What Is Caramelisation? Sugar Taken Apart and Rebuilt by Heat

The reaction begins when sugar is heated beyond its melting point and the molecules start to break apart. Water is driven off, rings open, fragments recombine, and the products include volatile compounds responsible for aroma, coloured polymers responsible for the brown, and acidic compounds that cut the sweetness. Hundreds of distinct compounds have been identified, contributing notes described as buttery, nutty, fruity, toasted and bitter, and the mixture changes continuously as heating proceeds, which is why the flavour at a pale amber stage differs entirely from the flavour at dark brown. Sweetness decreases throughout, since the sugar itself is being consumed, so a dark caramel is markedly less sweet than a light one and eventually becomes bitter. The process requires no ingredient other than sugar and heat, which distinguishes it from the browning reactions that require a protein or amino acid to proceed.

Different sugars start at different points and the stages are recognisable by colour and smell:

• •Fructose begins earliest, around 110 degrees Celsius, which is why honey and fruit sugars brown readily
• •Ordinary table sugar begins around 160 degrees, which is the figure most recipes assume
• •Maltose and lactose require higher temperatures still, which matters when working with malt syrups and dairy
• •Pale amber gives light flavour and full sweetness, suited to coating and to delicate applications
• •Deep amber to brown gives the classic flavour with reduced sweetness and useful bitterness
• •Past that the mixture darkens rapidly to black and becomes acrid, and the transition takes seconds rather than minutes

The reaction between sugars and amino acids, which produces most of the flavour in roasted meat, bread crust, coffee and chocolate, is a different process and the two are routinely conflated. That reaction requires both a reducing sugar and an amino group, begins at lower temperatures, and produces a different set of compounds with the savoury and roasted character that pure sugar browning lacks. Both happen simultaneously in most cooking, since almost every food contains both sugars and proteins, which is why the distinction is hard to observe outside a pan of pure sugar. The practical consequence is that onions, which are frequently described as caramelising, are doing both, and that bread crust and seared steak are dominated by the protein reaction rather than by sugar breakdown. Keeping them separate matters when reasoning about why something browns or fails to, since the two have different requirements.

The reaction is responsible for a wider range of foods than the word suggests. Caramel sauce, toffee, brittle and the top of a creme brulee are the obvious cases, made by taking sugar to a chosen stage and stopping it. Caramel colouring, produced industrially by heating sugar with various catalysts, is among the most widely used food colourings in the world and gives colas, sauces and spirits their colour. Roasted malt in dark beers and bread contributes both colour and flavour from sugar breakdown. Dulce de leche and similar milk preparations involve both sugar breakdown and the protein browning reaction, since milk supplies both. Roasted vegetables develop sweetness because starch is converted to sugar by heat before any browning begins, and the browning that follows is partly this reaction. Even the crust on a well-made rice dish depends on the same chemistry at the base of the pan.

The technique has well-known hazards and each has a standard handling. Crystallisation is the common failure in wet caramel, where a single seed crystal on the pan wall triggers the whole mass to solidify, and the preventions are washing down the sides, adding a small amount of acid or glucose syrup to interfere with crystal formation, and not stirring once the sugar has dissolved. Burning follows from the narrow window at the dark end, and since the pan retains heat the reaction continues after removal from the stove, which means stopping it requires adding liquid or moving to a cold surface rather than merely turning off the heat. Adding liquid to hot caramel produces violent spattering and steam, which is the most likely route to a serious burn in this process. Dry caramel, heating sugar with no water, is faster and less prone to crystallising and gives much less margin for error.

Heat breaks sugar molecules apart and the fragments recombine into hundreds of compounds, giving colour, aroma and acidity while consuming the sweetness, so dark caramel is far less sweet than pale. It needs nothing but sugar and heat, unlike the browning reaction that needs amino acids and dominates bread crust and seared meat. One stray crystal can solidify a whole wet caramel.