Improvement of peroxyacetic acid spray bar interventions to control Listeria monocytogenes on fresh apples

Xiaoye  Shen

doi:10.7273/000005056

Apples are one of the most valuable fruit crops in the United States. However, the caramel apple outbreak and multiple recalls due to potential Listeria monocytogenes contamination had a serious economic impact on the apple industry. This dissertation investigated intervention strategies for enhancing peroxyacetic acid (PAA), a commonly used sanitizer during apple packing, against L. monocytogenes during brush-bed spray bar wash intervention and further explored potential factors contributing to the enhanced L. monocytogenes resistance to PAA intervention after attached to apple surfaces. Including lauric arginate (LAE) or commercially available fruit cleaners significantly enhanced (P < 0.05) the effectiveness of 80 ppm PAA intervention against L. monocytogenes on fresh apples. A 30-sec spray intervention of 80 ppm PAA with 0.05% LAE at 22C and 46C caused 0.75 and 0.53 log10 CFU/apple higher reduction than 80 ppm PAA treatment alone at 22C and 46C, respectively. Including commercially available fruit cleaners, Acidix Duo and Nature’s Shield 220-ACL, also enhanced (P < 0.05) the antimicrobial efficacy of 80 ppm PAA during the spray-bar intervention and caused 1.51-1.63 log10 CFU/apple reduction of Listeria after 30-60 sec brush-bed spray wash compared to 0.94-1.03 log10 CFU/apple reduction of 80 ppm PAA treatment alone. PAA efficacy against L. monocytogenes dramatically reduced (P < 0.05) after L. monocytogenes was attached to fresh apple surfaces compared with L. monocytogenes in the aqueous solution. This could be due to the starvation stress, biofilm formation, and flagella expression in L. monocytogenes after attach to apple surfaces as supported by the increased mRNA expressions involved in respective biological process. Given that Listeria’s ubiquitous nature in the processing and packing environment, apples can be potentially contaminated with L. monocytogenes before wax coating application and during waxing application. The study further evaluated L. monocytogenes cross-contamination risk during wax coating application and the fate of L. monocytogenes on waxed apples during subsequent cold storage. There was up to 0.28% of L. monocytogenes transfer rate between apples during waxing. L. monocytogenes introduced on the waxed apples remain stable during 12 weeks of cold storage; there was only 1.8-2.0 log10 CFU/apple reduction of L. monocytogenes on waxed apples, regardless of wax type. The die-off rate of L. monocytogenes on waxed apples contaminated during wax coating was comparable to that contaminated on apples before wax coating. L. monocytogenes transferred onto waxing brushes during wax coating remained relatively stable during the 2-week ambient holding. Data from this dissertation generate useful, practical, and science-based information for the apple industry to ensure the safety of fresh apples.

Improvement of peroxyacetic acid spray bar interventions to control Listeria monocytogenes on fresh apples

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