Outbreaks of E. coli O157:H7 infections have been a continuing food safety challenge for the produce industry. Previous studies showed that... Show moreOutbreaks of E. coli O157:H7 infections have been a continuing food safety challenge for the produce industry. Previous studies showed that contamination that originated in the farm can spread during postharvest processing. The objective of this study was to examine the spread of E. coli O157:H7 during postharvest washing of contaminated lettuce and to determine factors affecting the efficacy of sanitizer use in preventing cross-contamination. A bench-scale washing system was established to simulate industry operations. This system was equipped with a submersible pump and instruments to measure wash water properties including pH, temperature, chlorine level, oxidation reduction potential (ORP), turbidity and total organic carbon (TOC). Fresh-cut romaine lettuce (8 or 20 g) inoculated with approximately 8 log CFU/ml of E. coli O157:H7 with green fluorescence protein (GFP) were added into 40 L of tap water or industry water together with uninoculated lettuce (800 or 2000 g). The wash procedure lasted for 2 minutes. Washing operations were performed at two temperatures (3°C and 20°C) combined with different levels of chlorine treatments (0, 5, 20 and 30 ppm). Smaller-scale (50 – 100 mL) washing experiments were performed separately to determine the effects of organic contents and solid contents on the efficacy of sanitizer. Without chlorine treatment, the spread of E. coli O157:H7 occurred in both tap water and industry water at both 20°C and 3°C. With 20 ppm chlorine, no E. coli O157:H7 was detected in either wash water or uninoculated lettuce after washing in tap water. In industry water, chlorine level at 30 ppm or above could prevent crosscontamination of E. coli O157:H7. Neither the lettuce load nor the wash water temperature was proved to affect the efficacy of sanitizer. At 5 ppm of chlorine, increases in organic carbon (0% to 20%) led to the drop of free chlorine which resulted in a decrease in the microbial reduction from 2.51 to 0.01 log CFU/g. Increases in solid contents (0 g/L to 20 g/L) also caused a decrease in the microbial reduction from 2.52 to 1.17 log CFU/g but it did not change the free chlorine concentration. The utility of ORP as a measure of the antimicrobial efficiency of wash water was evaluated. ORP readings increased with increasing chlorine levels but reached a plateau and failed to correlate with the concentration of chlorine at chlorine levels > 20 ppm. M.S. in Food Safety and Technology, December 2011 Show less
