using a homogenizer at 25 MPa. Earlier experiments gave at this homogenizing
pressure droplets ofd 32 & 0 : 6 mm, which would make droplets of a quite high shear
modulus. From a graph like that in Figure 17.20a, it is expected that the gels formed
from these solutions will have a modulus of at least four times that of the solutions
without oil, and we hope much higher, since some aggregation of the droplets would
occur. However, the emulsion made with the 1.5%protein solution did not give a gel
at all; the 3%one gave a gel, but the modulus was only 1 kPa. What can have been
wrong in the reasoning?
Answer
(a) Because protein is needed to cover the emulsion droplets made, there will be less
left in solution. The amount needed to cover the droplets would be 6Gj=d 32 , and
assumingG¼3mg?m^2 , this would amount to 12 mg per ml emulsion, whereas the
‘‘1.5%protein’’ emulsion would only contain 15(1j)¼9 mg per ml. In other
words, there is not enough protein present. This means that the drops obtained will
be larger and that the aqueous phase will contain very little protein, insufficient to
make a gel. On heating, the protein-covered droplets will aggregate, and the
aggregates will cream, thereby disturbing any very weak gel that may have formed.
For 3%protein, about 30 12 ¼18 mg of protein per ml is left in solution, and such a
solution would make a gel, presumably of a modulus of about 0.2 kPa. The emulsion
gel hadG&1 kPa, i.e., about 5 times as large. (b) The ratio ofGp/G 0 will depend on
the droplet size and on the value ofG 0 , both of which were not constant. Since the
ratio will be quite high in all cases, variation would not greatly affect the results, as
suggested by Figure 17.20a. (c) The size and the number of any aggregates of
droplets in the gel may well depend on variables like protein content and droplet size.
17.3 PLASTIC FATS
A plastic fat consists of a space-filling network of triglyceride crystals, where
the continuous phase is triglyceride oil. At room temperature, the volume
fraction of the crystals is generally below 0.5, and a plastic fat thus is a gel
according to the definition given at the beginning of this chapter. However,
the properties of a plastic fat are rather different from those of most gels,
and the formation is a complex process; this is the reason why it is given a
separate treatment in this chapter. The treatment proceeds from Chapter 15,
and knowledge of the matter in Section 15.4 is especially needed to
understand what follows.
Properties of plastic fats are very important in some foods:
The consistency of the fat determines its shape retention during
storage, plastic deformability during handling (e.g., when spreading
on a slice of bread, or mixing it into a dough) and during eating.