WEDNESDAY, Oct. 14, 2015 (HealthDay News) -- Caramel apples with dipping sticks -- sometimes handed out as Halloween treats -- can make you sick if they're not refrigerated, researchers warn.
The researchers studied the growth of Listeria monocytogenes bacteria on caramel apples stored at either room temperature or in the refrigerator. Some of the apples in both groups had dipping sticks and others did not.
After three days, the amount of listeria on unrefrigerated apples with sticks increased 1,000-fold, while listeria growth on unrefrigerated apples without sticks was delayed, the investigators found.
Refrigerated apples with sticks had no listeria growth for a week, but then had some growth over the next three weeks. Refrigerated apples without sticks had no listeria growth over four weeks, the findings showed.
The study was published online Oct. 13 in the journal mBio.
Because caramel has a low amount of water and apples are acidic, neither are normal breeding grounds for listeria, explained study author Kathleen Glass, associate director of the Food Research Institute at the University of Wisconsin-Madison.
However, piercing an apple with a dipping stick causes a bit of apple juice to leak out and become trapped under a layer of caramel, creating an environment that aids the growth of listeria already present on the apple's surface, Glass explained.
This growth occurs much more quickly when a caramel apple is stored at room temperature compared to refrigeration, she said.
To be safe, you should buy refrigerated caramel apples or eat them fresh, Glass advised.
In 2014, prepackaged caramel apples from three manufacturers were linked with a listeriosis outbreak in which 35 people in 12 states were infected and seven died, the researchers said.
Listeriosis symptoms include fever, headache, stiff neck and gastrointestinal problems, and the study authors noted that symptoms may not surface for several weeks after eating contaminated foods.
The U.S. Centers for Disease Control and Prevention has more about listeria.