Why a Cut Apple Turns Brown, and How Lemon Stops It

Slice an apple, get distracted for ten minutes, and come back to find the pale, juicy surface turning a tired shade of brown. It still tastes fine, but it looks as though it has had a long day. The apple is not rotting. It is doing chemistry, and it started the moment the knife went in.

The key player is an enzyme called polyphenol oxidase. Inside an intact apple, it is kept apart from the substances it works on. Those substances are polyphenols, a group of natural chemicals that plants make in large amounts and that are often praised as healthy antioxidants. As long as the cells are whole, the enzyme, the polyphenols and the oxygen in the air do not meet.

Cutting changes everything. The knife breaks open thousands of cells, and suddenly the enzyme, the polyphenols and oxygen are all mixed together on the cut surface. The enzyme gets to work at once, using oxygen to change the polyphenols into new compounds that are colourless at first. Those then react further and link up into brown pigments that belong to the melanins, the same family of pigments that gives colour to human skin. Scientists call the whole process enzymatic browning.

Why would a plant bother with this? It seems to be a kind of defence. When an apple is damaged, some of the chemicals made by this reaction help to fight off invading organisms, such as the microbes that might move into a wound. So the brown patch is less a sign of decay and more a hastily applied bandage. Avocados, potatoes, raisins and prunes brown in the same way, and so does apple cider.

That also explains the old kitchen tricks. Lemon juice works because it is acidic and contains natural antioxidants: the low acidity makes the enzyme far less active, and the antioxidants intercept the oxygen. Pineapple juice does a similar job. Coating the slices in a sugar syrup helps by keeping air away from the surface, and blanching apples in boiling water for four or five minutes can nearly wipe out the enzyme altogether, though you end up with softer apples. Varieties also differ, since some carry more of the enzyme or more polyphenols than others, which is why some apples brown much faster than their neighbours in the fruit bowl.

Browning is not always unwelcome, either. The same kind of enzyme reaction, carefully managed, is part of what gives tea, coffee and cocoa their rich brown colours during processing. In your fruit bowl it looks like a flaw; in a tea factory it is part of the recipe.

We know how it works because food scientists have traced each step, from the enzyme and where it sits in the cell, to the colourless first products, to the brown pigments at the end, and they have tested which tricks slow it down. So the next time a slice of apple blushes brown, you can forgive it. It is just a fruit trying to protect itself, one sliced cell at a time.