The main reason powdered sugar clumps is that it readily absorbs moisture from the air. The surface area between the ground sugar crystals is much larger than that of regular granulated sugar; when this larger surface contacts moisture, the sugar crystals stick together and form small clumps. A small amount of cornstarch is generally added to powdered sugar to delay clumping, but sugar left for a long time in a humid environment after the package is opened may still harden and form clumps.
How do storage conditions make a difference?
Storing powdered sugar in an airtight, tightly sealed container greatly slows clumping. Sugar kept in a steamy cupboard near the stove or dishwasher hardens much faster than sugar in a cool, dry cupboard. A few grains of dry rice or a ready-made desiccant packet placed in the container can absorb excess moisture and help keep the sugar free-flowing for a long time.
Can clumped powdered sugar still be used?
Lightly clumped powdered sugar can be restored by passing it through a fine-mesh sieve; the sieve separates the large pieces while keeping the rest of the sugar smooth. Sugar that has hardened completely into a rock-like mass generally cannot be sifted and should be discarded. To prevent clumping from the start, keep the sugar in a small container that is used often and store the larger package in a cool place where it will be opened less frequently.
Does homemade powdered sugar also clump?
Homemade powdered sugar made with granulated sugar and a blender clumps much faster than store-bought powdered sugar because homemade sugar generally contains less cornstarch. Adding one 5 ml measure of cornstarch for every 200 grams of homemade powdered sugar significantly delays clumping. Storing sugar near steam from a damp dishwasher or a tea or coffee maker is also a common mistake that speeds up clumping; keep the sugar container as far as possible from these sources of moisture. If powdered sugar is prepared immediately before use, clumping is almost never a problem. Rather than preparing a large amount in advance, the most practical solution is to grind granulated sugar in small batches as needed.
Granulated sugar is not a perfect substitute for powdered sugar; its grainy texture does not give the desired result in recipes that need a smooth texture, such as cream or glaze. If you do not have powdered sugar, granulated sugar can be processed in a blender as a temporary substitute. When making powdered sugar at home, it is important for the blender jar to be completely dry; even the smallest trace of moisture can make the sugar clump while grinding. Before sifting powdered sugar for use, collecting the larger pieces left in the sieve in a separate container also prevents waste; these pieces can be stirred into hot liquid to make a syrup-like sweetener. Closing the sugar container immediately after each use also greatly reduces the amount of moisture that gets in from the air.