Banana Peel Slipperiness

10 October 2016

They're yellow, radioactive, high in potassium and very slippery. (Spoiler Alert - some of what you just read is wrong.) Yes, they're bananas and we have just found out why they are so slippery. It's because of carbs and proteins, which are trapped inside tiny bumps on the inner layer of the banana skin.

OK, most of the bananas we see are indeed yellow.

But there are two caveats.

First, if you go looking around the world, you'll find that they usually come in other colours. When ripe, a banana can be green, red, brown or even purple. But yes, the bananas which we Westerners mostly see are yellow - mainly Cavendish and Gros Michel. Second, the yellow colour we see is actually more intense than 'natural', thanks to the artificial ripening process. Many bananas are picked while immature, and then stored and transported at around 13-15oC. Once they arrive close to their final destination, they are held at 17oC, and exposed to the ripening gas, ethylene.

Yes, bananas are slightly radioactive.

Bananas contain potassium, the second least dense metal (after lithium). The 'average' 70kg human carries about 150g of potassium. It occurs naturally as three isotopes: potassium-39 (93.26%) and potassium-41 (6.73%) are stable, but potassium-40 (0.0117%, or roughly just one out of every 8,550 potassium atoms) is unstable - it's radioactive. It has a half-life of 1.25 billion years (about 9% of the age of the Universe). It decays to either stable calcium-40 or stable argon-40. (This nuclear decay reaction allows us to 'date' the age of rocks.)

In your body, about 4,400 atoms of radioactive potassium-40 decay each second. It's our major source of natural human radioactivity, greater than from the decay of carbon-14. Mind you, the potassium-40 atoms don't accumulate in our bodies. Potassium is very dynamic in our human metabolism, continually entering and leaving our body.

The actual dose from a single banana (the Banana Equivalent Dose - BED - a real term used to express ionising radiation exposure) is about 0.1 microSievert (µSv). This is about 1% of your average daily exposure to radiation. A CT scan of the chest delivers 7 milliSieverts (7,000µSv, or 70,000 BED). The lethal dose of radiation is about 35 million BED (or 3,500mSv.)

But in terms of eating bananas, their radiation dose is soooo low that it's not harmful.

Bananas do in fact contain potassium - about 358mg per 100g. But they are not especially rich in potassium.

Out of some 8,019 food items that were rated for their potassium content, bananas came in around 1,600 down the list.

Potato (650mg per 100g), yam (495mg), avocados (507mg), soybeans (515mg), wheatbran (1,182mg), kidney beans (419mg), lima beans (508mg), sunflower seeds (850mg), cashew nuts (660mg), brazil nuts (659mg), and Spanish peanuts (744mg) are all higher in potassium than bananas. (I am waiting for potatoes to be spruiked as the next 'Superfood'.)

So, slippery?

Unless you work at a big fruit market you probably haven't seen a real person slip on a banana peel - let alone a bride! One fruitologist told me he had seen a forklift truck spinning its wheels on banana peels, totally unable to get any traction.

Of course, in cartoon-land, banana skins are very slippery indeed. Bugs Bunny, for example, tosses the skin of a peeled banana onto the floor, and then Elmer Fudd steps on it, windmills his legs and falls to the floor. The wascally wabbit wins once again.

What's odd is that we all 'know' that the banana peel is slippery, even though most of us have never seen it in action.

Suppose you have a block of leather, weighing 1,000g, resting on a wooden surface. To move it, you have to push it with an equivalent weight of 350g. The frictional coefficient of leather-on-wood is 350 divided by 1,000, which works out to 0.35. For rubber sliding on concrete, it's about 1.02.

Japanese tribologists (scientists who study interacting surfaces in relative motion) measured the frictional coefficient of banana peel on a linoleum surface at 0.07. This was amazingly low - better than metal on metal lubricated by oil. The kings of low friction are ice-on-ice and metal-on-ice (around 0.02) and the articular cartilage in your joints (0.01).

Under the microscope, they found that there were little tiny bumps, or follicles, on the inner banana skin - the surface that touches the delicious fruit. When your foot puts some pressure on these follicles, the follicles release polysaccharides (which are carbohydrates) and protein. These then unite (or combine) to form a homogeneous gel. Thanks to the bumpy nature of the skin, this homogeneous gel gets trapped between the banana and the floor, providing a wonderfully low friction state.

For their groundbreaking research, the Japanese tribologists won the 2014 IgNobel Prize in Physics.

And their ethics approval came with the proviso that no cartoon wabbits were harmed or injured during the course of their study - but they were unable to say the same for Elmer Fudds ...