Nucleation. Nucleation is the first and the most important step
of crystallization. Since the number and extent of nuclei determine
the amount and size of crystals in the finished product, controlling
crystallization starts with controlling nucleation (Hartel 2001).
Crystals can form only from a supersaturated solution, in which
the sweetener concentration exceeds the solubility concentration.
At a molecular level, nucleation occurs when enough sugar
molecules come together in an arrangement that minimizes their
free energy, leading eventually to formation of a crystal lattice.
While in the liquid state, sugar molecules associate with water
molecules through hydrogen bonds, with the number of molecules
of solvation water depending on the chemical structure of the
sugar and concentration of the dissolved sugar. For nucleation to
occur, these water molecules (the hydration layer) must de-solvate
from the sugar molecules to allow sugar-sugar molecular interactions,
which happen with increasing frequency as concentration
increases above the solubility concentration. A stable nucleus is
formed when a cluster of sugar molecules reaches the critical size,
which decreases with increasing supersaturation (Mullin 2001).
The propensity of different sugars to nucleate can be quite different;
for example, mannitol and maltitol nucleate readily compared
to sucrose and xylitol (Bensouissi and others 2010). How easily
nucleation occurs depends on the physicochemical properties of
the sweetener, such as solubility, viscosity (diffusivity), surface tension,
and hydration number (number of water molecules associated
with the sweetener molecule) (Bensouissi and others 2010).