To figure out how the desserts developed the bacteria, the team from the University of Wisconsin-Madison’s Food Research Institute swabbed the apples with the bacteria, then dipped the apples in caramel and, using either sticks or tongs, allowed them to cool.
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The study’s lead author and director of the institute, Kathleen Glass, explained listeria bacteria don’t normally grow on apples or on caramel, which is why it was such an unusual and hard outbreak to track.
After three days, there was a lot of Listeria growth on the room-temperature apples with sticks in them, Glass said.
“No growth of L. monocytogenes occurred on refrigerated caramel apples without sticks, whereas slow growth was observed on refrigerated caramel apples with sticks”, they wrote. Apple juice seeped to the surface and became a breeding ground for contaminants.
The 2014 outbreak left at least seven dead and 35 infected, according to the U.S. Centers for Disease Control and Prevention.
Listeriosis is usually caused when a person ingests listeria monocytogenes bacteria, but an infection is hard to pinpoint because the incubation period can be weeks. The outbreak was ultimately linked to prepackaged caramel apples, prompting several manufacturers to voluntarily recall their products. Half of the apples had sticks inserted; the others did not.
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.
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 a few growth over the next three weeks. The bacterial growth was less in the refrigerated apples and it took much longer – 28 days versus 3 days – but it was still significant. The researchers speculated that when the stick is poked into the apple, juice comes out, and that juice could get trapped between the apple and its caramel coating, she said. Additionally, in the apples with sticks, researchers found that bacteria concentrations were found around the stick inside the apple.
According to Glass, “If someone ate those apples fresh, they probably would not get sick”.
That acidity, combined with the low amount of water available in caramel, should have meant that the food was safe, Glass told Live Science. The findings were published this week in mBio, an online open-access journal of the American Society of Microbiology.
What does this mean for consumers this fall season? Stickless apples stored in the fridge had no listeria growth over the course of four weeks. For my end, I’d say coat them completely in caramel and put them on a fork when you’re ready to eat.
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Mark Bello is the CEO and General Counsel of Lawsuit Financial Corporation, a pro-justice lawsuit funding company.
