Why do cut apples turn brown?

KohakuLeave a cut apple out, and before you know it, it has turned brown. That can be bothersome when packing a lunch box, can't it?

The answer in 30 seconds

The cause is the enzyme inside the apple, together with the air.

A cut apple turns brown because an enzyme in its flesh meets oxygen in the air and changes the polyphenols in the flesh into brown substances. This change is called "enzymatic browning." If you pour lemon juice on it, the vitamin C reacts with oxygen first, and the sourness also makes the enzyme work poorly, so the apple is slower to turn brown.

The cut face of an apple sliced in half — An apple cut in half. As time passes, the cut surface slowly turns brown.Photo: Forest and Kim Starr / CC BY 3.0 us (cropped) / Wikimedia Commons

Cutting brings enzyme and oxygen together

A food science professor at the University of Massachusetts Amherst explained in the magazine Scientific American that when an apple is cut or bruised, oxygen gets into the damaged tissue. Then an enzyme called "polyphenol oxidase" changes the polyphenols in the flesh into other substances.

Where the brown color comes from

The substances made by the enzyme have no color at first. They turn brown when they join with things like proteins, or link together into bigger molecules. An explanation from the University of Wisconsin calls this reaction "enzymatic browning" and notes that the same thing happens with pears, bananas, and eggplants.

Some varieties brown faster

Almost all plants have this enzyme, but how strongly it works, and how much of the starting ingredients there are, depends on the variety, how the fruit was grown, and how ripe it is. That is why some apples turn brown quickly and others do not. Even when an apple turns brown, what has changed is its color and look.

KohakuEven among apples, how fast they turn brown depends on the variety. Isn't that interesting?

Two reasons lemon juice works

According to an explanation from the University of California, Santa Barbara, the vitamin C (ascorbic acid) in lemon juice reacts with oxygen before the enzyme can. Lemon juice is also very sour, and this enzyme does not work well in sour conditions. According to Penn State University, a vitamin C solution is said to be even better than lemon juice at preventing browning.

Figure: How apples turn brown, and why lemon juice helps

Want to know more?

Coating apple slices with sugar or syrup makes it harder for oxygen to reach the surface, so they turn brown more slowly.
Heating apples stops the enzyme from working, so apples cooked into jam or sauce do not turn brown the way freshly cut ones do.
The brown color of tea, coffee, and cocoa also comes from this enzyme working while they are being made.

KohakuWhen enzyme and oxygen meet, the apple turns brown. Lemon juice works on both the oxygen and the enzyme. It is a time-honored bit of wisdom.

Quick quiz: test yourself

Why does a cut apple turn brown?

Show the answer

The flesh dries out
An enzyme works with oxygen
The apple starts to go bad

An enzyme works with oxygen An enzyme in the flesh meets oxygen in the air and turns polyphenols into brown substances. This is called enzymatic browning.

Sources

Why do apple slices turn brown after being cut?Scientific American (Lynne McLandsborough, University of Massachusetts Amherst)
Carry Out Science: Apple BrowningUniversity of Wisconsin–Madison Division of Extension
Why does lemon juice prevent an apple from turning brown?UCSB Science Line (University of California, Santa Barbara)
Preserving Color and Preventing Browning of FoodsPenn State Extension

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.