production. This concerns especially puff pastry, and also several types of
biscuits (cookies). The gas cells are for the most part due to the expansion of
air and the evaporation of water. They can be seeded by entrapping air
bubbles during kneading, as in bread dough, but fat often plays an essential
role. Use is made of butter or a vegetable shortening (a partly hydrogenated
oil). These fats contain triglyceride crystals, especially at low temperature,
and it appears that tiny air cells adhere to these crystals and are thereby
carried into the dough or paste. The contact angle fat–water–air is quite
large, and, especially if the crystals, or lumps of crystals present, have some
concave surface regions, adhering air cells can be very persistent. Using oil,
rather than a plastic fat in the recipe, does indeed result in a dense product.
14.5 RECAPITULATION
Table 14.1 gives an overview ofphase transitions. The formation of a new
phase generally demands undercooling, overheating, or supersaturation
beyond the equilibrium temperature or the concentration at saturation. A
very small region of the new phase, called an embryo, can always be formed
by chance, but it will generally dissolve. Although the free energy of the
material will be smaller in the new phase, there is also an interface between
the phases, which goes along with an increase in interfacial free energy. Only
if the decrease mentioned exceeds the increase mentioned can an embryo
grow out and form a new phase; this means that it has become a nucleus.
This often needs considerable undercooling, etc. A phase transition is thus
subject to nucleation. Nucleation theory is quite uncertain in a quantitative
sense, but the trends can be well predicted. The factors involved are
discussed in general, but nucleation of crystals has the main emphasis.
The various nucleation mechanisms are indicated in Figure 14.11.
Homogeneous nucleation occurs in the absence of any surface, and it needs
considerable undercooling, mostly by 20–40 K. The smaller the transition
enthalpy (e.g., the heat of fusion), and the higher the interfacial tension
between the phases, the deeper the undercooling needed. Nucleation rate is
very strongly dependent on temperature (or supersaturation). In practice,
nucleation can occur at much smaller supersaturation. Then nucleation
occurs at a surface, be it of foreign particles (called catalytic impurities when
they cause nucleation) or the vessel wall. The efficiency of the impurities
depends (a) on the contact angle of the new phase on the foreign surface
(which depends on the nature of the materials involved) and (b) on the shape
of the surface (a crevice being far more effective than a flat surface). The
number of catalytic impurities as well as the rate of nucleation increase with
increasing supersaturation. Heterogenous nucleation rate is generally much