When a Wisconsin state epidemiologist called food microbiologist Kathleen Glass last fall to inquire if she thought caramel apples would be a source of listeriosis, her immediate reaction was no.
“The water availability of the caramel is just too low – bacteria doesn’t grow in it. That’s why it’s shelf-stable,” says Glass, associate director of the University of Wisconsin’s Food Research Institute. “And the reason why you can put apples out at room temperature for a while until they shrivel up is because they’re acidic and won’t support [bacterial] growth.”
Yet three people in Wisconsin were affected by an outbreak of listeriosis in late 2014 that eventually was tied to caramel apples. Thirty-five people from 12 states were infected and seven people died, according to the Centers for Disease Control and Prevention. 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.
“It didn’t make any sense to us,” Glass says. “We wondered what kind of conditions might happen to allow growth,” because outbreaks usually result from multiple things going wrong. By studying listeria growth on a group of Granny Smith apples dipped in caramel and stored at either room temperature or in the refrigerator, Glass and colleagues discovered that puncturing the top of the apple with a dipping stick caused a little bit of juice to migrate to the surface, 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.”
In a study published in mBio this week, Glass and colleagues prepared a cocktail of four L. monocytogenes strains associated with the listeriosis 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 degrees Celsius (77° F) or 7 degrees Celsius (44.6° F) for up to four weeks.
The average population of 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. Both moisture transfer and microbial growth are accelerated at room temperature compared to refrigeration, Glass says.
Dipping the apples in hot caramel killed off a lot of the surface bacteria, she says. “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.”
Consumers should look for refrigerated caramel apples or eat them fresh, Glass says. And 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 says.
-- Karen Blum