How apples plus caramel equals Listeria risk

In late 2014, an outbreak of listeriosis in the USA infected 35 people, resulting in seven deaths. 90% of ill people interviewed reported eating commercially produced, pre-packaged caramel apples (similar to toffee apples but dipped in caramel and often rolled in nuts) before becoming ill, prompting a voluntary recall of pre-packaged caramel apples by three manufacturers.

The outbreak has prompted a study by researchers from the University of Wisconsin-Madison’s Food Research Institute to discover how a food that is made from two ingredients considered low risk for Listeria growth — caramel because of its low amount of water, and apples because of their acidity — could have been contaminated.

The researchers studied Listeria growth on a group of Granny Smith apples dipped in caramel and stored at either room temperature or in the refrigerator. They found the average population of L. monocytogenes increased 1000-fold on toffee apples with sticks stored at room temperature for three days. By contrast, listerial growth was delayed on 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, then some growth over the next three weeks. Those without sticks had no listerial growth during four weeks of storage.

Lead study co-author Dr Kathleen Glass, associate director of the institute, explains that inserting a stick into the apple causes a little bit of juice to migrate to the surface; 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.

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

For the study, published in mBio, an online open-access journal of the American Society for Microbiology, Dr Glass and colleagues prepared a cocktail of four L. monocytogenes strains associated with the outbreak and swabbed it on the skin, stem and calyx regions of a group of Granny Smith apples. They inserted wooden sticks through the stems of half of the apples. They dipped all apples into hot caramel using either the sticks or tongs, then allowed them to cool. The apples were then stored either at 25°C or 7°C for up to four weeks.

Dipping the apples in hot caramel killed off a lot of the surface bacteria, Dr 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.”

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, she said.