This concerns especially butter, margarine, shortening, and cocoa
butter. The key rheological parameter is mostly the yield stress.
Oiling offis generally undesirable; it can occur if the permeability of
the fat is high and its yield stress is low (see the question at the end
of Section 5.3.1).
The crystal network canimmobilizeother structural elements in the
system. A good example is given by margarine, where the network
prevents coalescence of the aqueous droplets.17.3.1 Network Formation
When making a plastic fat, the triglyceride mixture is brought to a
temperature where it is fully liquid, and it is then cooled to a temperature
where at least part of the triglycerides are greatly supersaturated. Then
nuclei form, crystal growth occurs, and crystals start to aggregate, in that
order. After a little while, however, all three processes (and some others, see
below) happen simultaneously. This will be illustrated by an example,
shown in Figure 17.21.
The example concerns a fairly simple system, 12%fully hydrogenated
palm oil in sunflower oil. As seen in Figure 15.22, this mixture can be cooled
to such a temperature that it is greatly supersaturated in theb^0 -form, but not
in thea-form. In this mannerb^0 -crystals are formed that do not (or do only
very sluggishly) transform intob-crystals. Results are given for two values
of the initial supersaturation. By and large, the various events occur very
soon after cooling. Crystal growth starts almost immediately. A soon as the
nominal crystal radius is about 25 nm, the van der Waals attraction is
sufficiently large to induce aggregation, since there is no repulsion (except
hard core repulsion): see Question 2 in Section 12.2.1. Fractal aggregates are
formed, the dimensionality being about 1.7. This low value implies that
virtually no rearrangement occurs in the aggregates (see Section 13.2.3).
From Eqs. (13.15) and (13.20), the time needed for gel formation is
calculated at 60 and 250 s for lnb 0 values of 3.5 and 2.75, respectively, in
good agreement with the observed times needed for a measurable modulus
to develop.
Conditions change during the process, because lnb decreases with
increasing fraction solid. The initial supersaturation of 3.5 is after 200 s
already decreased to a value of 2.1 (or in terms of the supersaturation ratio
from 33 to 8.2). This has a large effect on growth rate and especially on
nucleation rate.
The initial value of the supersaturation has a large effect; see especially
the development of the modulus with time. It is also seen that for the higher