Thermal inactivation and growth of Listeria monocytogenes during production and storage of caramel apples

- Dipping apples in hot caramel decreased L. monocytogenes populations about 99%.
- L. monocytogenes caramel apple outbreak strains were more heat resistant than others.
- Listeria migrated and grew in caramel apples during extended storage.

During the fall of 2014, commercially produced pre-packaged caramel apples were linked to 35 cases of listeriosis in 12 states. In response, this study aimed to assess 1) the reduction of different outbreak and non-outbreak strains of Listeria monocytogenes during caramel dipping of apples, and 2) subsequent growth of the apple outbreak strains within caramel apples during storage at 22 and 4 °C. In aim 1, three unwaxed Jonathan apples were dip-inoculated with three different 4-strain L. monocytogenes cocktails (apple outbreak, unrelated outbreak or unrelated environmental) at ∼8 log CFU/apple, dried for 1 h, dipped for 5 s in caramel at 82, 88, 93 or 99 °C, cooled for 1 h at room temperature and assessed for survivors. In aim 2, Jonathan apples were spot-inoculated with the apple outbreak cocktail (∼3 log CFU/apple) at the stem juncture, dried for 1 h, pushed onto wooden sticks, and dipped in caramel at 82 °C. During storage at 4 and 22 °C for 28 and 14 days, respectively, four different apple sections (top, middle, bottom and core) were cut from three apples, homogenized and plated for Listeria. After dipping apples in caramel at 82 and 99 °C, the apple outbreak, unrelated outbreak and environmental Listeria strains decreased 2.0 ± 0.6 and 2.7 ± 0.1, 1.8 ± 0.3 and 2.6 + 0.1, and 1.7 ± 0.1 and 2.9 ± 0.2 logs, respectively, with the environmental cocktail significantly less heat resistant (P < 0.05) at 99 °C compared to the other two cocktails. After 14 days of storage at 22 °C, Listeria populations were significantly higher (P < 0.05) in the core (7.4 ± 0.6 log CFU/g) compared to the other three sections (4.9-5.4 log CFU/g). The same trend was seen for the core (7.7 ± 0.6 log CFU/g) and the other three sections (5.0-5.4 log CFU/g) after 28 days of storage at 4 °C. Since dipping in hot caramel cannot ensure pathogen elimination, producers of caramel apples should implement good agricultural practices, post-harvest preventive controls and refrigeration of the final product to minimize the risks from Listeria.