Validation of sanitizer efficacy in commercial apple dump tanks using a Listeria monocytogenes surrogate

Cross-contamination during postharvest apple dump tank processing presents a significant food safety risk. To mitigate this risk, chemical sanitizers such as chlorine and peroxyacetic acid (PAA) are commonly used, yet data on their practical anti-Listeria efficacy under commercial conditions remain limited. This study assessed the effectiveness of both sanitizers in mitigating cross-contamination of Enterococcus faecium NRRL B-2354, a surrogate for Listeria monocytogenes, during commercial apple dump tank operations at four packing facilities in Washington State. Initial assessments using commercial process water in a pilot-scale dump tank showed that 25-100 ppm free chlorine (FC) and 40-80 ppm PAA reduced E. faecium on inoculated apples during 2-5 min washes. Subsequent in-plant studies with 50 ppm FC and 60 ppm PAA revealed variable efficacy across facilities. A 2-5 min process using 50 ppm FC and 60 ppm PAA, respectively, reduced E. faecium by 0.9-1.9 and 1.2-2.2 log₁₀ CFU/apple on inoculated apples, with corresponding transfers of 1.7-3.6 and 1.6-3.3 log₁₀ CFU/apple to non-inoculated apples and residual levels of 0.1-0.8 log₁₀ CFU/250 mL in spent dump tank water. These findings provide valuable insights for apple packers on the practical efficacy of sanitizer-treated dump tank processing, underscoring the importance of maintaining effective sanitizer levels and monitoring water quality during commercial operations.