Factors Affecting Salmonella Cross-contamination During Post harvest Washing of Tomatoes

This study examined Salmonella cross-contamination during postharvest washing of tomatoes and determined factors that affect the spread of Salmonella. The use of the oxidation reduction potential (ORP) as a measure of the antimicrobial efficacy of wash water was evaluated. A bench-scale washing system was designed to simulate commercial operations. It consisted of a 15-gal tank equipped with a submersible pump and instrument to measure the chemical and physical properties of the wash water. Field-harvested tomatoes inoculated with S. Typhimurium were added to 40 L sterile tap water along with uninoculated tomatoes. After 5 min, all tomatoes and wash water samples were analyzed for Salmonella counts. The same washing operations were repeated at different temperatures and chlorine levels using tomatoes inoculated with different levels of the pathogen. Industry spent dump tank water was used to provide conditions with higher solid and organic contents. Uninoculated tomatoes became uniformly contaminated at levels of 3.02 ± 0.51 or 0.10 ± 0.19 log cfu/tomato after washing with tomatoes inoculated with 7 or 4 log cfu/tomato, respectively. Lower uptake of Salmonella was observed when wash water was kept at a higher temperature than that of tomatoes. No transfer of Salmonella was observed with 100 or 200 ppm of chlorine. At 5 or 30 ppm, low levels of transfer occurred and the extent of spreading was greater in the industry dump tank water. No linear correlation could be established between ORP and chlorine level at chlorine concentrations > 5 20 ppm. At 5 or 30 ppm of chlorine, the ORP readings were frequently higher than the recommended threshold value (650 mV). Salmonella could spread during washing operations and the use of sufficient sanitizer could prevent pathogen crosscontamination. Maintaining an ORP value at 650 mV may not guarantee the antimicrobial efficacy of the wash water. Introduction Fresh tomatoes have been associated with 14 outbreaks of salmonellosis in the U.S. since 1996 (1). Contamination that occurs in the farm can persist and further spread during postharvest processing. In packinghouses, field-harvested tomatoes are subjected to a series of washing steps. Past outbreak investigations have suggested that postharvest washing could serve as a point where contamination spreads. The risk is greater especially during dump tank washing where large quantities of tomatoes are comingled. As the wash water is often re-circulated, any contamination present in the water could further spread to subsequent batches. Although the potential of cross-contamination during postharvest washing has been recognized, quantitative analysis of the transfer of pathogenic microorganisms from contaminated tomatoes to wash water and other tomatoes in the batch have not been thoroughly investigated. Postharvest washing is also the most practical point of control. The use of sufficient sanitizer in wash water is critical to minimize potential cross-contamination. It has been suggested that tomatoes submerged in cooler water could result in the infiltration of bacteria. Maintaining a higher water temperature is considered an important control to reduce potential contamination. Industry and government guidelines (1, 2) have recommended specific performance criteria to maintain the quality of wash water (e.g., keeping the water temperature at least 10F warmer than the pulp temperature of tomatoes), yet these standards have not been adequately validated. Oxidation-Reduction Potential (ORP) has increasingly been used to monitor the antimicrobial efficacy of wash water as it provides real-time measurements and can be incorporated with automated sanitizer injection systems. The recommended threshold value of 650 mV was established based on municipal drinking water standards. Tomato dump tank water typically contains high levels of organic matters and particulates which may affect the antimicrobial efficacy of chemical disinfectants. It is not clear whether the recommended ORP threshold value correlates with an effective level of chlorine. Objectives  Study Salmonella cross-contamination during postharvest washing of tomatoes.  Determine factors affecting the spread of Salmonella during washing  Microbial load  Temperature  Chlorine levels  Organic/solid contents  Determine whether water quality measurements (e.g., ORP) can provide a real-time assessment of the antimicrobial efficacy of the wash water. Salmonella in sample Chlorine level (ppm) Inoculated tomato prior to washing (log cfu/tomato) Wash water (log cfu /ml) Uninoculated tomato after washing* (# positive / # total) 0 7.32 ± 0.06 3.95 ± 0.14 18/18 5 7.43 ± 0.30 < -2 11/18 30 7.76 ± 0.67 < -2 2/18 100 7.34 ± 0.13 < -2 0/18 Chlorine concentration 0 ppm 5 ppm 30 ppm 100 ppm Sterile tap water TCL initial (ppm) 5.0 ±0.3 32.0 ±1.4 96.5 ±0.7 TCL at t=5min (ppm) 4.7 ±0.1 28.5 ±0.7 90.5 ±9.2 ORP initial (mV) 266.6 ±39.8 792.0 ±32.5 807.0 ±8.5 786.5 ±5.0 ORP at t=5min (mV) 233.4 ±39.6 803.0 ±19.8 828.5 ±2.1 780.5 ±9.2 Industry spent dump tank water TCL initial (ppm) 5. 5 + 1.2 29.5 + 5.0 95.0 + 1.4 TCL at t=5min (ppm) 5.1 + 1.4 18 + 7.1 85.0 + 1.4 ORP initial (mV) 287.5 ±137.9 631.5 + 9.2 739.5 + 61.5 830.5 + 9.3 ORP at t=5min (mV) 196.5 ±2.1 545.5 + 27.6 737.0 + 73.5 842.5 + 7.8 Spread of Salmonella as affected by chlorine use (industry spent dump tank water, Room Temp) *Presence of Salmonella was detected after culture enrichments ORP reading during washing with different levels of chlorine TCL = total chlorine level The ORP reading measured as a function of chlorine levels. ORP vs. total chlorine level 0 200 400 600 80