Powdered sugar is the type of sugar that commonly includes a small amount of cornstarch to prevent lumping while mixing.
Also called confectioners’ sugar or icing sugar, powdered sugar is granulated sugar milled into extremely fine particles. Commercial versions usually contain about 2 to 5 percent of an anti-caking agent, commonly cornstarch. The starch absorbs small amounts of moisture and helps keep the fine sugar particles from sticking together during storage and use.
Powdered sugar dissolves quickly, which makes it especially useful for frostings, icings, glazes, whipped cream, and dusting finished desserts. Its fineness is often described with grades such as XXX, XXXX, or 10X, with higher numbers generally indicating smaller particles.
Powdered sugar is not interchangeable with ordinary granulated sugar in every recipe. The starch can affect texture, and the fine crystals dissolve differently. Home cooks can make a substitute by grinding granulated sugar, although the result may be less smooth and may lack the commercial product’s anti-caking ingredient.