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HONEYCOMB, or cinder toffee, is simple to make, but is a great example of the complex science involved in transforming sugar into confectionery.

The process begins by heating sugar and water. While pure water boils at 100°C (212°F), the boiling point of a sugar solution is higher. As the solution boils, water evaporates, but the sugar remains, increasing the concentration and raising the boiling point further. At 170°C (338°F), the sugar starts to caramelise: the molecules break apart and recombine, turning it brown and producing delicious flavour molecules.

By measuring the temperature of a boiling sugar solution, you can tell how much water is left, indicating what texture the sugar will have when it cools. For a hard toffee, like honeycomb, you want to get to 150°C (300°F). Without a thermometer, you can keep track by dropping a small amount of hot sugar solution into cold water. At 110°C (230°F), it will form soft threads; at 125°C (260°F), a hard ball. At 150°C, it forms hard threads that crack easily. The boiling point rises more rapidly as the sugar gets more concentrated, so watch carefully to avoid burning it.

With less water than when it started, the hot sugar solution is supersaturated, containing more sugar now than can remain dissolved within it. As it cools, the sugar molecules will tend to bond together into orderly crystal structures. To make hard and brittle confectionery, you want to avoid this, as it makes the texture grainy. One way is to cool the syrup quickly so the sugar molecules can’t arrange themselves into crystals – instead, they form a disorganised mass with a hard, smooth texture like glass.

Other ingredients can prevent crystallisation. Table sugar, also known as sucrose, is made of glucose and fructose bonded together. If a sucrose solution also contains unbonded glucose and fructose – called invert sugars – they will get in the way of sucrose molecules and stop crystals from growing. Honey and golden syrup are sources of invert sugars. Also, acid, such as lemon juice, breaks down sucrose into glucose and fructose, so adding it is another way to supply invert sugars.

To make honeycomb, line a baking tin with greaseproof paper. Place sugar, honey and water in a saucepan over medium-high heat and stir with a wooden spoon. When it reaches 150°C, add and mix in the bicarbonate of soda.

The heat causes the sodium bicarbonate to break down to produce sodium carbonate and carbon dioxide. The bubbles of CO₂ get trapped in the candy as it cools, creating a light texture. Sodium bicarbonate is also alkaline, which lets caramelisation occur at a lower temperature.

Pour the syrup into the tin and leave it to cool before breaking it into chunks. Store in an airtight container, as the sugar may absorb moisture from the air.

Sam Wong is assistant news editor and self-appointed chief gourmand at New Scientist. Follow him @samwong1

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