Madison researchers study listeria in caramel apples

The study’s authors tested batches of caramel apples, a few with sticks and a few without.

They wondered if putting a stick into the apples might release enough juice to make a moist place for Listeria to grow.

But she said inserting a stick into the apple causes a little bit of juice to migrate to the surface, and that moisture, trapped under a layer of caramel, “creates a micro-environment that facilitates growth of any listeria monocytogenes cells already present on the apple surface”.

Last December, health officials warned consumers to avoid eating prepackaged caramel apples after the popular fall treats were linked to a multi-state listeria outbreak, resulting in at least 35 people infected and seven deaths, acccording to the U.S. Centers for Disease Control and Prevention. In Canada there was one confirmed case.

The source of that damage turned out to be none other than the stick inserted into each apple to turn the caramel-dipped fruit into a handheld treat, according to the new finding.

Now, thanks to a new study, scientists think they have cracked the code on how Listeria bacteria can quickly grow on caramel apples even though it rarely grows easily on apples or caramel.

So researchers from the University of Wisconsin-Madison’s Food Research Institute chose to try and figure out how and why it happened. So they ran an experiment, putting Listeria bacteria on Granny Smith apples – the type implicated in the outbreak. You could also just substitute a regular apple for a caramel one because it will cut down on both your risk of Listeria as well as diabetes. Significant growth was observed within 3 days at caramel apples with sticks that were stored at room temperature.

“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”, said Glass.

The temperatures-and sticks-seemed to play a role in listeria growth. Three firms, Happy Apple, California Snack Foods, and Merb’s sweets all recalled caramel apples that were made with Bidart apples. For four weeks, the apples were either stored at 77° F (around room temperature) or in a fridge at 44.6° F.

The hot caramel killed a lot of the surface bacteria, “but those that still survived were the ones that were able to grow”, Glass said. They put wooden sticks through the cores of only half of the apples but coated all of them in a hot caramel mixture.

According to Glass, “If someone ate those apples fresh, they probably would not get sick”.