Cooking science

Caramelisation

Caramelisation is sugar decomposing under heat with nothing else involved. It produces colour, a set of aromas that are entirely its own, and — past a point — bitterness, which is a feature rather than a fault.

Sugar alone

Heat sugar past its melting point and it does not simply melt and stay sugar. The molecules break apart and recombine into a large family of new compounds — some coloured, some volatile and aromatic, some bitter.

It is genuinely distinct from the Maillard reaction, which requires an amino compound as well and produces a different family of aromas. Most browned food is doing both at once, which is why the two names are so often used interchangeably — including, misleadingly, for onions, where the Maillard reaction is doing at least as much of the work as caramelisation is.

Different sugars caramelise at different temperatures, and fructose does so at a considerably lower one than sucrose. This is part of why honey, which is high in fructose, browns so much faster than sugar and burns so readily.

The stages of a sugar caramel

By colour, which is the only practical guide without a thermometer.

Clear and molten

Melted, no colour, no caramel flavour yet.

Pale straw

The first flavour appears. Good for a delicate caramel sauce.

Amber

The classic caramel. Sweet, rounded, aromatic.

Deep copper

More complex and noticeably bitter. Better in a salted caramel or a sauce that will be diluted.

Dark brown, smoking

Bitter and increasingly acrid. Used deliberately in some cuisines; a mistake in most.

Black

Burnt. Start again.

Making a caramel

• Use a heavy, light-coloured pan so you can see the colour and there are no hot spots.
• Dry or wet. Dry caramel is sugar alone, faster and less forgiving. Wet caramel is sugar with a little water, slower and more controllable.
• Do not stir once it is boiling. Agitation encourages crystallisation. Swirl the pan instead.
• Brush the sides with water to dissolve stray crystals, which are what seed a crystallised caramel.
• A little acid or glucose helps prevent crystallisation.
• Stop it early. It keeps darkening off the heat, so take it off a shade lighter than you want. Adding cream or butter stops it, violently — stand back.
• Clean the pan with hot water, which dissolves set caramel without any scraping.

Where caramelisation happens outside a caramel pan

• Roast vegetables, especially carrots, parsnips, onions and squash, which are high in sugar. Much of what people call "caramelised" here is genuinely caramelisation.
• Caramelised onions, alongside the Maillard reaction — the sweetness is caramelisation and the savoury depth is not.
• Coffee and cocoa roasting, where both processes run together.
• A brulee top, which is pure caramelisation applied to the surface with a blowtorch or a grill.
• Balsamic vinegar reduced, which concentrates its sugars and then browns them.
• Bread crust and biscuit edges, where the sugars in the dough contribute alongside the proteins.
• Anything glazed with honey or syrup, which is why glazes go on late — they caramelise and then burn well before the food is done.

How it differs from the Maillard reaction

• Caramelisation is sugar alone, breaking down under heat. No protein is involved.
• The Maillard reaction needs both sugar and protein, and produces a different and much larger family of compounds.
• They usually happen together. A roasting joint, a loaf of bread and a fried onion are all doing both at once, which is why the two are so often confused.
• Caramelisation needs higher temperatures — around 160°C and up for table sugar — while Maillard browning starts appreciably lower.
• Caramelisation is what you get from pure sugar: a caramel sauce, a crème brûlée lid, a sugar syrup taken too far.
• Both produce bitterness if pushed, and in both cases the change from good to acrid happens over a few seconds.

Common questions