Physical Properties of Fats and Oils

145

is thermodynamically the most stable form, nucleation and crystal growth rates are
higher for the 

β′

 polymorph, and especially for the 

α

 polymorph [8]. Figure 6.7

shows that the activation free energy of nucleation (

∆

G

#

) for the 

α

 polymorph is

lower than for the more stable polymorphic forms.

Polymorphic stability depends on van der Waals interactions and packing

density [1]. When multicomponent fats do transform, they exhibit monotropic rather
than enantiotropic polymorphism. Transformations proceed only from the less to the
more thermodynamically stable forms [47]. To return to a less stable polymorph, the
crystal structure must be completely destroyed through the melt as shown in Figure 6.6.
TAG polymorphic transitions to more stable polymorphs can occur through either
the melt or the solid state [69]. In monoacid short-chain TAGs, the 

α

 form can

transform to the 

β′

 form during heating either via melting and then recrystallization or

directly without being melted. The direct transformation is preferred in short-chain
TAGs (C12) while it occurs via melting for longer chain TAGs (C18) because of the
relatively high stability of the 

α

 form [1]. At least some order remains during transitions,

as evidenced by persistent short spacings in continuous XRD measurements [50].

FIGURE 6.6 Dynamics of polymorphic transformations in fats.

FIGURE 6.7 Free energies of nucleation for the 

α

, 

β

, and 

β

′

 polymorphs in fats.

α

β′

β

Melt

α

β′

β

∆G

n

∆G

#

n

Melt

2162_C006.fm  Page 145  Monday, June 20, 2005  12:29 PM

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