Caramel Apples Can Harbor Dangerous Bacteria

Although caramel apples are a favorite autumn treat, they can be dangerous, according to a new study. If left unrefrigerated for a few weeks, they may harbor the Listeria bacterium.

The finding was published in mBio®, an online open-access journal of the American Society for Microbiology.

Researchers from the University of Wisconsin-Madison’s Food Research Institute studied a group of Granny Smith apples dipped in caramel and stored at either room temperature or in the refrigerator.

They found that the average population of listeria (L. monocytogenes) increased 1,000-fold on caramel apples with sticks stored at room temperature for three days. By contrast, listerial growth was delayed on caramel apples without sticks stored at room temperature.

Listerial growth was significantly lessened among apples stored in the refrigerator: those with sticks had no listerial growth for up to a week but then some growth over the next three weeks. Those without sticks had no listerial growth during four weeks of storage.

Neither caramel because of its low amount of water nor apples because of their acidity are normal breeding grounds for listeria, said lead study coauthor Kathleen Glass, PhD, associate director of the institute.

But inserting a stick into the apple causes a little bit of juice to migrate to the surface, she said, and that moisture, trapped under a layer of caramel, “creates a microenvironment that facilitates growth of any L. monocytogenes cells already present on the apple surface.” Both moisture transfer and microbial growth are accelerated at room temperature compared to refrigeration, she said.

To be safe, she said, consumers should look for refrigerated caramel apples or eat them fresh.

The study was prompted by an outbreak of listeriosis in late 2014, in which 35 people from 12 states were infected and seven people died, Glass said. Twenty-eight (90 percent) of 31 ill people interviewed reported eating commercially produced, prepackaged caramel apples before becoming ill, prompting a voluntary recall of prepackaged caramel apples by three manufacturers.

Listeriosis symptoms include fever, headache, stiff neck and gastrointestinal illness and may not appear until three to four weeks after eating affected foods.

Caramel apple manufacturers may wish to thoroughly disinfect apples before dipping them in caramel, add growth inhibitors to the caramel coating or apple wax, or use better temperature-time controls to inhibit the growth of L. monocytogenes, Glass said.

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