Listeria can't grow on apples, caramel; Why caramel apples?

Mmm caramel. The only thing that makes it better is when you eat it with apples.

And if you're making this combo happen — maybe with some extra toppings, like nuts or chocolate (you animal, you) — make sure you refrigerate it, too.

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Because no one wants listeria bacteria. And that's a risk you take if you don't let those apples chill.
Last year's caramel apple listeria outbreak came as a bit of a surprise because, as a new study says, apples are thought to be too acidic to have listeria — and caramel doesn't have enough free water for the growth.

But, somehow, this combination killed seven people and made at least 35 ill. Listeria causes fever, muscle aches, diarrhea and other unpleasant symptoms.

Related: The riskiest foods to eat:

In that study published in the journal mBio, researchers discovered it wasn't the apple or the caramel, but the stick inserted into the apple that caused the problem.

"Inserting a stick into the apple releases juice to the interface between the apple and caramel, providing a more hospitable environment than either component alone," a researcher said.

To test this, researchers put sticks in some apples and left some without. The refrigerated caramel apples without sticks were clean, but the refrigerated apples with sticks had slow growth. And the unrefrigerated caramel apples with sticks had significant growth. It's a good idea to refrigerate anyway, as most caramel contains milk.

And you wouldn't leave a gallon of milk sitting out either, right?

This video includes images from jlwelsh / CC BY 2.0, photogmateo / CC BY 2.0 and therealbridgetpalmer / CC BY ND 2.0.