Salmonella populations were observed; significant mean declines of Salmonella were achieved (mean 5 log CFU/onion [range 3.49 to 6.25]) when the water bath was covered. Short exposure to hot water can significantly reduce pathogens on the surface of whole onions; reductions are more consistent when the root end is submerged, or when the water bath is covered.