Efficacy of chlorine and peroxyacetic acid to control Listeria monocytogenes on apples in simulated dump tank water system

Chlorine and peroxyacetic acid (PAA) are commonly applied in dump tanks and flume systems in commercial fresh apple packing lines; however, little is known about their practical efficacies in dump tank water systems. This study evaluated the efficacies of chlorine and PAA to control Listeria monocytogenes on fresh apples and cross-contamination in simulated dump tank water (SDTW). Efficacies of chlorinated water with initial free chlorine (FC) of 25–100 ppm against L. monocytogenes on apples were significantly impacted by the presence of organic matter, especially for chlorine with 25 ppm initial FC. Chlorine with initial FCs of 50–100 ppm and 2 min contact reduced L. monocytogenes on apples by ∼0.9 log₁₀ CFU/apple in SDTW with 1000 ppm chemical oxygen demand (COD). However, 2–5 min wash of chlorine with 25 ppm initial FC only led to ∼0.3 log₁₀ CFU/apple reduction of L. monocytogenes on apples in SDTW compared to ∼0.9 log₁₀ CFU/apple reduction in clean water. The impacts of organic matter on the antimicrobial efficacy of PAA are concentration dependent. At 20–80 ppm and tested contact times (2–5 min), efficacies of PAA against L. monocytogenes were not influenced by organic matter presented in SDTW; 2–5 min wash with PAA 80 ppm caused 1.7–1.8 log₁₀ CFU/apple log reduction. However, the anti-Listeria efficacy of 10 ppm PAA was significantly lower in SDTW than in clean water. Sanitizers at the tested concentrations reduced L. monocytogenes transferred from contaminated apples to uncontaminated apples and SDTW but did not eliminate it. There were 1.7–0.6 and 1.0–0.9 log₁₀ CFU/apple of L. monocytogenes transferred to uninoculated apples in SDTW treated with 50–100 ppm FC and 60–80 ppm PAA, respectively, for 2 min, while 3.6–3.7 log₁₀ CFU/apple of L. monocytogenes were transferred to uncontaminated apples in SDTW without any sanitizer treatments. Data indicated that sanitizer treatments in SDTW are effective but can be further improved to ensure the microbial safety of apples.