Caster sugar is the British name for cane or beet sugar that’s been milled more finely than granulated sugar, but not quite as fine as powdered sugar. Known as superfine sugar in the United States, caster sugar dissolves quickly and is often used in pudding, custard, mousse, meringue and other smooth-textured desserts.
Caster sugar is nutritionally comparable to traditional granulated sugar -- both provide about 16 calories and 4 grams of carbohydrates per teaspoon. Like other highly refined sugar products, caster sugar is not a source of fat, protein or dietary fiber. It doesn’t contain measurable amounts of vitamins or minerals, either.
Caster sugar and granulated sugar are virtually indistinguishable from a nutritional standpoint, and both are considered equally unhealthy when consumed in excess. The primary benefit of caster sugar, therefore, is strictly a culinary one -- its smaller crystals blend more easily and caramelize more evenly.