Purpose: To determine the difference between indicator microorganism groups and E.coli O157:H7 in PAA inactivation in water, and level of cross-contamination in lettuce wash using PAA-water.
Methods: Inactivation of indicator microorganisms [total viable count (TVC), Enterobacteriaceae, total coliforms] and E. coli O157:H7 was determined when exposed to PAA (0-150 ppm) in wash water with lettuce juice added as additional organic load. Cross-contamination of indicator microorganisms and E. coli O157:H7 from inoculated lettuce (~ 5 log CFU/cm2) to water was evaluated in 50-ml scale washing with different level of PAA (20, 40, 60, and 80 ppm). Bacterial survival or transfer to wash water was quantified by plating on appropriate media.
Results: For microbial inactivation using PAA, fewer numbers of Enterobacteriaceae and total coliforms than E. coli O157:H7 survived when exposed to the same levels of PAA. These two indicator groups were completely inactivated at 40 ppm, while E. coli O157:H7 had a reduction of only 0.7 log CFU/ml. At 80 ppm PAA, E. coli O157:H7 was completely inactivated; however, TVC was inactivated only when PAA increased to 150 ppm. For lettuce wash with 60 ppm PAA-water, approximately 1 log CFU/ml E. coli O157:H7 was detected in wash water after 60 second wash while Enterobacteriaceae and total coliforms were not detectable. After 80 ppm PAA wash, E. coli O157:H7 was completely eliminated and transfer of TVC to wash water was still observed.
Significance: TVC, Enterobacteriaceae and total coliforms may not be good indicator groups for evaluating control of E. coli O157:H7 during lettuce washing using PAA.