Caster sugar is the British term for a fine sugar. It's called caster sugar because the sugar granules are tiny enough to be sprinkled through a condiment dispenser known as a "caster."
In the United States, this sugar is called superfine.
Because of its ability to dissolve well, both chefs and bartenders use the fine sugar.
Caster sugar has such a fine grain that it dissolves almost instantly. Its texture is slightly thicker than confectioner's sugar, yet finer than that of table sugar.
Caster sugar is made by pulverizing granulated sugar into finer grains (but not as fine as confectioner's sugar).
Caster sugar can be purchased in its refined and unrefined forms.
When to Use It
Caster sugar quickly liquefies when added to beverages. This is particularly helpful when mixing sugar into cold drinks such as iced tea, in which mixing sugars of heavier granules is more difficult.
Since caster sugar is so light, it is ideal for making fluffy, whipped desserts such as meringues, mousses and soufflés as well as sorbets, custards, cakes, cookies and for sprinkling on fruit.
If a mixed drink recipe calls for sugar to be added, bartenders often use caster sugar because it won't ruin the drink by leaving a syrupy layer behind.
Where to Find It
Authentic caster sugar can be purchased in specialty food stores and online.
In the United States, superfine sugar is comparable to the British caster sugar, and they are interchangeable.
If you do not have superfine or caster sugar on hand for a recipe, you can blend your own by processing regular granulated sugar in a food processor for about a minute, or until it is powdery.
When a recipe calls for caster sugar in a baked good, such as a cake, granulated sugar can be used as a substitute.
Caster or Castor
Caster sugar is also sometimes called "castor" sugar.
Which is the correct spelling? According to AskOxford, "Both are right. The spelling castor sugar used to be the prevailing one, but caster sugar seems to be more usual now, perhaps because it is used by some sugar manufacturers on their packaging."
Do not use regular granulated or powdered (confectioner's) sugar to replace caster sugar. Granulated sugar is too thick and will change the consistency of a mixture.
Powdered sugar contains small amounts of cornstarch, which is added to keep it from clumping. The added cornstarch could potentially alter a recipe