Purpose: The purpose of this study was to assess the efficacy of a chlorine-based sanitizer against MS2 coliphage, as a surrogate for enteric viruses on fresh-cut Romaine lettuce during simulated commercial production.
Methods: Batches of Romaine lettuce (6 kg) were submerged for 15 min in 80 liters of water containing coliphage MS2 at 109 - 1010 PFU/ml and then processed using a small-scale commercial leafy green processing line. A commercial chlorine-based sanitizer (XY-12, Ecolab) was added to ~800 liters of water adjusted to pH 7.0 - 7.5 to obtain 25 ppm free chlorine with sanitizer-free water serving as a control. Triplicate 50 g lettuce samples were collected after inoculation, mechanical shredding, flume washing (2 min), shaker table dewatering, and centrifugal drying. Viruses on the lettuce were recovered in Tris-glycine buffer and determined as plaque-forming units with Escherichia coli Famp serving as the host in a double agar overlay assay. An unpaired two sample t-test was used to compare virus removal during the various stages of processing.
Results: The inoculated lettuce contained MS2 at 104 to 106 PFU/g before processing. The greatest reduction in MS2 occurred between shredding and flume washing, ranging 0.45 to 1.02 log PFU/g and 0.56 to 0.98 log PFU/g for sanitizer and sanitizer-free treatments, respectively. Numbers of MS2 virus remained relatively stable following flume washing with or without 25 ppm free chlorine. Total reductions of 0.98 and 0.79 log PFU/g were seen after processing with and without the sanitizer, respectively, with no statistical difference observed between the two treatments (P > 0.05).
Significance: These findings suggest that currently recommended commercial production practices are unable to effectively decrease virus on leafy greens during commercial processing.