Neither caramel because of its low amount of water nor apples due to their acidity are normal breeding grounds for listeria, said lead study coauthor Kathleen Glass, PhD, associate director of the institute.
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The study, published by the online journal mBio, was prompted by an outbreak of listeriosis in America previous year, in which 35 people from 12 states were infected and seven people died.
The authors infer that “insertion of a stick into the apple accelerates the transfer of juice from the interior of the apple to its surface, creating a microenvironment at the apple-caramel interface where Listeria monocytogenes can rapidly grow to levels sufficient to cause disease when stored at room temperature”. Researchers at the University of Wisconsin-Madison’s Food Research Institute found that the sticks poked into caramel apples, if left at room temperature, may eventually harbor toxic levels of Listeria monocytogenes.
The researchers suspected that the problem “had to be from damaging the apple”, she said. All the apples were then dipped into hot caramel.
Caramel apples are a classic fall favorite, served up in a variety of ways. [Top 7 Germs in Food that Make You Sick]. She also suggests that manufacturers take steps to ensure their products’ safety by thoroughly disinfecting apples before dipping them in caramel; adding growth inhibitors to the caramel coating or apple wax; and using better temperature-time controls to prevent listeria growth.
“Then, we made caramel apples”, Glass said.
When apples were left unrefrigerated, the researchers report, this growth happened even more quickly.
Glass noted that the dipping process actually provides protection against bacteria. The trouble lies in the spots on the apple that aren’t submerged in the sauce, such as the stem area and the calyx region (on the opposite end of the apple from the stem, where the flower blossom would be).
They dipped all of the apples into hot caramel and after a cooling period, stored the apples at either room temperature or refrigerated them for up to four weeks.
Her team tested a batch of apples to see what factors might allow the bacteria to grow and thrive.
“In contrast, significant pathogen growth was observed within three days at room temperature on caramel apples with sticks inserted”.
“When you puncture the apple with the stick, a few of the juice from the apple can cause an environment, when you cover it with caramel, where the bacteria can grow”, McDowell said.
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Compared with the other apples, the ones on sticks that had been stored at room temperature grew lots of the bacteria.
