Unrefrigerated caramel apples may pose deadly listeria risk, study suggests

Caramel apples are arguably as much a part of Halloween as trick-or-treating and trying to find a non-“sexy” version of a costume at your local pop-up Halloween shop.

Researchers from the University of Wisconsin-Madison’s Food Research Institute did this study.

It isn’t the first time: in 2014, an outbreak of listeria linked to caramel apples resulted in seven deaths and 35 sicknesses.

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.

According to the Department of Health’s Food Safety division, listeria is a genus of bacteria typically found in soil, water, and a few poultry and cattle. The outbreak was particularly troubling because Listeriosis can have a long incubation period from three to 70 days, and grows really well at refrigerated temperatures.

The symptoms of Listeria monocytogenes food poisoning include flu-like fever and muscle aches, upset stomach, diarrhea, stiff neck, headache, loss of balance, confusion, or convulsions.

Although researchers agree that pre-packaged caramel apples should remain refrigerated until ready to eat, they just be warmed up to keep the hardened caramel from possibly chipping a tooth. The key to the Listeria growth was the sticks inserted into the apples. Testing had revealed the presence of Listeria at the company’s apple plant.

Both moisture transfer and microbial growth are accelerated at room temperature compared to refrigeration, she said.

For the study, Glass and her colleague created a cocktail of four L. monocytogenes strains linked to the outbreak. They inserted wooden sticks into the stems of half of the apples.

If you’re planning to enjoy a delicious caramel apple this fall, it might be smart to eat it fresh – or pop it in the fridge for safe keeping.

They dipped all of the apples into hot caramel and after a cooling period, stored the apples at either room temperature or refrigerated them for up to four weeks.

After three days, there was a lot of Listeria growth on the room-temperature apples with sticks in them, Glass said.

For this study, researchers began by deliberately covering the plain apples with Listeria to analyze its growth in a number of different environments. Listeria growth significantly decreased on apples with sticks stored in the refrigerator for up to a week. “If someone ate those apples fresh, they probably would not get sick”.