Caster sugar is fully refined white sugar milled to a very fine, even grain — finer than ordinary granulated sugar but still free-flowing and grainy rather than powdery. Its small crystals dissolve fast and cleanly, which is why it is the standard baking sugar for meringues, custards, sponge cakes and delicate creamed batters. It tastes of clean, neutral sweetness with no molasses colour or flavour at all.