Icing sugar vs caster sugar
Icing sugar is sugar ground to a powder with starch added — and the starch is why it cannot simply be swapped for caster by weight.
The short answer
Icing sugar is sucrose milled to a powder, with a few per cent of starch blended in to stop it caking. It dissolves instantly, which is what icings need, and it carries that starch into anything it is used in. Swapping it into a creamed batter removes the abrasive crystals creaming depends on and adds starch nobody accounted for; the cake comes out denser and slightly pasty.
Where they differ
| Dimension | First | Second |
|---|---|---|
| What it is | Powdered sucrose plus an anti-caking starch. | Sucrose crystals. |
| Dissolving | Instant, in cold liquid. | Fast, but it needs heat or time. |
| Creaming | Poor. There are no crystals to tear pockets into the fat. | Good — this is what it is for. |
| Icings and frostings | What they are made with. Anything coarser is gritty. | Unsuitable unless dissolved first. |
| Shortbread and very short biscuits | Sometimes used deliberately, for a smooth close texture. | The default. |
| Added starch | A few per cent, and it does show in a large quantity. | None. |
The substitution arithmetic nobody mentions
They are not equal by volume. Icing sugar is far less dense than caster, so a cup of one and a cup of the other are quite different weights of sugar. Recipes stated in cups are where this goes wrong.
By weight the sweetness is very nearly equivalent, minus the starch. So a weight-for-weight swap in a wet application — an icing, a syrup — is close enough; a volume-for-volume swap is not.
The reverse swap, caster into an uncooked icing, does not work at any ratio. The crystals will not dissolve in cold butter or cold liquid and the result is gritty however long it is beaten.
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