may even within a minute be transformed intob^0. Theb^0 -form generally has
a longerlifetime. In multicomponent fats,acrystals tend to live much
longer, and it is no exception that (part of the)b^0 crystals remain almost
indefinitely. Again, the greater supersaturation for compound crystals
causes the driving force for transition to be small or even negative.
Moreover, the transitionsa!b^0 !b must go along with a change in
crystal composition, since compoundb^0 crystals can host fewer different
molecules thana, and compoundbcrystals hardly exist. This also implies
that the polymorphic transitions are not true solid-state transitions: they
have to proceed via the liquid state. This will also hinder changes in
polymorphic form, especially if very little liquid phase is left (i.e., at low
temperature).
Solid Solutions. It is now well established that in a
multicomponent fat true solid solutions involving many different
triglycerides can be present. The phase diagram would be comparable to
that depicted in Figure 15.23b. The number of componentsnwill, however,
be (much) larger than two, and the phase diagram would thus ben-
dimensional. This does not necessarily imply that it concerns one continuous
series of compound crystals over the whole temperature range. Crystals
containing different groups of triglyceride may be present at the same time
or at various temperature ranges. For instance, there may be a group of
saturated triglycerides of relatively constant chain length formingb^0 -L2
compound crystals; another group of triglycerides incorporating one short
chain per molecule may formb^0 -L3 crystals (like SCS in Figure 15.19); a
third group may incorporate one oleyl chain per molecule and form otherb^0 -
L3 crystals (like SOS in the figure); finally, some other molecules may form
compounda-crystals. Moreover a solid solution will not always be formed
over the full compositional range possible.
The presence of solid solutions has some important implications,
as will be illustrated for a highly simplified example of two components,
depicted in Figure 15.24. Assume that we have a liquid fat of compositiona 3
at temperatureT 1. Cooling it to T 2 , crystals will form (provided that
nucleation occurs). Now a liquid phase of compositiona 2 will result and a
solid phase (crystals)a 5. The molar ratio of solid to liquid will be given by
the ratio (a 3 a 2 )/(a 5 a 3 ). Assume now that the mixture is cooled further
toT 3. The crystals of compositiona 5 remain, and the liquida 2 will separate
into a liquid and crystals of compositiona 1 anda 5 , respectively. Note that
the composition of the system differs from that resulting from cooling
directly toT 3. In other words, there is no equilibrium (i.e., within the
polymorphic form, probablyb^0 ). There is, however, a ‘‘surface equilibrium’’:
the crystalsa 4 will probably be formed at and around the existing crystals