It is surely one of fall and winter’s decadent treats—crisp, juicy apples dipped in warm and buttery caramel. Caramel Candied apples provide the perfect taste of sweet and crunchy. So naturally, it was disturbing to learn that this classic fall favorite could be harboring listeria.

A new study published in the American Society for Microbiology’s journal mBio, alerts consumers that if the apples are made incorrectly, sickening bacteria, Listeria monocytogenes, could grow just under the caramel-coated surface. Researchers from the University of Wisconsin-Madison’s Food Research Institute found the best way to avoid turning the seasonal treat into a breeding ground for bacteria.

Researchers began investigating the cause of the bacteria after a 2014 outbreak that saw the infection of 35 people and seven deaths. The study’s lead author and director of the institute, Kathleen Glass, explains that listeria bacteria don’t normally grow on apples or on caramel. That is why it was such an unusual and difficult outbreak to track. But when a stick is inserted into the apple, it causes just enough juice to surface and get trapped under a layer of caramel.

According to the CDC, Listeria, a serious infection usually caused by eating food contaminated with the bacterium Listeria monocytogenes. It can be found in soil and water and some animals, including poultry and cattle. It can be present in raw milk and foods made from raw milk.

Be sure to do your research before indulging in your favorite fall treat. Or learn to make them at home!

Read the full study over at mBio.

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