Peeling back the skin on the science behind bananas

Bananas are enjoyed by people the world over, and in fact, about 100 megatonnes are produced globally each year. We’re kind of obsessed.

So, you’ve probably heard some interesting myths and rumours about our beloved wonky yellow fruit, like they’re all clones of each other and that there’s a deadly banana disease threatening their continued existence.

Let’s peal back the skin to reveal the tasty science centre of bananas.

Bananas are actually berries

It sounds fake, but bananas are the berries of the different flowering plant species in the genus Musa, which are native to tropical South-East Asia.

There are over 1,000 varieties, but we can eat the cultivated ones because of a genetic mutation that produced the seedless fruit we enjoy today.

But these bananas are parthenocarpic, which means that the fruit grows without seed development, pollination, and fertilisation. That means they’re sterile, so to make more of them you need to clone them.

Banana plants produce “suckers” – shoots that develop from the stem – that can be cut off and planted to grow into an identical fruit-producing plant.

Unfortunately, this means that there’s very little genetic diversity between plants, so this makes them incredibly vulnerable to disease outbreaks.

Cultivated banana varieties are vulnerable to disease

Until the 1950s the Gros Michel variety of bananas was the most popular grown in commercial banana plantations. But a fungal disease called Fusarium wilt, or Panama disease, nearly wiped them out.

It’s caused by a soil pathogen, called Fusarium oxysporum f. sp. cubense, which infects the root and vascular system and makes them unable to transport essential water and nutrients.

The spread of this disease forced the banana industry to switch to cultivating the Fusarium-resistant Cavendish varieties we see today. Now, they dominate the market – making up around 50% of global banana production and about 95% of exports.

But thanks to the emergence of an extremely virulent strain of Fusarium wilt called Tropical Race 4, the Cavendish has become vulnerable to going the same way as Gros Michel.

First detected in South-East Asia in the 1990s, the strain has since spread to Australia, East Asia, Africa, and recently South America.

What’s being done to save bananas?

Well, unfortunately once the fungus is established in an area, it can’t be removed through chemical or physical means and can survive in the soil for more than 30 years. The only way to protect bananas is to prevent the fungus from spreading, or to develop resistant banana varieties.

Scientists are improving diagnostic tools to locate TR4, and are using breeding and genetic engineering to try to create resistant varieties. One approach has scientists introducing antifungal genes from other species of plant, while in another project, a resistance gene from the wild banana Musa acuminata malaccensis, has been introduced into Cavendish varieties.

A recent study published in PLOS One found that scientists could induce resistance by inoculating Cavendish bananas with a non-virulent form of the Fusarium strain called Race 1.

Why do bananas cause other fruits to ripen quicker?

Have you ever wondered why putting your brown bananas next to green ones and some other fruits cause them to ripen quicker?

It happens because bananas are what’s known as a climacteric fruit – which also includes pears and avocados – that continue to ripen after picking.

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These fruits begin to ripen when exposed to a plant hormone called ethylene (C₂H₄), which kick-starts the process.

Through ripening, energy stored as starch – a long carbohydrate polymer that doesn’t taste sweet – is converted into simpler sugars that do taste sweet.

Could eating too many bananas kill you?

Have you heard that eating too many bananas could kill you, or at least make you very sick?

Bananas are a good source of potassium, but too much potassium can sometimes lead to a fatal condition called hyperkalemia. This can be caused by kidney failure, heavy alcohol use, low red-blood-cell count, or overdosing on potassium supplements.

A medium banana contains about 420 milligrams of potassium, so you would have to eat 11 in one day just to meet the recommended daily allowance of potassium for adults and children over 4 years.

There’s no established upper limit of potassium intake, but oral doses of more than 18 grams may lead to hyperkalemia. But, since that’s approximately 42 bananas eaten in a short period of time, its highly unlikely to occur.

But what about radiation? Aren’t bananas radioactive?

Only very slightly, thanks to the presence of a particular radioactive isotope called potassium-40.

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Radioactive isotopes aren’t stable, and they break down, releasing energy as radiation. Potassium-40 makes up about 0.012% of the total amount of potassium found in nature, so eating one banana will expose you to the equivalent of roughly 1% of your average daily exposure to background radiation.

But it would take eating 35 million bananas in a day to get a high enough dose of radiation for it to become lethal.

Why are banana peels so slippery?

Pop culture will tell you that banana peels are slippery suckers, but in a 2012 study, Japanese researchers from Kitasato University found out just how slippery, by measuring the friction coefficient under Cavendish banana skins.

Their experiment simulated a slipping accident under a sliding shoe and found that banana peels have a friction coefficient of 0.07. A lower coefficient of friction implies that a surface is more slippery, so surfaces with a friction coefficient of less than 0.1 are considered well lubricated, like ice on ice.

Further inspection under a microscope revealed that banana skins are lined with lots of tiny sacs of a gooey substance called polysaccharide follicular gel.

When the peel is crushed under foot, these substances are released and combine to make a slippery solution that forms a lubricating film between the shoe and surface.

This discovery earned the team the 2014 Ig Nobel Prize in physics.

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