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I put a piece of damp terracotta into my brown sugar to keep it crumbly. Why does the same treatment make icing sugar fuse into a solid brick?

鈥� Brown sugar is a mix of crystalline sugar and up to 10 per cent molasses, a brown syrup obtained during refining. Soft brown sugar is moist owing to the hygroscopic (water-absorbing) properties of the molasses. If this water evaporates, the sugar hardens. To prevent this, you can add damp terracotta, a few marshmallows or a slice of bread to supplement the moisture.

Icing sugar is an entirely different kettle of fish. It is a finely ground sugar that, because of its small particles, is prone to clumping if it gets moist. Such highly refined, granulated sugars have a water content of around 0.05 per cent and contain anti-caking agents such as calcium phosphate to absorb moisture and prevent clumping. The correspondent鈥檚 damp terracotta overcame the anti-caking agent, so the sugar particles clumped, then crystallised together.

Experienced bakers know that success depends on using the right sugar for each job. That鈥檚 the way the cookie crumbles.

David Muir, Edinburgh, UK

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