Why Does a Cut Apple Turn Brown?

Step 1 of 5

The reaction needs three things that were kept apart.

An enzyme, a group of compounds it acts on, and oxygen. Inside an intact apple the first two are stored in separate compartments.

Step 2 of 5

Cutting breaks the walls between them.

Now the enzyme, its target and the air all meet on the cut face. Within minutes the surface starts to change colour.

Step 3 of 5

The brown is a real pigment being built.

The reaction links small molecules into large brown ones, similar to the pigment in tanned leather. The apple is not dirty, it is painted.

Step 4 of 5

Cold, acid and no air all slow it down.

Lemon juice supplies acid the enzyme cannot work in, cold slows every reaction, and cling film limits the oxygen supply.

Step 5 of 5

It probably protects the fruit.

The same pigment seals a wound and tastes bad to insects and microbes. Bruises brown for exactly the same reason.

Some apple varieties have been bred with far less of the enzyme, so slices stay white in a lunchbox all day.