Chlorine and peroxyacetic acid inactivation of Listeria monocytogenes in simulated apple dump tank water

During postharvest processing, apples are transported via dump tanks and flumes with recirculated water treated with sanitizers. This study examined the efficacies of chlorine and peroxyacetic acid (PAA) in controlling Listeria monocytogenes in simulated dump tank water (SDTW) and cross-contamination, and evaluated the strengthening effects of octanoic acid (OA), a Generally Recognized as Safe compound, on the efficacy of PAA. Anti-Listeria efficacy of chlorine at 25 ppm free available chlorine (FC) diminished significantly with organic load in SDTW. The efficacies of chlorine at 50-100 ppm FC were less impacted by organic load, causing 4.9-5.8 and 4.1-5.1 log₁₀ CFU/ml reduction of L. monocytogenes in SDTW with 1000 and 4000 ppm chemical oxygen demand (COD) after 5-min contact, respectively. The efficacies of PAA at 40, 60, and 80 ppm were not influenced by organic matter in SDTW at tested contact time and led to 5.9–6.3 log₁₀ CFU/ml reductions after 5-min exposure at 1000 and 4000 ppm COD, which was in alignment with the relatively stable PAA concentration in SDTW. However, a 5-min contact with 20 ppm PAA only led to 3.1–3.2 log₁₀ CFU/ml reduction of L. monocytogenes in SDTW. Incorporating 800 ppm OA to 20 ppm PAA solution significantly improved PAA's efficacy and resulted in 5.8 log₁₀ CFU/ml reductions after 6-min contact. The chlorine and PAA application in SDTW with 1000 ppm COD lowered transference of L. monocytogenes but failed to prevent cross-contamination from spiked SDTW to clean apples during the 2-min exposure. A 2-min contact of apples in SDTW spiked with 6.8 log CFU/ml L. monocytogenes and supplied with 100 ppm chlorine and 80 ppm PAA resulted in 2.9 and 2.6 log₁₀ CFU/apple of L. monocytogenes contamination, respectively. When SDTW was loaded with apples, the efficacies of sanitizers against L. monocytogenes in SDTW decreased; 100 ppm FC and 80 ppm PAA only reduced the L. monocytogenes level in process water by 5.0 and 5.4 log₁₀ CFU/ml after a 2-min exposure, respectively. Incorporating 800 ppm OA significantly strengthened the effectiveness of 80 ppm PAA in controlling cross-contamination from SDTW to apples, but 2.4 log₁₀ CFU/apple of L. monocytogenes was still recovered after 2-min exposure. Collectively, this study provides the industry with practical information about the effectiveness of chlorine and PAA against L. monocytogenes in SDTW and highlights the importance of monitoring water quality and maintaining an effective level of sanitizers.