Why does a cut apple turn brown — and can you stop it?

Slice an apple and leave it out. In a little while the white flesh goes brown. People say a splash of lemon juice keeps it fresh — but does it really, and if so, what is the lemon doing? Let's cut one open and find out.

1What it takes to make brown

Brown needs two things to bump into each other

Browning isn't the apple "going bad." It's two ingredients meeting. Watch each one:

Oxygen in the air

An invisible gas floating all around you. It bumps into anything you leave out — including the wet inside of a freshly cut apple.

The apple's helper

An enzyme already inside the apple. Cutting lets it meet oxygen and other apple molecules. It speeds up reactions that eventually make brown pigments.

2One thing changes: can the helper work?

The bare cut vs a lemon coat

The whole mystery is about whether the helper gets to do its job. The bare slice we already know about — but what could a coat of lemon change? Here are the two set-ups, side by side:

Bare cut

Oxygen reaches the helper

Nothing stops it. The helpers grab oxygen all over the surface — so it slowly turns brown. (This is the part we already know.)

Lemon coat

Lemon's sour acid soaks in

Lemon is acidic — and acid can gum up molecules like the helper. So would a lemon coat actually stop the browning, or not? That's what you'll test next.

3Your turn — fast-forward time

Leave a bare slice out and slide time forward

Here's one plain apple slice on the counter. Drag the clock and watch the helpers grab oxygen all over the wet surface — the longer it sits, the browner it gets.

Time on the counter just cut
JUST CUTHOURS LATER

4Now race two slices

What if the lemon arrives late?

Both slices have already formed brown patches. Now brush lemon on only the right slice. Can slowing the browning reaction erase the pigment that has already formed?

Guess before you watch

Two slices already have brown pigment. After lemon is added to one, what happens to those existing brown patches?