in a more even and compact network. The same mechanism may explain the
steepness of the relations with logj, since for a higher value ofj, the
supersaturation will be less reduced at the gel point.
So far, we have only considered network formation in one system. It
appears likely that for other fat compositions or other process conditions
the same phenomena occur, although additional changes may occur and
quantitative relations will be different. The exponents ofjin the relations
betweenBorG^0 andjare generally about the same, although the absolute
values ofBorG^0 may vary significantly. Some important considerations
are
- Fats of awider compositional rangetend to give more secondary
nucleation and hence smaller crystals; the crystals also tend to be more
anisometric (more slender). Moreover, slow changes can occur into another
polymorph, which generally goes along with a change in compound crystal
composition. Such changes can greatly enhance sintering between crystals.
In systems where only one triglyceride, e.g., tripalmitate, can crystallize,
sintering does not occur, and the modulus of the fat tends to be relatively
small. - Often thesupersaturation is smaller, leading to larger crystals and
a weaker network. Isothermal recrystallization, e.g., into another poly-
morph, generally occurs at very low supersaturation, and it may cause the
formation of larger and more isometric crystals (e.g., spherulites) that make
very weak networks. - It has been implicitly assumed so far that crystallization occurs
isothermallyin a quiescent system. In practice, this is generally impossible.
Either thetemperature rises, due to release of the enthalpy of fusion, which
will decrease the supersaturation; or the system isagitated, which breaks up
networks formed. The latter point will be briefly discussed later on.
17.3.2 Rheological Properties
The effects of some variables on the modulus have already been mentioned,
and to some extent these variables also affect large deformation properties,
although other factors will also play an important role. It must be assumed
that the bonds in the network are due to van der Waals attraction (relatively
weak) and to sintering of touching crystals (often quite strong). Moreover,
crystals can be bent under stress.
Linearity. The strain at which the modulus is no longer
proportional to the applied stress is always quite small for plastic fats.
Defining the linear region as that over which the change in modulus is at