Purpose: To comprehensively evaluate survival of L. monocytogenes on fresh apples at recommended and abusive storage temperatures over a three-month storage period.
Methods: Unwaxed apple fruits of selected varieties (organic Granny Smith apples (GSA), conventional GSA, and Fuji apples) were dip inoculated in a three-strain L. monocytogenes cocktail. Inoculated apples (24 h postinoculation, ~6.0, 5.0, or 4.0 log CFU L. monocytogenes/apple) were subjected to 1, 4, 10, or 22°C storage for up to three months. Apples under the different storage treatments were sampled at one and four days, and one, two, four, eight, and 12 weeks for survivor enumeration.
Results: During the two-week short-term storage, L. monocytogenes population on fresh GSA apples stored at 1, 4, or 10°C remained stable. Reduced L. monocytogenes population was observed when GSA were stored at 22°C; there was 0.5~1.5-log CFU/apple reduction 14-day post storage depending on initial inoculation level. Listeria monocytogenes showed a similar survival trend on GSA apples during 12-week cold storage under 1, 4, and 10°C; there was 0.5~1.5-log reduction (for both high and low inoculations) with apples in 1°C storage conditions, which showed the least reduction while 10°C showed the most reduction. Furthermore, L. monocytogenes demonstrated a similar survival curve on GSA (organic or conventional) and Fuji apples.
Significance: A limited reduction of L. monocytogenes on apple surfaces occurred during 12 weeks of refrigerated storage. This data will provide industry with important information to assess the potential risk of L. monocytogenes on fresh apples.