The analysis was conducted by a team of experts from the University of Wisconsin-Madison’s Food Research Institute. The findings were published on October 13, in the journal mBio.
Previously, according to the U.S. Centers for Disease Control and Prevention, no explanation had been found for last year’s Listeria infection that had affected 35 people, killing 7 of them.
Although all the people who had contracted the bacteria had consumed caramel apples, such products were considered to provide an unfavorable environment for Listeria monocytogenes.
Apple juice is normally too acidic to act as an effective breeding ground for the bacteria. Moreover, Listeria needs water in order to develop, and there is too little of it present in the caramel. As a result, the caramel apples should’ve been safe.
Study authors suspected however that the fruit had been damaged, in a way that made the bacteria flourish. Their hypothesis was that the stick that is inserted into the candy apple as a handle might perturb its usual acidic environment, and push Listeria inside.
As juice begins to sip between the fruit and the caramel, its acidity is neutralized in the presence of sugar. Through this process, candy apples turn from highly unlikely sources of contamination into ideal breeding grounds for the Listeria bacteria.
In order to test the validity of this theory, researchers conducted an experiment which included 12 dozen Granny Smith apples, chosen for their exceptionally high acidity. Such fruit had also been involved in the 2014 outbreak, and in the trial experts had them contaminated with 4 strains of Listeria, like the ones identified last year.
Afterwards, sticks were inserted in half of the apples, while the rest of the fruit remained intact. In the next step, all of the apples were dipped into hot caramel. According to experts, this process usually destroys all the bacteria on the surface of the fruit, although some germs might persist in the stem and calyx region.
Finally, half of the caramel apples were placed in containers and kept at room temperature (77 degrees Fahrenheit), while the rest were kept refrigerated (at 45 degrees Fahrenheit).
When assessing Listeria growth, researchers discovered that it only took 3 days for the germs to develop on room-stored candy apples that had sticks. When these treats had no stick Listeria appeared in a week.
For refrigerated apples with sticks, the dangerous bacteria manifested itself after 2 weeks, while those without sticks never showed signs of Listeria.
Based on these findings, study authors suggest that it is essential to avoid keeping candy apples at room temperature. Instead, the sugary treats should be consumed fresh, straight from the fridge, so that a potential outbreak can be prevented.
Moreover, experts suggest that candy apple producers should pay closer attention to the stem and calyx when sanitizing the fruit, so that the bacteria can be flushed out.
These recommendations are particularly important, since Listeria is an insidious attacker, which can pose severe health risks, like meningitis, septicemia, miscarriage and stillbirth.
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