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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. The outbreak was particularly troubling because Listeriosis can have a long incubation period from three to 70 days, and grows really well at refrigerated temperatures.

Listeriosis is usually caused when a person ingests listeria monocytogenes bacteria, but an infection is hard to pinpoint because the incubation period can be weeks. Symptoms can include gastrointestinal distress, fever and muscle aches. In severe cases, people can develop encephalitis, swelling of the brain, or bacterial meningitis, inflammation of the membrane surrounding the brain and spinal cord. Listeria can cause particular harm among the elderly people, pregnant women or anyone with a compromised immune system.

Since the outbreak, researchers have been puzzled how the Listeria bacteria could grow so quickly on caramel apples. Apples are typically too acidic for Listeria to grow quickly and caramel doesn’t often grow the bacteria because of low water content. Kathleen Glass, associate director of the Food Research Institute, figured it must be something about the caramel coating process that allowed the Listeria growth.

Her hypothesis: When the sticks (used to hold the apple) punctured the contaminated apples, they spread small amounts of apple juice over surface of the otherwise dry outer skin. Then, once the caramel coating was applied, it locked in the juice and created a micro-environment in which the Listeria on the surface of the apple could grow.

To test her hypothesis, Glass and a team of researchers swabbed 144 apples with four different strains of the bacteria, then inserted wooden sticks into the stems of half of the apples. All the apples were then dipped into hot caramel. After a cooling period, the apples were stored at either room temperature or refrigerated for four to six weeks.

The study found that the bacteria growth increased at room temperature 1,000-fold on caramel apples with sticks stored at room temperature for three days. Listeria growth was delayed on caramel apples without sticks stored at room temperature. By contrast, Listeria growth was significantly lessened among apples stored in the refrigerator; those with sticks had no bacteria growth for up to a week but then some growth occurred over the next three weeks. Caramel apples without sticks had no Listeria growth during four weeks of storage. Additionally, in the apples with sticks, researchers found that bacteria concentrations were found around the stick inside the apple. The researchers theorized that the stick pushed the bacteria into the apple where it was protected from hot caramel, allowing the bacteria to grow.

According to Glass, “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 two weeks, it is enough time for the bacteria to grow.” The study was recently published in the medical journal mBio, an online open-access journal of the American Society of Microbiology.

What does this mean for consumers this fall season? Glass recommends eating caramel apples fresh or ones that are refrigerated. She also suggests that manufacturers take steps to ensure their products’ safety by thoroughly disinfecting apples before dipping them in caramel; adding growth inhibitors to the caramel coating or apple wax; and using better temperature-time controls to prevent listeria growth.

Mark Bello is the CEO and General Counsel of Lawsuit Financial Corporation, a pro-justice lawsuit funding company.

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