Caster sugar

Sucrose ground to a crystal of about 0.2 to 0.35 mm, against 0.5 to 1.0 mm for granulated. The finer crystal dissolves several times faster, because dissolution rate follows surface area and surface area runs inversely to crystal diameter. In a custard base worked at only 48 degrees C, that is the difference between five minutes and twenty. On a brûlée top the fine crystal melts as one population and forms a continuous sheet. A coarse crystal browns on its surface while its centre is still solid, which gives burnt shells in a bed of grit. Granulated sugar can be used for both jobs. Rub the vanilla seeds into it while it is coarse, because larger crystals grind the seed clumps apart better. Then grind the sugar fine before it is used.