Survival of Listeria innocua on Fuji Apples under Commercial Cold Storage with or without Ozone Gaseous

Introduction: Listeria monocytogenes was linked to a caramel apple outbreak and reported to grow on caramel-coated apples during storage. Ozone, an FDA approved GRAS, has broad antimicrobial effect but no residual effects. However, there is only limited knowledge and information about the fate of Listeria on fresh apples during commercial cold storage and the efficacy of continuous low doses of gaseous ozone application during the cold storage in controlling Listeria on apples.

Purpose: This study examined the fate of Listeria innocua on apples during commercial cold storage to further evaluate antimicrobial efficacy of low dose gaseous ozone against L. innocua.

Methods: Unwaxed Fuji apples were dip inoculated in a three-strain L. innocua cocktail. Inoculated apples (24h postinoculation, 6.0 log CFU L. innocua/apple) were subjected to refrigerated atmosphere (RA), controlled atmosphere (CA), or CA with low doses of ozone during storage in a commercial facility. Survivor enumeration occurred at 1-, 3-, 6-, and 12-weeks. Simultaneously, a set of uninoculated Fuji apples were subjected to aforementioned storage treatments and sampled every six weeks for total bacterial count (TPC) and yeast/mold count (Y/M).

Results:   Commercial cold storage (RA and CA) had little effect on TPC, but led to ~ two-log reduction of L. innocua on apples over 12-weeks of storage. Listeria survival and TPC count of apples under RA or CA are similar; Y/M was slightly decreased on apples under CA storage compared to the RA storage. Low dose gaseous ozone application enhanced L. innocua reduction, resulting in an additional one to two-log reduction over 12-weeks storage; reduced TPC and Y/M by ~ 0.5 and 0.7 log during 12-weeks storage, respectively.

Significance: Low dose gaseous ozone application in commercial CA storage had a positive effect on fruit microbial safety against Listeria on apples, as well as on fruit decay.