A study conducted at the University of Wisconsin-Madison and published in mBio has found that caramel apples can support the growth of Listeria monocytogenes bacteria. The huge 2014 outbreak linked to caramel apples was unexpected because apples have a pH too low for bacterial growth, and the available water in caramel is too low as well.
In the study, Granny Smith apples were inoculated with Listeria monocytogenes bacteria on the skin, stem, and calyx. Half of the apples had sticks inserted; the others did not. The apples were dipped into hot caramel and stored at either 7°C or 25°C.
The study revealed that apples with inserted sticks had significantly more growth of Listeria bacteria than apples without sticks stored under both temperature conditions. The bacterial growth was less in the refrigerated apples and it took much longer – 28 days versus 3 days – but it was still significant.
No growth occurred on refrigerated caramel apples without sticks. Significant growth was observed within 3 days at caramel apples with sticks that were stored at room temperature.
The authors infer that “insertion of a stick into the apple accelerates the transfer of juice from the interior of the apple to its surface, creating a microenvironment at the apple-caramel interface where Listeria monocytogenes can rapidly grow to levels sufficient to cause disease when stored at room temperature.” They conclude that “food producers should consider interfaces between components within foods as potential niches for pathogen growth.”
That 2014 Listeria outbreak, linked to apples produced and distributed by Bidart Brothers, sickened 35 people. Thirty-four people were hospitalized because their illness was so serious. Listeriosis contributed to at least three of the seven deaths reported. Eleven illnesses were pregnancy-related, with one resulting in a fetal loss. And three invasive illnesses of Listeria meningitis occurred in healthy children aged 5 to 15.
Bidart Brothers recalled all of their Granny Smith and Gala apples because environmental testing found the Listeria bacteria at the company’s apple-packing facility. Three firms, Happy Apple, California Snack Foods, and Merb’s Candies all recalled caramel apples that were made with Bidart apples.
The symptoms of Listeria monocytogenes food poisoning include flu-like fever and muscle aches, upset stomach, diarrhea, stiff neck, headache, loss of balance, confusion, or convulsions. Pregnant women are usually only mildly ill, but listeriosis can cause miscarriage, stillbirth, and infection in the newborn baby.
The biggest concern with Listeria infections is the incubation time. While most people get sick within two to three weeks of ingesting the bacteria, some people don’t get sick until two months later. This can make both diagnosing the illness and pinpointing the source of the bacteria very difficult. Listeriosis is treated with antibiotics, but the earlier the disease is diagnosed and treated the better the outcome.
If you have had symptoms of listeriosis, see your doctor. This illness can spread to the nervous system and cause meningitis, which can be life-threatening.