Purpose: Determine how the presence of solids may affect chlorine inactivation of Salmonella and the potential for pathogen cross-contamination during washing of fresh-cut lettuce.
Methods: Small-scale experiments were conducted to determine the impact of different types of soil in affecting the efficacy of chlorine in disinfecting Salmonella. 100 ml sterile DI water containing 2 – 20 g of play sand or 1 – 5 g of sandy soil was inoculated with 6 log CFU/ml Salmonella and treated with different levels of free chlorine (0.5 - 20 ppm) for 15 sec. Lettuce washing experiments were also performed to determine whether the presence of solids affected the efficacy of chlorine in preventing Salmonella cross-contamination. Eight g of cut romaine lettuce inoculated with 7 log CFU/g of Salmonella was added to 40 l of sterile tap water together with 800 g of uninoculated cut lettuce and washed for 2 min. Washing trials were performed with different levels of chlorine (0 and 10 ppm) and play sand (0 and 4 kg).
Results: In water containing 8 ppm of chlorine, the level of Salmonella changed from not detectable in the presence of < 5 g of play sand to 0.3 or 1.4 log CFU/ml in the presence of 10 or 20 g of play sand, respectively. At 10 ppm of chlorine, Salmonella level changed from not detectable to 1.0 log CFU/ml when the level of sandy soil increased from 0 to 5 g. In 40 l washing trials without chlorine treatment, Salmonella transfer occurred and resulted in the contamination of wash water and uninoculated lettuce at levels of 2.9 ± 0.1 log CFU/ml and 2.9 ± 0.3 log CFU/g, respectively. With 10 ppm of chlorine, no Salmonella was detected; but when 4 kg play sand was added, although no Salmonella was detected in the lettuce samples, it was found in the wash water after enrichment.
Significance: The presence of solids provides protection for Salmonella thus avoiding inactivation by chlorine, and may affect the sanitizer’s ability to prevent cross-contamination.