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Rapid large-volume concentration for increased detection of Escherichia coli O157:H7 and Listeria monocytogenes in lettuce wash water generated at commercial facilities

Kearns, Elizabeth A., Gustafson, Ryann E., Castillo, Sonia M., Alnughaymishi, Hamoud, Lim, Daniel V., Ryser, Elliot T.
Food control 2019 v.98 pp. 481-488
Escherichia coli O157, Listeria monocytogenes, chlorine, hydraulic flumes, lettuce, microbial detection, pathogens, probability, quantitative polymerase chain reaction, sodium, tap water, thiosulfates, ultrafiltration
In pilot-scale testing, dead-end ultrafiltration concentration (DEUF-C) sampling increased detection of pathogens in lettuce wash water. This study investigated DEUF-C performance when scaled up to commercial processing levels. Two sets of experiments were done to: 1) compare pathogen detection probability in lettuce wash water generated from commercial pilot-scale processing using DEUF-C versus standard grab sampling, and 2) evaluate DEUF-C to concentrate pathogens inoculated into commercially-generated flume and centrifuge lettuce wash water. In pilot-scale runs, one lettuce head inoculated to contain 2–8 × 104 CFU of Escherichia coli O157:H7 and 3–10 × 105 CFU of Listeria monocytogenes was processed along with 907 kg of uninoculated lettuce using sanitizer-free tap water. Two to four 40-L volumes of the wash water were concentrated to 400 mL by DEUF-C, and pathogen detection probability in these samples was compared to standard grab samples after 24 h–48 h of enrichment using qPCR. In the second set of trials, chlorine in flume and centrifuge water from a local commercial processing facility was neutralized with sodium thiosulfate (100 mg/L), spiked with both E. coli O157:H7 and L. monocytogenes at 102 CFU/mL, and processed by DEUF-C. Total filterable volume was determined for DEUF-C samples, and detection (qPCR) probabilities in unenriched DEUF-C and grab samples were determined. E. coli O157:H7 and L. monocytogenes were detected in 100% of the DEUF-C samples from pilot-scale water samples. Comparable grab samples yielded 6.7% detection for E. coli O157:H7, and 20 and 60% for 24 h- and 48 h-enriched L. monocytogenes samples, respectively. The total filterable volume in commercially-generated waters was significantly higher for flume than for centrifuge water. All unenriched DEUF-C samples yielded E. coli O157:H7 and L. monocytogenes compared to 11.1 and 22.2%, respectively, for the unenriched grab samples. These findings indicate DEUF-C can improve the probability of detecting E. coli and Listeria in commercial wash water. However, filtration of commercial lettuce wash water remains challenging, with more work needed to ensure DEUF-C is feasible for commercial use.