Caramel is two reactions and only one of them is caramelisation

Browning is not a single process

The word caramel covers a range of things: a clear amber syrup, a chewy sweet, a pale sauce with cream in it, the coating on a crème caramel. They do not all get their colour and flavour from the same chemistry, and the distinction is worth having because it explains why some caramels taste sharp and bitter while others taste of butterscotch and toffee.

Caramelisation, strictly speaking, is sugar decomposing under heat with nothing else involved. The Maillard reaction is something else: a reaction between reducing sugars and amino acids that produces a different and much larger family of aromatic compounds. Add dairy to a sugar and you have both running at once, and the flavour you recognise is the combination.

What happens when sucrose alone gets hot

Heat sucrose past roughly 160°C and it begins to break apart. The molecule first splits into glucose and fructose, and those then undergo a cascade of dehydration, fragmentation and recombination reactions that generate hundreds of new compounds — some volatile and fruity, some nutty, some frankly bitter — along with the brown polymers that give caramel its colour.

The process is progressive rather than a single event, which is why colour is such a useful guide. A pale amber caramel is sweet and simple. A deeper one has lost sweetness, because sucrose is being destroyed, and gained complexity and bitterness. Push it further and the bitterness dominates, and past that it is burnt. There is no way back down the scale, which is why professionals pull a caramel off the heat before it looks right — the residual heat in the pan carries it several shades further.

Milk and butter change the reaction, not just the texture

Introduce cream, milk or butter and amino acids arrive with them, along with lactose, which is a reducing sugar. The Maillard reaction then proceeds alongside caramelisation, and it starts at considerably lower temperatures — it is running well below the point at which sucrose decomposes. Its products are different in kind: the compounds responsible for the aromas we describe as malty, buttery, toasted and meaty in savoury cooking.

This is why a dairy caramel tastes rounder and less sharp than a water caramel taken to the same colour, and why dulce de leche, which is made by heating sweetened milk for a long time at a temperature nowhere near 160°C, browns and develops flavour at all. Almost none of that is caramelisation. It is Maillard, running slowly, for hours.

Butter contributes on a third front as well. Browning butter drives off its water and toasts the milk solids suspended in it, which is a Maillard reaction happening before the butter ever meets the sugar, and those toasted solids carry their own nutty compounds into whatever they join. A caramel built on browned butter therefore arrives with flavour that did not come from the sugar at all, which is worth knowing when a sauce tastes deeper than its colour suggests it should.

Dry and wet methods reach the same place by different routes

A dry caramel is sugar melted alone in a pan. It is fast, it gives good control over the final colour because there is nothing to obscure it, and it is prone to lumps and to scorching wherever the pan has a hot spot. A wet caramel starts with sugar dissolved in water, and the water buys time: it must all boil away before the temperature can climb past 100°C, so the sugar melts evenly and the whole process is slower and steadier.

The wet method carries its own hazard, which is crystallisation. A concentrated syrup will seize into a grainy mass if a stray crystal seeds it, hence the wet brush around the pan walls and the instruction not to stir. A little glucose syrup or acid interferes with crystal formation and makes the whole thing considerably more forgiving. Bakers disagree about which method is better and the disagreement is mostly about which failure they would rather risk.

Stopping it, and what the addition does

A finished caramel has to be arrested, because the pan holds enough heat to keep the reaction running after the flame is out. Adding cream or butter does this and does it violently: the liquid hits sugar well above the boiling point of water, flashes to steam, and the mixture erupts. Warming the cream first reduces the shock and the spitting, and it also reduces the risk of the sugar seizing into a solid lump as it is chilled below its melting point.

What goes in also determines what the caramel becomes. Water gives a sauce that sets to nothing. Cream gives a pourable sauce that thickens as it cools. Cream plus butter plus a further cook to a specific sugar concentration gives a chewy caramel, and the chew is decided by that concentration rather than by the browning. Salt is not merely a fashion here either: it suppresses the perception of bitterness and sharpens the roasted notes, which is why a dark caramel takes salt better than a pale one.