The 2014 listeria outbreak in the United States may be linked to caramel apples, the researchers found in a new study.
A number of 35 American citizens were infected with the listeria monocytogenes bacterium, and seven people died because of the infection.
Previously, scientists believed that listeria monocytogenes could not breed properly in caramel apples, since apples have an acidic juice and bacteria do not like acidic environments because they hinder their growth, Kathleen Glass, the associate director of the Food Research Institute at the University of Wisconsin-Madison, said.
The low amount of water in the caramel and the level of acidity meant that the caramel apples were safe for consumption; or so scientist thought.
They found that the fault was in the wooden stick, which people insert into the apple and use as a handle. According to Glass, the environment within the apple changes once the stick is inserted.
One plausible theory is that the juice that comes out of the apple – once the wooden stick is inserted – gets neutralised by the caramel, turning it into a hospitable breeding ground for the listeria monocytogenes bacteria.
In the study – published October 13 in the journal mBio – the researchers tested this hypothesis. They bought 144 Granny Smith apples, and proceeded to contaminate them with four different strains of listeria monocytogenes.
Although in the 2014 outbreak Gala apples were also involved, the researchers only used Granny Smith apples in their experiment because they had a higher level of acidity, making it harder for bacteria to grow.
When they made the caramel apples, the researchers inserted sticks into half of the apples before dipping them into caramel, while the remaining apples were left ‘stick-less’.
The next step was to store half of the apples – both with and without sticks – at room temperature (about 77 degrees Fahrenheit, or 25 degrees Celsius) and the other half in refrigerators at 45 degrees Fahrenheit (7 degrees Celsius).
Three days after, the results showed that the apples with stick in them that were stored at room temperature had high levels of Listeria growth, compared with the refrigerated apples with stick in them which only showed signs of bacteria growth after two weeks.
In the case unrefrigerated apples without sticks, Listeria grew after one week, while no Listeria growth was observed on the refrigerated ‘stick-less’ apples even after a three-week period, the researchers said.
Glass advises retailers to avoid keeping the caramelised apples in an unrefrigerated environment for a long period of time.
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