Apples

[einsteinsdog]

If an apple goes brown when it is cut , then it is oxidising . Does this mean apple skin is not permeable for oxygen molecules?

[gen2]

Yes - it is oxidation, but it is an extremely slow reaction. The browning is caused by enzymes (released from damaged cells) which greatly speed up the oxidation reaction. You can easily stop it by deactivating the enzyme. One way to do that is to lower the pH by covering the cut surface with fresh lemon juice. The apple will then not go brown.

The skin plays no part in the process, If an apple is dropped onto a hard surface, the apple will bruise even if the skin remains intact. The damaged cells in the bruised area will release enzymes that will brown the flesh.

[darkmatter]

If you tear an apple, rather than cut or bite it, you can separate the cells instead of breaking them and the apple will take much longer to turn brown.