A newly published study by researchers at the University of Wisconsin-Madison Food Research Institute in the USA has found that Listeria monocytogenes is able to grow on unrefrigerated caramel apples that have had wooden sticks inserted into the apple.
The study was carried out in response to a fatal outbreak of listeriosis in 2014 that was linked to commercially produced, pre-packaged caramel apples and affected at least 35 people in 12 states, causing seven deaths.
The researchers used a cocktail of four different L. monocytogenes strains to inoculate the skin, stem and calyx regions of a batch of Granny Smith variety apples. Half of the apples then had wooden dipping sticks inserted through the stems. All were dipped in hot caramel and then allowed to cool before storage at 25oC or 7oC for up to four weeks.
They found that the numbers of Listeria on the unrefrigerated apples with sticks increased by a factor of more than 1,000 after three days, while growth on unrefrigerated apples with sticks was delayed. Growth on apples without sticks was delayed significantly at 25oC and did not occur at 7oC even after 28 days.
The researchers suggest that juice released by the stick becomes trapped under the layer of caramel and creates a microenvironment where Listeria cells are able to survive and grow at room temperature. They recommend that caramel apples are either eaten fresh or refrigerated.
The study is published in the open access journal mBio® and can be found here.