Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 18, 2011
Publication Date: March 1, 2012
Citation: Zhou, B., Feng, H., Luo, Y., Millner, P.D. 2012. Improved Design and Ultrasound-assisted Sanitation of Lettuce Harvesting Knives for Minimizing Escherichia coli O157:H7 Contamination. Journal of Food Protection. 75(3):563-566.
Interpretive Summary: Lettuce field coring harvesting is a recent industry development designed to reduce shipping cost and increase production yields; however, the design of the harvest knives currently used in the industry needs improvement in order to minimize pathogen contamination. This study developed two new harvesting knife designs with improved food safety features and tested the potential of ultrasound to improve harvest knife disinfection. Results indicate that the two prototype coring knives harbored significantly fewer harmful bacteria cells than the current commercially used ones, and were much easier to disinfect. Furthermore, the use of ultrasound significantly improved the disinfection efficacy of the chemical sanitizers used to clean the harvesting knives. The results of this research will be useful to growers and regulatory agencies.
Laboratory studies have shown that Escherichia coli O157:H7 can be transferred to lettuce during harvesting. Knives used for lettuce coring-in-field (CIF) harvesting are likely to contact soil and consequently could serve as a vehicle for the transmission of food-borne human pathogens from contaminated soil to harvested lettuce heads. The currently used harvesting knife consists of a wedge-shaped blade and coring ring welded together and positioned at opposite ends of a central plastic handle. The rough weld design enhances attachment of bacteria and soil/plant sap and makes difficult a thorough disinfection of the knife. Improving the design of the coring knife to aid sanitation and disinfection is critical for minimizing pathogen contamination and transference during CIF lettuce harvesting. This study was undertaken to develop prototypes of a new lettuce harvesting knife with improved food safety features, and to examine the effect of ultrasound on enhancing chlorine efficacy for reduction of E. coli O157:H7 on coring knives. The coring rings of the knives were dip-inoculated in a lettuce extract and soil slurry containing 106 CFU/ml E. coli O157:H7 cells followed by air-drying at 22°C for 2.0 hours. The rings were then submerged in a solution containing 1, 10, 50, 100 or 200 ppm free chlorine, with or without ultrasound (25 kHz, 500 W/L) for 30, 60 or 120 s. Results indicate that the two prototype coring knives harbored significantly fewer E. coli O157:H7 cells than the current commercially used ones, and significantly improved the disinfection efficacy. The ultrasound treatment reduced E. coli O157:H7 counts to below the detection limit on both the smooth areas and the rough weld joint within 30 s in 1 ppm chlorine solution.