Cooking science

Enzymatic browning

Cut an apple and it browns within minutes. This is not decay and it is not dirt — it is a specific enzyme meeting oxygen, and because it needs both, removing either one slows it down.

The reaction

Plant tissue contains an enzyme and a set of phenolic compounds, kept apart inside intact cells. Cutting, bruising or crushing breaks the cell walls and lets them meet. In the presence of oxygen the enzyme converts the phenolics into compounds that then link together into brown pigments — the same family of pigments that colours tea and gives cocoa its colour.

It needs three things: the enzyme, the phenolic compounds, and oxygen. Take away any one and the browning stops or slows, which is the whole basis of every remedy.

It is worth being clear that this is a quality change, not a safety one. Brown apple, brown avocado and grey potato are not spoiled. They look worse and taste slightly duller, and the flesh underneath is fine.

What slows it, and why

Acid — lemon, lime, vinegar

The enzyme works poorly below a certain pH. The most effective everyday measure, and the reason every guacamole recipe contains lime.

Excluding air

Cling film pressed directly onto the surface, submersion in water, or a film of oil. The key is direct contact with the flesh, not merely a covered bowl.

Cold

The enzyme works more slowly at fridge temperature. Slows it; does not stop it.

Heat

Cooking destroys the enzyme entirely. This is why blanching prevents browning permanently and why cooked apple does not darken further.

Salt

Mildly inhibits it. A salted water soak works for potatoes and apples.

Sugar syrup

Both excludes air and slows the reaction. The traditional treatment for fruit destined for a tart.

Leaving the stone in an avocado

Protects only the small area underneath it. It is a coincidence, not a technique.

Where it shows up

• Apples and pears — the classic. Acid or a salted water soak.
• Avocado and guacamole — lime, plus direct-contact covering. See grey guacamole.
• Potatoes — cut potatoes go grey-pink then brown. Keep them in cold water, which excludes air and rinses off surface starch at the same time.
• Bananas — the skin browns from bruising and from cold; the flesh browns on cutting.
• Mushrooms — bruise easily and darken quickly. Handle them gently.
• Artichokes, aubergines, celeriac, quince — all strongly prone. Acidulated water is standard practice.
• Tea and cocoa — the same enzyme family, deliberately encouraged rather than prevented.

Not the same as the other browning

Three different processes get called browning and only this one is enzymatic.

Enzymatic browning happens at room temperature, in minutes, on cut raw plant tissue, and needs no heat at all.

The Maillard reaction happens at high temperature between proteins and sugars, and it is what browns a crust or a seared steak.

Caramelisation is sugar breaking down under heat on its own.

The distinction matters practically: the remedies for one do nothing for the others, and a brown apple and a brown crust have almost nothing in common except the word.

Common questions