On Food and Cooking

Particles, Temperature, and Stirring
Influence Crystallization The crystal “seed”
is an initial surface to which sugar molecules
can attach themselves and accumulate in a
solid mass. The seed can be a few sugar
molecules that happen to come together
during random movements in the syrup.
Stirring and agitation have the effect of
bumping solution molecules together more
often than they otherwise would, and thereby
encourage the formation of crystal seeds.
Other things can also serve as seeds in a
cooling syrup and initiate crystallization.
Among the more common are the tiny crystals
that form when the syrup spatters on the side
of the pan or dries off on a spoon, and that
then are stirred back into the syrup. Dust
particles and even tiny air bubbles can also act
as crystal seeds. A metal spoon can induce
crystallization by conducting heat away from
local areas of the syrup, cooling them and so
leaving them super-supersaturated.