caster sugar [kas-ter, kah-ster shoog-er] noun
Also called superfine sugar or bar sugar. Extra-fine white granulated sugar commonly used in baking (shown far left in image. Center is conventional granulated sugar; right is decorative sugar with larger crystals). The name references the fact that the sugar is fine enough to flow through a sugar “caster” or shaker. This sugar dissolves more quickly than regular sugar and is very useful when making cold beverages and cocktails, hence the alternative name, bar sugar. Caster sugar is also often recommended in recipes such as angel food cake and meringues where it is important for the sugar to dissolve and be evenly dispersed within whipped egg whites.
Caster sugar can be made at home by grinding regular granulated sugar in a food processor fitted with a metal blade; however, if you need superfine sugar for its sparkly qualities, such as when making crystallized flowers or fruit, then this homemade version will not suffice. After being buzzed in the processor, it takes on a powdery quality and loses most of its sparkle.