Evaluation of chlorine and peroxyacetic acid efficacy in controlling Listeria innocua in a pilot-scale apple dump tank system

The dump tank and flume system, treated with antimicrobial sanitizers, is the first and most critical step to control the cross-contamination of foodborne pathogens on apples in commercial packing lines. Previous studies on the efficacy of chlorine and peroxyacetic acid (PAA) during apple washing have been primarily conducted in laboratory-scaled settings, which may not accurately reflect the efficacy of commercial dump tank operation due to variations in processing parameters. This study employed a pilot-scale dump tank system containing 102 L of pilot-simulated dump tank water (pSDTW) to evaluate the efficacy of chlorine and PAA in reducing Listeria innocua on apples and its cross-contamination between apples and water. Chlorine and PAA significantly reduced the transfer of L. innocua to uninoculated apples and water, with greater sanitizer concentrations and extended processing times enhancing their efficacy. Specifically, chlorine at 25–100 ppm free chlorine (FC) for 0.5–10 min resulted in the reductions of L. innocua by 0.4–1.1 log₁₀ CFU/apple on inoculated apples and 3.5–5.4 log₁₀ CFU/ml in inoculated pSDTW in the presence of 1000 ppm chemical oxygen demand (COD), leading to a transfer of 0.4–3.5 log₁₀ CFU/apple to uninoculated apples. For PAA treatment, 20–80 ppm achieved reductions of 0.5–1.5 log₁₀ CFU/apple on inoculated apples and 0.7–4.3 log₁₀ CFU/ml in inoculated pSDTW, resulting in transfers of 3.5–4.2 log₁₀ CFU/apple to uninoculated apples. Elevating the organic load in pSDTW from 1000 to 4000 ppm COD significantly reduced the effectiveness of chlorine in reducing L. innocua and its cross-contamination, and the anti-Listeria efficacy of PAA in water. This study provides valuable insights into optimizing dump tank operations for commercial apple packing lines.