Caramel apples may cause listeria, study reveals

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Fall inspires recipes with apples, cinnamon, and other tasty treats, but before handing a child a caramelized apple, make sure it's made fresh and had not been stored in room temperature, as it may have been harboring bacteria called Listeria monocytogenes.

In fact, Science Daily reports that in a 2014 outbrmBeak of listeriosis, 28 of the 31 persons admitted to consuming commercially produced, prepackaged caramel apples before falling ill. Seven victims died of the illness, and three manufacturers issued a voluntary recall of prepackaged caramel apples. Huffington Post reports that about 1,600 individuals fall victim to listeria annually.

"The outbreak took producers, public health officials, and food safety experts by surprise: caramel-coated apples are not a food on which Listeria monocytogenes should grow," Kathleen Glass, PhD, lead author and associate director of the the University of Wisconsin-Madison's Food Research Institute said, as per NBC News.

This unfortunate event prompted researchers from the University of Wisconsin-Madison's Food Research Institute to conduct a study on listeria growth on Granny Smith apples dipped in caramel and stored at either room temperature or in the refrigerator.

Researchers led by Glass prepared a several L. monocytogenes strains associated with the 2014 outbreak and swabbed it on the Granny Apple's skin, stem, and calyx. Wooden sticks were inserted into half of the apples, and all of them were dipped in hot caramel and allowed to cool. Researchers stored one set at 25° Celsius and another set at 7° Celcius for up to four weeks.

Researchers found that listerial growth was largely lessened in apples that were stored in the refrigerator, and those with sticks had no listerial growth for about a week, with some growth during week two to four. The apples without sticks had no listerial growth across four weeks of storage.

Glass explained that listeria bacteria won't breed in caramel because it has a low amount of water, and neither will it breed on apples because of its acidity. However, once a stick is inserted into an apple, some juice flows out, providing moisture on the surface, which gets trapped under the caramel, and thereby "creates a microenvironment that facilitates growth of any L. monocytogenes cells already present on the apple surface." She explained that moisture transfer and microbial growth speed up at room temperature, versus refrigerator temperature.

BBC reports that according to Glass, "It's low risk but it's not no risk. I don't want people to be worried, but it can happen, depending on how they are stored."

The study was published in mBio, an online open-access journal of the American Society for Microbiology.