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Half-brown banana

By David Shaw, 27 April 2022

Bananas go brown, but not usually just at one end

Unripe bananas are green, ripe ones are yellow and overripe bananas are brown. But can you make a banana that’s half brown and half yellow?

Safety: This activity uses a hot stove and boiling water. Ask an adult to help.

You will need

• Saucepan
• Skewer
• Banana
• Water
• Tongs
• Stove

What to do

1. Check that your skewer is wider than the saucepan. If it isn’t, see if you can find a longer skewer or a smaller saucepan.
2. Stick the skewer through the side of the banana, about halfway down its length.
3. Put the banana in the saucepan, so the skewer rests on the sides of the saucepan.
4. Take the banana out of the saucepan. Fill the saucepan so it’s ⅔ full of water. Don’t fill it all the way! The water level will rise when you add the banana.
5. Turn on the stove and then put the saucepan on. Wait until the water starts to boil.
6. Hold the banana with tongs and then put it on the saucepan, so one end of the banana is in the water. You may need to support the banana with the tongs to keep it in place.
7. Wait for 30 seconds and then use the tongs to pull the banana out.
8. Look at the banana. Can you tell which part was in the water?
9. Wait for 5 minutes and then look again at the banana. Did it change?
10. Peel the banana. Did the water affect the edible part of the fruit, or just the peel?

What’s happening?

When they get old, bananas go brown. They also go brown when they are bruised. But what causes the brownness?

It’s all down to a chemical called polyphenol oxidase (PPO). This chemical oxidises (adds oxygen to) a type of chemical called a phenol. These oxidised phenols then join together with other compounds to make brown colours.

Polyphenol oxidase is normally contained inside certain parts of plant cells. When the cells are damaged, the PPO escapes and starts the browning process. That’s why a bruised banana goes brown, and it’s also what happens in this activity.

There are ways to stop the browning. PPO can’t oxidise anything without oxygen. In this activity, it’s why the banana doesn’t really go brown until it’s out of the water for a while. This exposes the banana to air and the oxygen it contains.

You can also stop the browning by destroying the PPO before it can start oxidising. One way to destroy PPO is to heat it up for several minutes. Why not try boiling a banana for 10 minutes and see if it turns brown?

Real-life science

Browning a banana skin is fun, but there’s also a serious side to all this. Lots of fruit is wasted because it goes brown from a small bruise or from being cut and left in the air. To help fight food waste, CSIRO developed an anti-PPO gene that can prevent browning. It’s currently being used in special Arctic varieties of apples. They’re only available in the Americas for the moment, but maybe sometime soon, you’ll be able to eat some non-browning apple slices!
Read more about CSIRO’s non-browning technology

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