Caramel apples, a staple of the Halloween holiday, can become incubators for dangerous listeria bacteria, researchers are warning.

The holiday treats, once punctured with their dipping sticks and if left unrefrigerated, can develop and then harbor a bacterium called Listeria monocytogenes, they explain.

An outbreak of L. monocytogenes in 2014, traced to prepackaged caramel apples, infected 35 people and caused seven deaths.

Neither raw apples nor hot caramel are usually considered usual bacterial culprits, so researchers at the University of Wisconsin-Madison's Food Research Institute set out to determine how the outbreak may have come about.

Dipping sticks used to coat the apples and used as a handle when consuming them quickly became the prime suspects, the researchers report in the journal mBio.

The act of inserting the stick can cause apple juice to leak to the surface, they found, creating a microscopic environment between the apple and its caramel coating where any bacteria already present bacteria can thrive.

For the study, they took samples of the listeria strain involved in the 2014 outbreak and coated Granny Smith apples with the bacteria.

Half the apples were pierced with dipping sticks while the other half remained unpunctured. All of the apples were dipped in caramel, then stored for 4 weeks either at room temperature or refrigerated.

No listeria growth was seen in the refrigerated apples without sticks, but caramel apples with sticks at room temperature saw a 1,000-fold increase in bacteria in 3 days, they report.

Refrigerated apples with sticks showed no bacterial growth during the first week but did display some growth in subsequent weeks.

"If someone ate those apples fresh, they probably would not get sick," says lead study co-author Kathleen Glass. "But because caramel-dipped apples are typically set out at room temperature for multiple days, maybe up to two weeks, it is enough time for the bacteria to grow."

The researchers emphasize that they deliberately introduced Listeria into the apples to monitor growth rates, and that home cooks preparing caramel apples in a reasonably clean home kitchen probably should not worry about bacteria.

Symptoms of a listeria infection, which can take 3 to 4 weeks to develop, can include fever, headache and gastrointestinal upset.

Researchers suggested consumers look for refrigerated caramel apples or ones prepared fresh.

Photo: Joshua Ganderson | Flickr

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