You start eating an apple, and just then you friend calls you up about homework, and you're speaking for hours together. When you come back to your apple, it's gone brown all over. What happened?
Rusting in apples
The brown colour is because your apple has rusted! That's because apples are rich in iron, which is present in all their cells. When you cut an apple, the knife damages the cells. Oxygen from the air reacts with the iron in the apple cells, forming iron oxides. This is just like rust that forms on the surface of iron objects. An enzyme called polyphenol oxidase (that's present in these cells) helps make this reaction go faster.
If you cut a browned apple into two again, you'll notice that the insides are still white. That's because the cells inside were intact, and did not let oxygen enter right inside.
Lots of other fruits and vegetables also turn brown when cut. These include bananas, pears and even potatoes.
Keeping an apple from turning brown
There's no harm in eating an apple that has turned brown, for the iron oxide will not affect you. But when you're making a fruit salad or apple pie, the browning may make it look unattractive. Here are some things you can do to stop or slow the browning:
Cut and keep the apples under water. This prevents air from reaching the iron. But it may cause some vitamins to leach into the water.
Rub the cut apples with lemon juice. The acid in the lemon juice stops the polyphenol oxidase from working.
If you're making apple pie, you can dip the apples in boiling water for a few seconds and take them out. This is called blanching, and it stops the browning enzyme.
You can add some salt to the apples; the salt stops the enzyme. Do this if you don't mind the salt-and-sweet taste that will result.
Keep the apple pieces in an airtight jar, or wrap them in cling wrap very tightly. This also stops air from getting to them.
And finally, the method we like the most. Turn your apple into apple juice. The iron oxide gives it the special golden-brown colour, and it's a tastier way to consume an apple!