Why does seaside rock go soft?

Fresh seaside rock is hard, glassy and snaps cleanly. Leave it a while, especially once it has been opened, and it turns soft, bendy and sticky. It has not gone off. It has simply been drinking the air.

Rock is almost pure sugar with almost no water

Rock is made from sugar and glucose syrup, roughly three parts to one, boiled with just enough water to dissolve the sugar. The mixture is cooked to about 147 °C, the stage sugar boilers call hard crack. By then almost all the water has boiled away. What is left sets into a hard sweet that is about 98% sugar, with a smooth, glassy structure rather than a grainy one.

Sugar loves water

Sugar is hygroscopic, which means it attracts and holds water from the air around it. Honey and caramel are the same. A stick of rock is a solid block of very dry sugar, so the moment it meets damp air, the surface starts to take in moisture.

A little water goes a long way. As the outside of the stick absorbs moisture it softens and turns sticky, and over time the softening works its way in. That is why rock bought on a dry summer day can feel like a different sweet by the damp end of autumn.

Why the wrapper matters

Rock is sold wrapped in clear paper for a reason. While the wrapper is sealed, very little damp air can reach the sugar. Once it is opened, or if the wrapper is torn, the rock is exposed and softens much faster.

It can go grainy too

Softening is not the only change. The smooth, glassy sugar in rock is not in its most settled state, and over time, helped along by moisture, it can start to crystallise, turning cloudy and grainy. Sugar boilers call this graining off. Chocolate suffers something similar, known as sugar bloom: see why chocolate goes white.

How to keep rock hard

• Keep it sealed in its wrapper until you want to eat it.
• Once opened, put it in an airtight tin or jar.
• Store it somewhere cool and dry, away from steamy kitchens and bathrooms.
• Avoid the fridge: cold sweets taken out into warm air can collect condensation on the surface, and that moisture is exactly what softens sugar.

The same rules apply to other boiled sweets, which go sticky for the same reason: see how long do sweets last?

Is soft rock safe to eat?

Softening is a change in texture, caused by water from the air. Rock is almost entirely sugar, which is why sweets like it keep for a long time, so soft rock is usually still fine to eat unless it looks or smells wrong. It just will never snap the way it did at the seaside. For more on how rock is made, see how seaside rock is made.

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