Purpose: The purpose of this study was to identify the important elements relevant to produce washing processes and identify methods that will be used in surrogate selection.
Methods: The behavior of three (3) non-pathogenic microorganisms (generic E. coli Nissle1917, Pediococcus pentosaceus and lettuce isolate 813-F1) were examined in comparison to E. coli O157:H7 based on phenotypic similarities. Chlorine inactivation kinetics of E. coli O157:H7 and the non-pathogenic strains were evaluated with varying pH levels (6.5 and 8.0) and exposure times (3-30 seconds). Detachment of leaf-bound E. coli O157:H7 and non-pathogenic strains at different inoculation levels (~2 log, 6 log) and drying conditions (aging time, temperature) in wash water was examined.
Results: Chlorine inactivation at pH 6.5 resulted in a range of viability corresponding to E. coli O157:H7 and the non-pathogenic strains; demonstrating a sharp inactivation curve for E. coli O157:H7, EcN and P. pentosaceus. Whereas, inactivation at pH 8.0 allowed more survival relating to exposure time for all microorganisms. Detachment from inoculated leaves at 2 log and 6 log inoculation showed steady survival levels in wash water at 0 ppm and significantly lower survival at 1 ppm for all strains excluding 813-F1; 813-F1 was consistently less chlorine-sensitive. Aging time was not seen to have remarkable effects on bacterial transfer during washing.
Significance: These results suggest assay methods of chlorine inactivation at pH 6.5 and detachment with 6 log initial inoculation may be useful in selecting appropriate surrogates for fresh produce washing.