WELLNESS
10/14/2015 04:12 pm ET Updated Oct 14, 2015

Your Caramel Apple Might Be Teeming With Bacteria

Caramel apples are delicious, but may come with listeria.

Caramel-covered apples are a tasty fall treat. Unfortunately, this staple of autumnal fairs and Halloween parties may also be a welcome host for listeria monocytogenes, a bacterium that often grows in unpasteurized foods and can cause mild to severe symptoms such as diarrhea, vomiting and fever.

It's not exactly what you hope for when treating yourself to a sugary snack. 

About 1,600 people are sickened by listeria each year, and it's clear from past outbreaks that caramel apples are one of the culprits. In 2014, 35 people in the U.S. were stricken by a listeriosis outbreak, seven of whom died, according to the Centers for Disease Control and Prevention. Ninety percent of those affected by this outbreak reported eating a packaged caramel apple before falling ill. 

Researchers from the University of Wisconsin-Madison's Food Research Institute just found that the storing practices of these packaged apples significantly affects the likelihood of their contamination. 

Their study, which was published last week in the microbiology journal mBio, evaluated Granny Smith caramel apples that were inoculated with a strain of L. monocytogenes. Some were refrigerated, while others were left at room temperature, and half of each group was put on popsicle sticks.

It turns out, the wooden stick used to puncture the apple is the culprit behind the bacteria's growth: When the stick breaks the fruit's skin, a small amount of apple juice moves to the fruit's surface. That moisture, which is then trapped under the caramel, "creates a microenvironment that facilitates growth of any L. monocytogenes cells already present on the apple surface," study coauthor Dr. Kathleen Glass said in a statement. 

And, indeed, the study found that apple punctured with a wooden stick and kept at room temperature carried the highest risk for contamination. 

Refrigeration seemed to help: Apples stored cold with sticks showed no signs of listerial growth for up to a week, though they did show some bacterial growth over the next three weeks. Better yet, caramel apples that were refrigerated without sticks showed no listerial growth at all during the experiment's four-week storage period. 

To ensure your caramel apple is safe, Glass suggested purchasing the refrigerated kind or eating them fresh (you can make them at home). 

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