A listeria outbreak from 2014 linked to unrefrigerated caramel apples ended in 35 cases of listeria infections from 12 different states. Seven people died from eating these fall favorites. It turns out commercially produced, prepackaged, unrefrigerated caramel apples can be deadly, if the apples on a stick aren’t stored in a refrigerator after production.
Scientists studied the listeria outbreak and published the results of their research in mBio.
— CBS News (@CBSNews) October 13, 2015
Caramel apples that have been punctured with dipping sticks and left at room temperature over the course of a couple of weeks turn out to be the perfect breeding ground for a bacterium known as Listeria monocytogenes. Last year, three different manufacturers declared a voluntary recall of the prepackaged caramel apples during the outbreak because listeriosis can end in death. Symptoms include headache, stiff neck, fever, and stomach ache. It can take up to three to four weeks after eating a caramel apple to exhibit signs of being infected with listeria. After the connection between listeria and caramel apples was made, Dr. Kathleen Glass and her colleagues from the University of Wisconsin-Madison’s Food Research Institute studied listeria growth on juicy Granny Smith apples by making them into caramel apples that no one would want to eat.
— Seventeen (@seventeen) October 16, 2015
The team had prepared a mix of four different L. monocytogenes strains and swabbed the Granny Smith apples’ skin, stem and calyx regions with the mixture. They used wooden sticks shoved through the stems of half of the apples. Then, the apples were stored either at 77 or 44.6 degrees Fahrenheit for up to four weeks.
The average population of the listeria increased 1,000-fold on caramel apples that had been poked with sticks and then stored at room temperature for a mere three days. Apples without sticks had delayed growth, even when stored at room temperature. When refrigerated, the listerial growth was significantly less; they showed no growth of the listeria for up to an entire week, followed by a little growth over the next three weeks, according to Medical News Today.
— Popular Science (@PopSci) October 14, 2015
The authors said that caramel and apples are generally not a good breeding ground for bacteria, but when the apple is pierced with the stick and dipped in warm caramel and then left out at room temperature, the juice from the apple comes to the surface and gets trapped under the caramel. The microenvironment to facilitate the growth of any listeria cells that could be already on the apple’s surface is created. The team explained,
“Dipping the apples in hot caramel killed off a lot of the surface bacteria. 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 2 weeks, it is enough time for the bacteria to grow.”
Given how long it can take listeria to sicken a person, any number of the holiday illnesses of last year could have come from the caramel apples.
Going forward, manufacturers are encouraged to disinfect all apples before dipping them in caramel and to add growth inhibitors to the caramel coating or apple wax. Better temperature controls were also suggested. Alternately, consumers could avoid listeria by making their own caramel apples at home and store any apples not eaten immediately in the refrigerator… as if there could ever be leftover caramel apples outside of a laboratory.
The caramel apples Taran and I made with caramel we made from scratch! (We feel very accomplished.) pic.twitter.com/pXOqCiJesZ
— Jenn Manley Lee (@jemale) October 12, 2015
[Photo via Pixabay]