Sugar turns to caramel

Section 1 of 4

Heat sugar in a pan and it melts, turns gold, then brown, and starts to smell of caramel. The heat is running chemical reactions: the sugar molecules break apart and join up into hundreds of new ones.

Table sugar is sucrose, which you have already met: a glucose ring joined to a fructose ring through one oxygen atom. Most caramel recipes start with sugar and a splash of water. In the hot syrup, some of the sucrose splits back into glucose and fructose. Each split takes in a water molecule: it is the condensation that joined the two rings, running backwards.

Sucrose, C₁₂H₂₂O₁₁Table sugar. A glucose ring and a fructose ring joined through one oxygen atom, with eight -OH groups around them.OOOOHOHOHOHOHOHOHOH plus Water, H₂OOne oxygen atom joined by single bonds to two hydrogen atoms, in a bent shape.OHH
make
Glucose, C₆H₁₂O₆A sugar. Five carbon atoms and one oxygen atom make a six-membered ring, and a sixth carbon hangs off the ring. Four ring carbons and the outside carbon each carry an -OH group.OOHOHOHOHOH plus Fructose, C₆H₁₂O₆Fruit sugar, an isomer of glucose. Four carbon atoms and one oxygen atom make a five-membered ring, the form fructose has in sucrose. Two ring carbons each carry a -CH₂OH group, and three -OH groups sit on the ring.OOHOHOHOHOH
Sucrose takes in a water molecule and splits into glucose and fructose: C₁₂H₂₂O₁₁ + H₂O → 2 C₆H₁₂O₆. Glucose and fructose are isomers: the same atoms, joined in a different order. Fructose is drawn as the five-membered ring it has in sucrose.

From syrup to caramel

The water boils off first, and as it goes the syrup gets hotter. At about 150 °C, the stage cooks call hard crack, the syrup is almost pure sugar, still clear, and it sets into glassy boiled sweets. From about 160 °C the sugar molecules themselves start to come apart. Glucose and fructose give off water, crack into smaller pieces, and the pieces join up again in new ways. Many reactions run side by side, and together they make hundreds of different molecules.

Sugar heated in a pan, from syrup to dark caramel A thermometer runs up the left side, with five temperatures marked from the bottom up. At 100 °C the syrup boils and its water steams away. At 150 °C, hard crack, the sugar is clear and glassy. At 160 °C the melted sugar starts to turn gold. At 170 °C it is brown caramel. At 180 °C it is dark caramel, dark and bitter. Beside each temperature, a round swatch shows the colour: clear, clear, pale gold, golden brown and dark brown. 180 °C dark caramel dark and bitter 170 °C caramel brown and rich 160 °C melted sugar starts to turn gold 150 °C hard crack clear, glassy sugar 100 °C syrup boils water steams away
Sugar heated in a pan. The colour comes in quickly between about 160 °C and 180 °C, so cooks watch the pan closely.

Some of the new molecules are small, and they drift up out of the pan as the smell and taste of caramel. Butanedione, C₄H₆O₂, a chain of four carbons with two ketone groups, tastes of butter. Others join into big molecules, built from sugar units that gave off water as they linked. Chemists have named three families of them: caramelans, caramelens and caramelins. A caramelin molecule is huge: C₁₂₅H₁₈₈O₈₀.

Many of these big molecules carry runs of alternating double bonds, the colour-makers of lesson 15. They absorb violet and blue light, so caramel looks golden brown, and the longer the sugar cooks, the darker it gets.

Quick check

Two sucrose molecules, C₁₂H₂₂O₁₁, can join into caramelan, C₂₄H₃₆O₁₈, one of the brown molecules in caramel. How many water molecules are given off?