Caster sugar
White sugar milled fine — the same chemistry as granulated, and a different result wherever dissolving speed matters.
Also called superfine sugar (US); baker’s sugar (US); berry sugar (AU)
Caster sugar is granulated sugar with a smaller crystal. Chemically the two are the same compound; what differs is surface area, and therefore how quickly the crystals dissolve.
That sounds trivial and is not. In creaming, the crystal edges are what cut air pockets into fat — finer crystals cut more, smaller cells, which is why caster gives a lighter, more even cake crumb. In a meringue, undissolved sugar weeps; caster dissolves into the foam readily and granulated often does not. And in a cookie, faster-dissolving sugar joins the liquid phase sooner, lowers viscosity and increases spread.
So the substitution is safe by weight and not by result. Granulated for caster gives a coarser crumb, a grittier meringue and slightly less spread.
What changes if it is left out
Substituting granulated by weight works chemically and changes texture: a coarser cake crumb, a greater risk of a weeping meringue, and marginally less cookie spread.
Follows from crystal size and dissolving rate. The direction is professional consensus; no quantity is claimed.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Aeration
Crystal edges cut air pockets into fat during creaming. Finer crystals cut more of them, which is why caster gives a lighter, more even crumb than granulated.
Can stand in for
Chemically identical, physically finer. Safe by weight; expect a lighter crumb, a more stable meringue and slightly more cookie spread.
Variables this controls
Sugar quantity and type
Crystal size is part of what the sugar-quantity variable actually controls — finer sugar dissolves faster, lowers viscosity sooner and increases spread at the same weight.
Covered in
The sugars, and when the difference matters
The crystal-size axis.
What happens when you reduce the sugar
Crystal size decides the aeration and spread functions specifically.
Used in
The abrasive that makes creaming work, as well as the sweetener.
Standard for a creamed batter.
Sweetness and tenderness.
Dissolves readily into the foam, which coarser sugar does not.
Stabilises the foam and must dissolve into it.
In a sweet paste, for tenderness as well as taste.
Sweetness and structure.
Caster rather than granulated because it dissolves into the whites far more readily, and undissolved sugar is what weeps.
Compared in
Caster sugar vs granulated sugar
One side of the comparison.
One side of the comparison.
Keep reading
Every ingredient, by what it does
Grouped by function rather than by shelf, so a substitution question has somewhere to start.