Granulated sugar vs icing sugar
The same sugar at two crystal sizes, plus an anti-caking starch that is invisible until it is the reason something tastes chalky.
The short answer
Chemically these are the same substance. What differs is crystal size and one added ingredient, and both matter more than they sound. Icing sugar is milled to a powder and carries a few per cent of starch to stop it caking — which is why it dissolves instantly, why it is the only sugar that works in an uncooked icing, and why it tastes faintly chalky in anything where it is not dissolved. Granulated sugar's crystals are what cut air into butter during creaming, so replacing it with icing sugar in a cake gives a denser, finer, slightly less risen result. Neither is a general substitute for the other, and the weight-for-weight swap is safe only in things that are cooked or fully dissolved.
Where they differ
| Dimension | First | Second |
|---|---|---|
| Crystal size | Coarse, and abrasive enough to cut air into fat. | Milled to a powder. |
| Added starch | None. | A few per cent, to prevent caking. |
| Creaming | The whole mechanism — the crystals do the aerating. | Poor. Too fine to cut air in. |
| Uncooked icing | Stays gritty; there is no water to dissolve it. | The only one that works. |
| Volume for the same weight | Denser in the cup. | Much lighter — a cup weighs considerably less. |
| Flavour | Clean. | Faintly chalky where the starch is not cooked or dissolved. |
The starch is why it is not simply finer sugar
Almost every icing sugar sold contains a small proportion of starch — maize, wheat or rice — added because pure powdered sucrose absorbs moisture from the air and sets solid in the packet.
It is invisible in an icing, where it is a tiny fraction of a mixture that is mostly sugar and is going to be eaten cold. It becomes noticeable in two places: dusted over something and eaten dry, where a careful palate finds it chalky, and used in quantity in a caramel or a syrup, where it clouds what should be clear and can catch.
This is the reason a recipe that specifies granulated for a syrup means it, and the reason substituting icing sugar there is one of the few sugar swaps that visibly fails.
Related
Compares
The same sugar at two crystal sizes, plus the anti-caking starch that decides where the swap fails.
The same sugar at two crystal sizes, plus the anti-caking starch that decides where the swap fails.