Blackening of banana peels a natural process, unrelated to parasites: experts

"Chinese scientists have recently discovered that the reason for the colour change in banana peels is a parasite that breeds inside the banana," reads part of the Thai-language caption of a video shared on Facebook here on October 5, 2023.

It says eating black bananas is "very dangerous" and "this is why I get diarrhoea every time I eat ripe bananas like this”.

The 29-second-long video shows a person cutting a thin slice off a banana with a black peel that they put under a microscope. Microscope footage appears to show living organisms in the sample.

Simplified Chinese text is also overlaid on the clip. The text reads: "It is scary when you put a banana under a microscope. Is a banana that has become dark and soft still edible?"

It goes on to say that bananas with black peels can still be eaten "if the pulp is normal", but if the inside and outside of a banana are both black, it should be thrown away immediately.

It also says blackened and rotten bananas are a breeding ground for microorganisms -- which can be seen when placed under a microscope.

"Blackened, rotten and spoiled bananas are basically contaminated as a whole," it adds.

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Screenshot of the false Facebook post, captured on October 16, 2023

Similar posts can be found on Facebook here and here and on TikTok here and here.

Comments on the posts suggest many users believed the claim.

"How to live safely, you must read this to know," read one comment.

Another said: "I have often eaten it by mistake because it tastes good and sweet."

But experts told AFP the colour change is not caused by parasites and that bananas that have turned black are still safe to eat as long as it does not attract fruit flies.

Natural process

Sasivimon Swangpol, an assistant professor at Mahidol University's Department of Plant Science, told AFP the colour-changing process is normal and has nothing to do with parasites (archived link).

"Banana peels normally turn black," she said on October 9. "When the cells are torn open, the enzyme in the cells comes into contact with the air and turns them black, which is normal for fruits and plants."

Benjakhun Sangtongpraow, an entomologist at Kasetsart University, also told AFP bananas turning black is a "normal process" (archived link).

But Benjakhun added that ripe bananas may attract fruit flies, which can lay eggs on the banana peel.

Once the eggs hatch, they develop into larvae that invade the banana and "make it rot faster", she told AFP on October 11.

"This process allows the living organisms on the banana peel, especially fungi and bacteria, to enter the inside of the banana through the wound caused by the larvae," she said, adding that consuming contaminated bananas could lead to symptoms such as diarrhoea.

But she stressed that the harm comes from the bacteria and fungi on the skin of the banana and not from the insects.

Tawee Saiwichai from Mahidol University's Department of Parasitology and Entomology also told AFP on October 12 that bacteria such as "Salmonella, E. coli, and Listeria, which can cause gastrointestinal illnesses, can be introduced to bananas through contact with fruit fly eggs" (archived link).

Fruit fly larvae

Both Benjakhun and Tawee told AFP the organisms seen in the footage are not parasites.

Benjakhun said: "Under the microscope, you can see that the tiny larvae have mouth hooks at the end of its head that can expand and contract... Breathing holes can be seen on the sides of its body, and there is a breathing tube leading from the side breathing hole to another at the end of its body.

"This is a characteristic of dipteran larvae."

Tawee said that while it cannot be definitively confirmed, the organisms are similar to the larval stage of fruit flies.

"These larvae are not parasitic and do not pose a threat to humans," he said.

"They are more likely to be dipteran fruit fly larvae, and their presence doesn't lead to Myiasis, a condition where fly larvae infest human tissue."

Benjakhun said even if a banana's peel has turned black, as long as there are no fruit flies buzzing about, "it is safe to eat".