Summary
Recently, there has been increasing interest and commitment to maintain the continuity of food safety preventive controls across the produce supply and marketing chain. Many retail foodservice and supermarket outlets are currently processing various fresh produce items in-store with the goal of delivering the freshest possible product to consumers. In store handling and food preparation for retail display and sales involves different modes of water use. In quick-serve-restaurant (QSR) outlets and supermarket backroom preparation areas it is common to have batch immersion as one or the only step in produce washing. Distinct from efficacy evaluations in dump and flume systems, small-scale batch immersion and washing, which involves extended contact time of product in water, was identified as an application where ozonated water could be an effective practice. This Proof of Concept project focused on washing of cored head lettuce, one of the most common produce items handled in retail stores, as the first application for assessment of a continuous ozone injection system. The specific objectives were to 1. Characterize the microbiological and oxidative-demand status under a representative operational retail washing and crisping; 2. Assess the total heterotrophic bacteria (HPC) and total coliform populations on cored crisphead lettuce from a wholesale distribution center to establish a short-term baseline of seasonal variation; and 3. Evaluate the efficacy of ozonated water treatment on HPC and total coliforms, as well in preventing cross-contamination by inoculated generic Escherichia coli, Listeria innocua (a surrogate for Listeria monocytogenes), and an attenuated strain of Salmonella enterica. Focusing on Objective 3, in each of 3 replicates, 24 heads of lettuce obtained directly from a single distribution center were removed of wrapper leaves and cored the day of processing. After filling a stainless steel tank with 75 L of tap water, 22 heads were submerged, cored side up, and covered with 5400 g of crushed ice. During filling and icing, a specifically designed small-scale ozone generator (Model SW-1-01, DEL Ozone, San Luis Obispo, CA) was used to charge the recirculating chilled water and left on for the duration of the sampling interval. Lettuce and water samples were taken at 20, 26, 32, and 38 minutes, with and without the addition of ozone in the batch wash water. Water samples were analyzed at a commercial retail outlet (Objective 1) and during each experimental batch wash setup for physicochemical parameters including: temperature, pH, oxidation-reduction potential, chemical oxygen demand, electro-conductivity turbidity, and dissolved ozone. For batch rinsing of uninoculated head lettuce with water (no ozone), populations of total heterotrophs in the wash water increased significantly (P < 0.05) by 2.90 log CFU/100 ml after 40 min. After 30 and 40 min of processing with the pilot-scale system, water with continual ozone injection contained lower populations (P < 0.05) of heterotrophs (2.33 and 3.81 log CFU/100 ml lower, respectively) and coliforms (2.30 and 2.77 log CFU/100 ml lower) than during processing with the water control. As expected, for inoculated water, a greater log-reduction was observed in the ‘cleanest’ batch water with low COD, exceeding 5-log reduction of pathogen numbers in two minutes. Pathogen surrogates released from the surface of individual inoculated lettuce leaves accumulated in water without ozone treatment but not in water when continually injected with ozone during the soak-interval. The accomplishments to date support an expectation that this ozone system approach is transferrable to other applications at points of retail produce handling and food preparation. Though preliminary relative to a standard optimization and comparative research study, this project has developed data which better defines and determines the performance of a small-scale ozonated wash water system. These results serve as a foundation for future studies that enhance guidelines for food service companies to use as an additional step for improving the continuity of produce safety from farm to fork.
Research Objectives
1. Characterize the microbiological and oxidative-demand status under a representative operational retail wash unit applied to heads of cored crisphead lettuce with ozonated water.
2. Assess the total heterotrophic bacteria (HPC) and total coliform populations on cored crisphead lettuce from a wholesale distribution center over time to establish a short-term baseline of seasonal variation.
3. Evaluate the efficacy of ozonated water treatment on HPC and total coliforms, as well as generic Escherichia coli, Listeria innocua (a surrogate for Listeria monocytogenes), and an attenuated strain of Salmonella. Determine whether efficacy in prevention of cross-contamination is maintained during continual batch soak-washing.
Findings & Recommendations
Benefit to industry Though preliminary relative to a standard optimization and comparative research study, this project has developed data which better defines and determines the performance of an ozonated wash water system pilot design. The ozonated wash maintained its efficacy in suppression of microbial accumulation in the water by continual injection during washing of iceberg lettuce heads, thereby preventing cross contamination. These results serve as a foundation for future studies that enhance guidelines for food service companies to use as an additional step for improving the continuity of produce safety from farm to fork.