How Halloween caramel apples can become infected with listeria


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We're on the heels of Halloween and that can only mean one thing: a whole heap of seasonal snacks and treats. But just because the food is fun doesn't mean you should take it any less seriously. A new study from the University of Wisconsin-Madison's Food Research Institute suggests that something very scary might be hiding in those Halloween apples.


Out of sight


Publishing this week in mBio, an online journal of the ASM, the researchers have found that caramel apples punctured with dipping sticks and then left unrefrigerated may develop a bacterium called Listeria monocytogenes. Studying listeria growth on a group of Granny Smith apples dipped in caramel, researchers at the Institute found that, at room temperature, the average population of L. monocytogenes increased a thousand-fold on caramel apples stored for three days when they had sticks while listerial growth was delayed on caramel apples without sticks.

Caramel apples are not natural breeding grounds for listeria — caramel because of its low water content and apples because of their acidity, says lead coauthor Dr. Kathleen Glass. Pushing a stick into the apple, however, causes a little bit of juice to leak to the surface and if it becomes trapped under a layer of caramel, it "creates a microenvironment that facilitates growth of any L. monocytogenes cells already present on the apple surface." If the apples are kept at room temperature, both moisture transfer and microbial growth are accelerated when compared to refrigeration, she says.

For the study, Glass and her colleagues used four L. monocytogenes strains associated with the outbreak and swabbed them on the skin, stem and calyx — the bottom end, opposite the stalk — regions of a group of Granny Smith apples. They then inserted wooden dipping sticks through the stems of half of the apples. All the apples were dipped into hot caramel using either the sticks or tongs, and then allowed them to cool. The apples were then stored either at 77 degrees Fahrenheit (25 degrees Celcius) or 44.6 degrees Fahrenheit (7 degrees Celsius) for up to four weeks.


On the surface


Initially, dipping the apples in hot caramel kills off a lot of the surface bacteria, Glass said. "But those that still survived were the ones that were able to grow. If someone ate those apples fresh, they probably would not get sick. But because caramel-dipped apples are typically set out at room temperature for multiple days, maybe up to two weeks, it is enough time for the bacteria to grow." Apples with sticks had no bacteria for up to a week but then some growth over the next three weeks — those without sticks, on the other hand, had no bacterial growth during four full weeks of storage. Refrigeration helps significantly, the team discovered. To be safe, they say, 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.