Listeria can infect toffee apples

The findings came after a listeriosis outbreak killed seven people across different US states and was linked to caramel-coated apples last year.

Researchers found that putting sticks into apples released apple juice into the caramel coating and created a breeding ground for the bacteria.

Their scientific study was published in the journal mBio ​on Tuesday (October 13) ahead of Halloween, a time when toffee apples are traditionally eaten.

‘Apples too acidic’​

“Neither caramel nor apples are foods where the pathogenic bacterium Listeria monocytogenes should grow, as caramel does not contain enough free water and apples are too acidic,”​ the paper said.

“We hypothesised that inserting a stick into the apple releases juice to the interface between the apple and caramel, providing a more hospitable environment than either component alone.”​

In the experiment, Granny Smith apples – with and without sticks  – were exposed to the Lm​ bacteria, dipped in caramel and stored.

Bacteria were found to multiply on the apples that had sticks in them, but not on the apples without sticks.

‘Potential niches’ warning​

The researchers warned producers about the dangers of potentially creating breeding grounds for bacteria that cause food poisoning.

“Food producers should consider interfaces between components within foods as potential niches for pathogen growth,” ​the paper said.  ​

Listeriosis, the illness associated with Lm, is relatively rare but causes more deaths from food poisoning in the UK than any other foodborne bugs, according to the Food Standards Agency.

Symptoms include fever, vomiting and diarrhoea and the infection can cause serious complications such as meningitis.