Caster sugar vs granulated sugar
The same sugar at two crystal sizes, and the size decides whether it dissolves before the structure sets.
The short answer
Chemically identical. Caster is milled to a smaller crystal, which dissolves faster and creams more effectively. In anything relying on the sugar dissolving into a mixture that sets quickly — a creamed sponge, a meringue, a delicate biscuit — that is a real difference. In anything wet or long-cooked, the two are interchangeable.
Where they differ
| Dimension | First | Second |
|---|---|---|
| Composition | Sucrose. | Sucrose. |
| Crystal size | Fine. | Coarse. |
| Creaming | Better. Smaller crystals tear more, smaller air pockets into the fat. | Poorer, and the batter can stay gritty. |
| Meringue | Dissolves into the foam. Essential. | Frequently fails to dissolve, which is the commonest cause of a weeping meringue. |
| Wet or cooked applications | Interchangeable. | Interchangeable. |
Differences that do not matter
These are real distinctions that are unlikely to change your decision. They are listed so you can stop thinking about them rather than left out so the comparison looks decisive.
- Sweetness. Identical. Identical.
When the substitution is genuinely fine and when it is not
Fine: anything with enough water and enough time for the crystals to dissolve — a custard, a syrup, a fruit filling, a wet batter that bakes for an hour.
Not fine: a meringue, where undissolved sugar draws water out of the foam and appears as syrupy beads; a creamed sponge, where the crystals are the tool that makes the air pockets; and a shortbread or biscuit judged on smoothness.
The workaround for granulated in a creamed batter is to cream for longer and accept a slightly coarser crumb. The workaround in a meringue is to grind it finer first, which works and is more trouble than buying caster.
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