Icing sugar
Sugar milled to a powder and cut with starch — which is why it dissolves instantly and why it is not a drop-in replacement by weight.
Also called powdered sugar (US); confectioners’ sugar (US); 10x sugar (US)
Icing sugar is sucrose ground to a powder, almost always with a small proportion of starch added to stop it caking. Both facts matter.
The fine particle means it dissolves essentially on contact, which is what makes it right for icings, glazes and anything that must not feel gritty. The starch means it is not pure sugar: substituting it for granulated by weight brings starch into the formulation, and in a batter that starch absorbs water and dulls the result.
It also creams poorly. There are no crystal edges left to cut air into fat, so a cake creamed with icing sugar is denser than the same cake creamed with caster — the sugar is doing the sweetening and none of the aerating.
Composition and specification
The added starch
Icing sugar conventionally contains a small proportion of starch as an anti-caking agent, so it is not chemically equivalent to an equal weight of granulated or caster sugar.
Stated without a passage-verified source. The proportion varies by manufacturer and BakeHQ claims no figure; the presence of starch is stated on the packaging of the products themselves.
RecordedNot yet sourced — a known gap
What changes if it is left out
In an icing, nothing else gives the same smooth texture — granulated stays gritty however long it is beaten. In a batter, its absence is unimportant and usually an improvement.
Follows from particle size and the creaming mechanism.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Moisture retention
Dissolves on contact, which is what makes it right for icings and wrong for creaming — there are no crystal edges left to cut air with.
Covered in
The sugars, and when the difference matters
Powdered, so it dissolves instantly and traps no air.
Icing, frosting, glaze, fondant: what the words mean
Fine enough to dissolve in the small amount of liquid an icing carries, which is the whole reason the grade exists.
Compared in
One side of the comparison.
Granulated sugar vs icing sugar
The same sugar at two crystal sizes, plus the anti-caking starch that decides where the swap fails.
Used in
Its fineness is the whole reason it is specified: sugar suspended in fat cannot dissolve, so the crystal size is the finished texture.
Nearly all of the finished mass. The set is this sugar left behind as the water goes.
Commonly confused with
Swapped for one another by weight constantly. Only one of them can cream, and only the other works in an uncooked icing.
Keep reading
Granulated sugar
Swapped for one another by weight constantly. Only one of them can cream, and only the other works in an uncooked icing.
Every ingredient, by what it does
Grouped by function rather than by shelf, so a substitution question has somewhere to start.