Castor (or caster) Sugar is fine powder sugar, so named because the grains are small enough to fit though a sugar "caster" or sprinkler. It is slightly coarser than icing sugar and at least 950 g/kg of Castor Sugar shall pass through a sieve having a mesh aperture size of 0.599 mm. It is called "superfine sugar" in U.S.A. & "berry sugar" in British Columbia. The fine sugar crystals of Castor Sugar incorporate more air than other types of sugar.
When used in baking, caster sugar gives more volume to the mixture, resulting in extra-light cakes. Castor Sugar is also perfect for light and crisp, melt-in-mouth puddings and for sprinkling over cereal, doughnuts and fresh fruit. Because of its fineness, it dissolves more quickly than regular white sugar, and so is especially useful in meringues and cold liquids.