14.2.3 Other Aspects
The importance of the magnitude of the rate of nucleation is (a) that it
determines the supersaturation needed to realize the desired phase transition
and (b) that it is the main factor determining the size of the regions of the
new phase obtained. The latter is specially important in crystallization.
Crystal Size. The size of crystals may be an important quality
variable in foods. In liquid foods it will often determine whether settling of
crystals occurs, which generally is undesirable. In several liquid and soft
solid foods it is undesirable that crystals be felt in the mouth, a sensation
often described assandiness. To prevent this, crystals generally have to be
smaller than about 10mm (the critical size can also depend on crystal shape).
Examples are ice crystals in ice cream, crystals of amino acid salts in some
cheeses, and lactose crystals in some concentrated milk products. The
rheological properties of a plastic fat depend, among other factors,
markedly on fat crystal size. Moreover, if crystallization is aimed at
harvesting the solid, as in sugar boiling or fat fractionation, it is usually
desirable to obtain large crystals, to ensure their efficient separation from
the mother liquid.
Figure 14.7 gives very roughly the effect of the value of the
supersaturation on the rates of nucleation and crystal growth. The scales
on the graph and the shapes of the curves will greatly vary with the system
considered, but the main point nearly always holds: nucleation rate depends
far more strongly on supersaturation (hence, on temperature) than does
growth rate. Consequently, many crystals are formed when supersaturation
is high (especially if catalytic impurities are absent), and many crystals
implies small crystals. At a low concentration of impurities that are catalytic
at small supersaturation, only a few, hence large, crystals are formed. If the
increase in supersaturation is very fast, which generally means fast cooling,
small crystals will nevertheless result.
In some situationsseed crystals, i.e., crystals of the material to be
formed, are added after a given supersaturation is reached. The seed crystals
then grow, without nucleation being needed. When adding a few crystals at
low supersaturation, large crystals will be formed, albeit slowly. On the
other hand, a large number of tiny seed crystals can be added at high
supersaturation, to ensure rapid formation of small crystals. This method is
used, for instance, if it is difficult to attain conditions at which homogeneous
nucleation occurs. Very small seed crystals are generally obtained by dry
grinding.