Purpose: Reduction of inoculated generic E. coli levels was observed on Gala and Golden Delicious apples with and without overhead cooling water application for up to three weeks.
Methods: A cocktail of rifampicin-resistant, generic E. coli was sprayed onto individual Gala and Golden Delicious apples in an orchard using a backpack sprayer after sunset. Galas (18) and Golden Delicious (at least 27) were harvested at each time point after inoculation (0, 8, 24, 32, 56, 176 h); Golden Delicious were also harvested at (344 and 512 h). Samples were enumerated on Chromagar ECC-rifampicin with and without filtration; pre-enrichment in TSB was performed as microbial counts declined. Uninoculated control apples were tested for indicator organisms on Chromagar ECC, as well as for pathogenic E. coli and Salmonella.
Results: Only one of forty uninoculated control apples was positive for generic E. coli (2.5 log CFU) and no pathogens were detected. Average generic E. coli levels were reduced 2.5 log CFU/apple within 8 h after inoculation; this time period reflects the minimum anticipated time between a final application of agricultural water and harvest. Average generic E. coli levels continued to decline at slightly more than a 0.5 log per day up to 3 days. Although average generic E. coli levels at the last sampling point for each variety were low (1.2 - 1.5 log CFU/apple), 3 of 15 Gala apples (20%) were above > 3 log CFU/apple after 1 week, and after 3 weeks, 14 of 57 Golden Delicious apples (25%) were > 3 log CFU/apple.
Significance: Preliminary data indicated that average generic E. coli levels were reduced at rates greater than or equivalent to the proposed 0.5 log per day reduction proposed by FDA for mitigation of water that exceeds the proposed standards.