Caster sugar is a fine grind of sugar which is used by bartenders and bakers, among others, for its quick dissolving properties. It is also sometimes seen spelled as castor sugar, and it may be labeled as “superfine sugar” as well. Much like regular table sugar, caster sugar can come in both refined and unrefined forms. Many grocery suppliers sell caster sugar, which can be replaced in recipes with partly ground conventional sugar if it cannot be obtained.

Sugar itself is an old culinary ingredient. The word comes from the Sanskrit sarkara, which means “pebble,” a reference to the granular nature of raw sugar. Both cane sugar and sugar beets have been historically processed to extract their sucrose. Some cooks have also traditionally used naturally sweet fruits such as mangoes for their fructose, another form of sugar.

Sugar cultivation appears to have begun in India, and it spread to Europe slowly, emerging as a recipe ingredient in the Middle Ages, although it took a long time for sugar to supplant honey as a sweetener. Initially, sugar was quite expensive, and it could only be eaten by members of the upper classes, especially in a heavily refined white form. Sugar also initially came in blocks which needed to be shaved, and the concept of granulated sugar is relatively recent.

The texture of caster sugar lies somewhere between conventional table sugar and confectioner's sugar. The fine grind makes it ideal for many baking projects, since it dissolves easily and creams readily. Meringues especially seem to benefit from the use of caster sugar. When bartenders add sugar to mixed drinks, caster sugar is often used since it is less likely to create a layer of undissolved crystals or thick syrup.

Refined caster sugar is made with sugar cane or beets which have been treated to remove the molasses. The refined form of this sugar is white, sweet, and does not usually discolor with baking, unless it becomes caramelized. Unrefined or golden caster sugar, on the other hand, is made with the molasses left in. As a result, it has a faint creamy to golden color, and it will darken with baking. The unrefined sugar cane also makes this type of caster sugar more complex, flavorwise. Most recipes specify which type should be used.

Powdered sugar should not be used to replace caster sugar, since it often contains cornstarch to keep it from clumping. Cornstarch may adversely impact a recipe, and it is better to use plain table sugar in this situation.