Caramel apples can be a fertile breeding ground for disease-causing bacteria if they are not refrigerated, according to a new study published today in mBio. The findings were surprising to researchers–they had long assumed that apples are too acidic and the caramel coating doesn’t have enough water activity to be hospitable to bacteria, said Kathleen Glass, a food researcher at the University of Wisconsin and one of the authors of the recent study, in a press release.
But in 2014, there was real-world evidence to the contrary. Thirty-five people were hospitalized after eating commercially prepared caramel apples. The culprit, it seemed, was Listeria monocytogenes, a type of bacteria that can cause fever, headache and vomiting weeks after being ingested.
The researchers decided to investigate the cause of the outbreak. They prepared mixtures of four different Listeria strains and put them on the stems, skin, and bottom (calyx) of 144 Granny Smith apples. They put wooden sticks in the cores of half the apples, and then coated them all with hot caramel. After the apples cooled, the researchers refrigerated half of them and left the other half out in room temperature.
After just two weeks, the researchers found that the apples left at room temperature with wooden sticks in them had significant bacterial growth. The hot caramel had killed most of the bacteria, they said, but those that were left grew easily because of the small quantity of juice that escaped when the researchers inserted the wooden sticks. If a consumer had eaten the apples raw, or eaten caramel apples that had been refrigerated, he might not have gotten sick, the researchers said.
These results are more relevant to commercially available caramel apples, which can be left out for weeks at a time, but contamination can also happen when caramel apples are prepared at home.
While parents tend to fret a lot about what ends up in their kids’ plastic pumpkin buckets on Halloween, it may be smart to throw away caramel apples if you’re not sure how long they’ve been sitting out. The researchers suggest that people who want to enjoy this autumnal treat should only eat those that have been refrigerated–or just eat the apples fresh.
