Why do peeled potatoes turn brown if you leave them for too long?

Answer

This has to do with the so-called enzymatic browning. It is therefore an enzymatic reaction that mainly takes place in vegetable matrices if you e.g. damaged the tissue. Just think of a bruised apple or banana: they also turn brown. When peeling the potato you also damage the tissue and this triggers the reaction.

Enzymatic reactions are reactions aided by enzymes; these are actually biocatalysts. They are special proteins, which can therefore allow reactions to continue under physiological conditions. Most reactions in biological systems are catalyzed by enzymes.
Oxygen is also needed for the enzymatic browning; it is actually an oxidation reaction – so by immersing the peeled potatoes you can slow down the reaction: you shield them from the oxygen in the air. The enzymes also work more slowly in an acidic medium: that’s why you add some lemon juice to a fruit salad to slow down the browning…

It is a reaction that is mainly of ‘aesthetic’ importance: we don’t like brown spots very much – when I tell this to our international students from Africa, they don’t understand this: they are used to this and don’t care – here with us, however, a brown spot is immediately seen as a serious defect…
By cooking the potatoes you no longer suffer from this reaction: proteins are modified by cooking (just think of boiling an egg!; we call this denaturing), so that enzymes – which are proteins – lose their activity – no more enzymatic browning in boiled potatoes or French fries and so on. French fries and baked potatoes may turn brown again by baking, but that is another reaction in food (the one between sugars and proteins; see also bread, biscuits, coffee, etc.)

Regards