What Is Caramelization? How Sugar Browns When Heated
Caramelization is the browning of sugar when heated, as its molecules break down and recombine into many new compounds that give caramel its colour and flavour.
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Caramelization is the browning of sugar caused by heat alone, with no other ingredient taking part. When sugar gets hot enough, its molecules break apart and recombine into many new compounds. Some of these are brown, and others carry the buttery, nutty and bitter notes that make caramel taste the way it does.
What does heat do to a sugar molecule?
Table sugar, sucrose, is a glucose unit joined to a fructose unit by a single chemical bond. Caramelization starts at roughly 160 to 170 °C for sucrose, around the point where the crystals begin to melt. Each step of the change removes water from the sugar and rearranges what remains.
Dissolved sugar follows a different path at first. Water in a syrup boils at 100 °C at sea level, and the syrup stays close to that temperature while water is still boiling off. Cooks therefore cook a sugar syrup until most of the water is gone before it can brown.
Once the sugar is hot and dry, its molecules begin to lose water. Each loss of water leaves behind a double bond between carbon atoms, and the molecule rearranges itself around it. Sugar ring structures break open, and the fragments join into larger molecules with alternating single and double bonds.
How does the colour develop?
The colour comes from those double bonds. When carbon atoms alternate single and double bonds along a chain or ring, the molecule absorbs visible light, especially blue light, and the light that remains looks yellow or brown. The longer and more connected these structures become, the darker they look.
Caramelization therefore moves gradually, from clear to pale gold, then amber, then deep brown. The table shows the usual progression.
| Stage | Appearance | Flavour |
|---|---|---|
| Early | Pale gold | Sweet and light |
| Middle | Amber | Richer and slightly nutty |
| Late | Deep brown | Roasted and bitter |
| Too far | Black | Burnt and acrid |
Timing matters because the change speeds up as it goes. A few seconds past deep brown can turn a caramel from pleasant to bitter, which is why cooks watch the colour closely. Adding cream or water stops the reaction, because the liquid cools the sugar and dilutes what has formed.
Different sugars brown at different temperatures. Fructose, the sugar that dominates honey and many fruits, browns at a lower temperature than sucrose. A sugar rich in fructose therefore darkens more readily in a hot pan than white table sugar does.
How is caramelization different from other browning?
The browning of bread crust, seared meat and roasted coffee often involves the Maillard reaction, which combines sugars with amino acids from proteins. Caramelization needs no protein at all. Maillard reactions can begin at lower temperatures than caramelization, which is one reason they dominate in bread and meat.
Many foods brown by both routes at once. Roasted onions and toasted bread contain sugars and proteins, so each adds its own compounds to the flavour. Browning of a cut apple is different again. It is driven by enzymes that react with compounds in the fruit, and it needs no heat at all.
Where do you see caramelization in cooking?
Caramel sauce, the hard glassy top of crème brûlée and toffee are the obvious examples. Recipes usually describe the colour rather than a fixed time, because the same pan and heat can give different results from one batch to the next. Roasted carrots, onions and sweet potatoes also brown partly by caramelization, because their natural sugars become concentrated as water leaves the vegetable. Caramel malts used in brewing are another source of caramelized sugar.
Coffee beans contain sugars that caramelize during roasting, alongside Maillard reactions. The roast level changes both the colour and the taste of the brew, as covered in the science of coffee extraction. Caramelization is also why a microwave oven seldom browns food. Microwaves heat the water inside food, which keeps the surface moist and too cool to brown, as explained in how microwave ovens heat food.
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chemistry cooking food science organic chemistry sugar