Purpose: This study was performed to identify hot water or lime juice treatments for whole (unpeeled) or diced onions, respectively, that significantly reduced levels of inoculated pathogens.
Methods: Separate cocktails of rifampin-resistant Escherichia coli O157:H7, Listeria monocytogenes, or Salmonella spp. were prepared in sterile water. The stem, equator, or root end of the outer papery skin of whole yellow onions (Allium cepa) was inoculated at six log CFU/onion. After drying for 30 minutes and storage for zero to six days, onions were immersed in water at 100°C for five seconds or at 85°C for 5 to 180 seconds. Freshly diced yellow onions (20 g) were inoculated at three log CFU/g, 15 ml of commercially-prepared lime juice were added, and the mixture held at ambient temperatures for zero to 30 minutes. Samples (n=6) were plated onto tryptic soy agar and CHROMagars (O157, Listeria, and Salmonella), all supplemented with 75 µg/ml rifampin.
Results: Exposure of whole onions at 100°C for five seconds reduced populations of Salmonella by > five log CFU/onion at the stem and equator, but not the root end. Reductions of > five log CFU/onion of E. coli O157:H7, L. monocytogenes or Salmonella spp. at the root end were achieved when onions were exposed to 85°C hot water for 60 seconds, but not consistently at 45 seconds. After 30 minutes, less than one log CFU/g (0.41, 0.33, or 0.82 log CFU/g) declines of E. coli O157:H7, L. monocytogenes, or Salmonella spp., respectively, were observed in diced onions mixed with lime juice.
Significance: Short exposure to hot water can significantly reduce pathogens on the surface of whole onions; but, soaking diced onions in lime juice had little effect.