This is the British term for sugar with small grains that are between granulated and icing sugar in terms of fineness. It is sometimes spelled castor sugar, and is known as ‘superfine’ sugar in America. Caster sugar is obtained from sugar cane or sugar beet, and is valued for its quick-dissolving properties. The usual refined white variety of caster sugar will have been treated to remove molasses, and so will be free-flowing.