Purpose: Consequently, this study aimed to assess 1) the reduction of different outbreak and non-outbreak strains of Listeria monocytogenes on caramel apples after dipping and 2) subsequent growth of the apple outbreak strains in caramel apples at 22 and 4°C.
Methods: In aim 1, three Jonathan apples were dip-inoculated with three different 3-strain L. monocytogenes cocktails (apple outbreak, unrelated outbreak or unrelated environmental) at ~8 log CFU/apple and then then dipped for 10 s in caramel at 82, 88, 93 or 99°C. One hour later, the apples were massaged in buffer and plated for survivors. In aim 2, Jonathan apples were spot-inoculated at the stem juncture with the apple outbreak cocktail at 3 log CFU/apple, dried for 1 hour, pushed onto wooden sticks, and dipped in caramel at 82°C. At various times during storage at 22 or 4°C, four different apple sections (top, middle, bottom, core) were cut from three apples homogenized and plated for Listeria.
Results: After dipping apples in caramel at 82 and 99°C, the apple outbreak, unrelated outbreak and environmental Listeria strains decreased 2.0 and 2.7, 1.8 and 2.6, and 1.7 and 2.9 logs, respectively, with the environmental cocktail significantly less heat resistant (P < 0.05) at 99°C than 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 + 0.2 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 + 0.1) and other three sections (5.0 – 5.4 log CFU/g) after 28 days of storage at 4°C.
Significance: Since dipping in hot caramel will not eliminate Listeria, caramel apples should be refrigerated to minimize pathogen growth.