Why does Edinburgh rock crumble?

Edinburgh rock shares a name with seaside rock, but the two could hardly be more different. Seaside rock is hard and glassy; Edinburgh rock is soft and crumbly, and breaks as you bite. Both are made mostly of sugar. The difference is in how that sugar sets.

Two ways for sugar to set

Food science sorts sugar sweets into two kinds.

• Glassy, or non crystalline. Hard sweets have a disorganised structure with no crystals. They are about 98 per cent sugar or more and set hard. They are sometimes called ungrained.
• Crystalline, or grained. These sweets contain tiny sugar crystals. The crystals give them a texture that is easy to bite or cut into, and a creamy texture that melts in the mouth.

Seaside rock is the first kind. Edinburgh rock is deliberately left to turn sugary, and in sugar work a sugary, grainy texture is the sign of sugar crystals forming.

How seaside rock stays glassy

Seaside rock is made with sugar and glucose syrup, roughly three parts sugar to one part glucose syrup, boiled to about 147°C, the hard crack stage. Rock makers take care over the water, because getting it wrong can leave the rock sugary and graining off, the word the trade uses when sugar starts to crystallise: see why is seaside rock so hard?

How Edinburgh rock turns crumbly

Edinburgh rock is made from sugar, water and cream of tartar, with colourings and flavourings, and no glucose syrup. A traditional recipe gives the method:

1. Dissolve the sugar in water, add a generous pinch of cream of tartar and boil to 126°C, well below the temperature used for seaside rock.
2. Pull the sugar quickly and evenly over an oiled hook until it turns cloudy and dull.
3. Cut it into sticks.
4. Leave the sticks in a warm kitchen for about 24 hours, when the rock becomes sugary and short when broken.

The same recipe warns that the sugar must be pulled enough, or it will stay sticky instead of turning short. That sugary, short texture is what makes Edinburgh rock friable, the word used to describe it: it crumbles rather than cracks.

The legend

There is a story, told as an apocryphal tale, that the rock's inventor, Alexander Ferguson, known as Sweetie Sandy, discovered it by accident. He is said to have found a tray of sweets that had lain forgotten for months, tried a piece of the dried out result, and liked it. Whether or not that is true, it fits the way the rock is made, left to rest until it turns sugary: see Edinburgh rock

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