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Caster sugar, also commonly called castor or superfine sugar, is basically granulated sugar that has been ground into much finer particles. The smaller grind and tinier granules allow it to dissolve faster and easier than the larger particles of granulated sugar. It commonly is used for its ability to dissolve quickly and for its texture in baked goods. The best caster sugar will have a fine, fairly even texture without clumps or chunks of larger granules. It also may be important to determine whether the sugar was made from sugar cane or sugar beets, as these can react differently in certain dishes.
Sugar quality can affect the outcome of a recipe. Sugar will typically sweeten food the same way regardless of brand, but the texture of the sugar will affect how it reacts in relation to other ingredients. Caster sugar is typically called for in recipes that need a quick dissolving sugar that will not feel grainy in a syrup or a liquid. Granulated sugar takes longer to dissolve and may leave a texture behind, while much finer caster sugar usually does not.
It may not be necessary to purchase special caster sugar if it is something you rarely use. Ordinary granulated sugar can be finely ground in a food processor to make caster sugar at home. The most important feature of this type of sugar is the size of the granules, so grinding regular sugar often works well.
The source of the sugar also is important — sugar comes from either sugar cane or sugar beets. These sugars are almost identical, but there is a small chemical difference that affects how they respond to such things as moisture and heat. When quick dissolving sugar is the priority, the source may not be as important. In baked goods, however, the texture and the browning properties can be affected by the choice. Many cooks believe that caster sugar from sugar cane works best in baked goods and creates the most pleasing texture.
The sugar cost is usually higher for brands made from sugar cane. Not all sugars will state on their labels whether they came from cane or beets, though most generic brands that cost far less can generally be assumed to come from sugar beets. If caster sugar is not available and you do not have the equipment to finely grind regular sugar, plain granulated sugar can be substituted in most baked goods with only a slight change in texture. Recipes that call for dissolving the sugar may not turn out as well, however, if granulated sugar is substituted.