Sugar crystallization

When sugar syrup is boiled, cooled or concentrated, it becomes supersaturated, holding more sugar than it normally could. Any seed, such as a stray crystal, a speck of dust or vigorous stirring, can start crystals forming, and they can spread quickly through the syrup. Candy makers control this to get the texture they want.

Fudge and fondant need many tiny crystals for a smooth, creamy feel, so they are cooled before beating. Hard candy, caramel and brittle must avoid crystals entirely, so cooks wash down pan sides and add corn syrup, cream of tartar or lemon juice to interfere. Rock candy is the opposite: large crystals are grown on purpose.