Evaluating serotype-specific survival of Listeria monocytogenes and Listeria innocua on wax-coated Granny Smith apples during storage

Listeria monocytogenes serotypes 1/2a, 1/2b, and 4b are implicated in over 90 % of human listeriosis cases; however, information regarding the serotype-specific survival of L. monocytogenes on apples remains limited. This study evaluated the survival dynamics of these serotypes, using two sets of strains, on Granny Smith apples (GSA) and examined the impact of wax-coating on their survivability during storage. The fate of Listeria innocua on GSA during commercial refrigerated air (RA) storage was also investigated. Results revealed distinct behaviors among L. monocytogenes serotypes. The culturable count of the serotype 4b strain on apples decreased significantly (P < 0.05) by 1.36 log CFU/apple within the first 48 h post-inoculation at ambient temperature. In contrast, counts of serotypes 1/2a and 1/2b strains significantly increased (P < 0.05) by 0.44 and 0.50 log CFU/apple, respectively, during this 48-h attachment period. Over the subsequent 12 weeks of cold storage, serotypes 1/2a and 4b remained stable on GSA, while 1/2b decreased by 1.68 log CFU/apple. Wax coating had a minor impact on L. monocytogenes survival on GSA under both cold and ambient storage conditions, regardless of serotypes. During 18 weeks of commercial RA storage, L. innocua exhibited a gradual decrease of 1.76-1.96 log CFU/apple on wax-coated GSA. Yeasts and molds showed no significant changes in populations due to wax coating, maintaining counts between 5.5 and 5.7 log CFU/apple by the end of the 18-week storage. Additionally, wax coating enhanced quality attributes, significantly preventing (P < 0.05) firmness loss and internal browning compared to unwaxed apples at the conclusion of storage. This study highlights the serotype-specific survival traits of selected L. monocytogenes produce-outbreak isolates on apples, emphasizing the importance of understanding the distinct behaviors of different serotypes.