Purpose: Consequently, this study assessed the spread of Escherichia coli O157:H7 during pilot-scale washing and drying of fresh-cut Romaine lettuce.
Methods: Using inoculated radicchio as a colored surrogate for uninoculated Romaine lettuce, radicchio leaves (~5 x 5 cm) were spot-inoculated at ~10-1, 101 and 103 CFU/leaf and mixed with uninoculated Romaine lettuce leaves (~5 x 5 cm) to obtain 5 kg batches having inoculated:uninoculated product ratios of 0.5:100, 1:100, 5:100 and 10:100. After 2 minutes of flume washing in sanitizer-free water followed by shaker table dewatering and centrifugal drying in a pilot-scale processing line, all radicchio leaves were removed. Each batch was then divided into ~20 225-g samples which were analyzed for presence/absence of E. coli O157:H7 using the E. coli O157 GeneQuence method (Neogen Corp, Lansing, MI). The percentage of E. coli O157:H7 lost from radicchio during processing was determined by direct plating before and after processing with E. coli O157:H7 populations also assessed in 50 ml water samples by membrane filtration.
Results: Based on triplicate experiments, lower radicchio inoculation levels led to decreased E. coli O157:H7 transfer to Romaine lettuce during processing (P<0.05). All lettuce samples yielded E. coli O157:H7 when radicchio was inoculated at 103 CFU/leaf. When radicchio was inoculated at 101 CFU/leaf, the percentage of positive samples decreased from 100-90.5% to 100-81.0%, 90.5-76.2% and 85.7-28.6% for inoculated:uninoculated ratios of 10:100, 5:100, 1:100 and 0.5:100 while at 10-1 CFU/leaf, 38.1-4.8%, 9.5-0%, 4.8% and 19-0% of the samples were similarly positive for the same inoculated:uninoculated ratios. Within each inoculation level, no significant differences (P>0.05) were found among the four inoculated:uninoculated product ratios. E. coli O157:H7 was non-quantifiable in the water samples.
Significance: These findings will provide important data for improving exposure assessment in risk assessments for leafy greens.