Cooking science

Browning without enzymes

Food browns by three different routes and they are constantly confused. Two need heat and are usually wanted; one needs no heat at all and is usually a nuisance. Telling them apart tells you whether to encourage it or prevent it.

The three brownings

Maillard reaction

Between amino acids and reducing sugars, at high temperature. Needs protein and sugar and a dry surface. Bread crust, seared meat, roast coffee, toast.

Caramelisation

Sugar alone, breaking down under heat. No protein involved. Caramel, caramelised onions, the edges of roast carrots.

Enzymatic browning

No heat at all. Enzymes in cut fruit and vegetables react with oxygen. Apples, avocados, potatoes, bananas, mushrooms.

Which are wanted

The first two, nearly always. The third, almost never — except in tea, coffee and cocoa fermentation, where it is essential.

How to encourage

Heat, dryness, and time for the first two.

How to prevent

Acid, cold, exclusion of air, or brief heat to destroy the enzymes, for the third.

Why the distinction matters

Every technique that helps one hinders another. Lemon juice on cut apples prevents enzymatic browning; lemon juice on a steak prevents the crust you wanted.

A wet surface stops Maillard browning entirely and does nothing to stop a cut apple going brown. Blanching prevents enzymatic browning by destroying the enzymes and simultaneously guarantees no Maillard browning at all.

So the first question when something is browning is which browning it is: did it need heat? If yes, it is probably wanted. If it happened on a board at room temperature, it is not.

Maillard and caramelisation, told apart

They usually happen together and they are not the same reaction.

• Maillard needs protein. A pure sugar syrup cannot do it. Meat, bread, dairy and pulses all can.
• Caramelisation needs only sugar, and it happens at a higher temperature than Maillard.
• Maillard produces far more distinct compounds — hundreds of them — which is why a bread crust smells complex and caramel smells mostly of caramel.
• A roast onion does both. The sugars caramelise and the amino acids react, which is why caramelised onions taste far more complex than the name suggests.
• Adding a little sugar or milk to a surface encourages both — this is why brushing a loaf with milk browns it.
• Alkalinity speeds Maillard. A pinch of bicarbonate of soda in an onion pan or on chicken skin genuinely accelerates browning.

Encouraging the browning you want

1. Dry the surface. Nothing browns while it is wet.
2. Get the surface genuinely hot. Both reactions need considerably more than boiling point.
3. Give it space. Crowding traps moisture and holds the temperature down.
4. Add something to react with if there is little — a brush of milk, a dusting of sugar, a little honey in a glaze.
5. Consider a pinch of bicarbonate of soda where a recipe allows it, which raises pH and speeds Maillard.
6. Be patient. Browning is not instant and it accelerates once it starts.
7. And to prevent enzymatic browning instead: acid, cold, cover it, or blanch it.

Common questions