Evaluation of chlorine and peroxyacetic acid for Listeria monocytogenes control in apple dump tank practice

Yuan Su

doi:10.7273/000007477

Recent outbreaks and recalls of fresh apples due to Listeria monocytogenes contamination highlight the need for effective foodborne pathogen control on fresh apples. During post-harvest packing, apples are first washed in sanitizer-treated dump tank water, which accumulates organic matter over time, potentially reducing sanitizer efficacy. However, the effectiveness of this process remains scientifically underexplored. This dissertation comprehensively evaluated the effectiveness of two commonly used sanitizers, chlorine and peroxyacetic acid (PAA), in controlling L. monocytogenes during apple dump tank practice at both bench and pilot scales and further developed mathematical models to predict the anti-Listeria efficacy of chlorine under various conditions. Laboratory-scale experiments using simulated dump tank water (SDTW) in a hand-agitation system demonstrated that both chlorine and PAA exhibited concentration-dependent efficacy against L. monocytogenes on inoculated apples. The anti-Listeria efficacy of chlorine at initial 25 - 100 ppm free chlorine (FC) was significantly impacted by organic load, especially at 25 ppm FC in SDTW with 1000 ppm chemical oxygen demand (COD). PAA’s efficacy was similarly affected at initial 10 ppm but remained stable at 20 - 80 ppm. To further assess these findings under commercial conditions, a 102-L pilot-scale dump tank system was employed. Chlorine and PAA significantly reduced the Listeria innocua cross-contamination between apples and water but did not eliminate it. A 0.5 - 10 min wash using 25 - 100 ppm FC resulted in reductions of 0.4 - 1.1 log CFU/apple on inoculated apples and 3.5 - 5.4 log CFU/ml in inoculated SDTW, leading to transfers of 0.4 - 3.5 log CFU/apple to uninoculated apples. PAA at 20 - 80 ppm achieved reductions of 0.5 - 1.5 log CFU/apple on inoculated apples and 0.7 - 4.3 log CFU/ml in inoculated SDTW, with transfer levels of 3.5 - 4.2 log CFU/apple to uninoculated apples. Elevating the organic load in SDTW from 1000 to 4000 ppm COD further reduced chlorine’s efficacy against L. innocua and PAA’s effectiveness in water. To quantify and predict chlorine’s efficacy, response surface methodology (RSM) models were developed using the pilot-scale dump tank system. These models incorporated key parameters including FC level, Listeria contamination level, and COD level, to predict the anti-Listeria efficacy of chlorine in apple dump tank processing. The RSM models demonstrated strong predictive accuracy (R2 = 0.91 - 0.98) and were validated using commercial apple dump tank water, showing high agreement between the predicted and observed responses. The findings provided valuable insights into the practical efficacy of sanitizer-treated apple dump tank processing and developed a reliable tool for assessing the anti-Listeria efficacy of chlorine, in compliance with federal regulations and ensuring the microbial safety of fresh apples.

Evaluation of chlorine and peroxyacetic acid for Listeria monocytogenes control in apple dump tank practice

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