Physical Chemistry of Foods

gas bubble, its shear modulus is simply given by the Laplace pressure

Gp¼pL¼

4 g

d

ð 17 : 19 Þ

For a protein-covered oil droplet of diameterd¼ 1 mm and with interfacial
tensiong¼10 mN?m
^1 , the modulus then is 40 kPa, i.e., quite high. Effects
of droplet size onGpand thereby onGmwill thus be considerable and have
been experimentally confirmed.
The predicted results often do not correspond to the experimental
results, as shown in Figure 17.20a. The results for nonbonded particles are
too high; as mentioned, this may have to do with the particles being
effectively bonded to a limited extent. The two curves for bonded particles
should coincide with the theoretical curve forGp/G 0 ¼90, but they differ
widely. This is discussed below.

4. Intermediate gel layer. Even for adsorbing flexible polymers, a
   kind of depletion layer tends to form around a particle. In this intermediate
   layer the polymer concentration, and thereby the modulus, is decreased, as
   schematically depicted in frame 3 of Figure 17.20. According to a modified
   van der Poel theory, this can explain the curve for PB in Figure 17.20a.


5. Particle shape. Anisometric particles tend to give a higherGm
   value than spherical ones at the same value ofj, although the effect may be
   small. Large deformation properties are greatly affected by particle
   anisometry, but published results appear to be conflicting.


6. Aggregation. Gels made of casein or a heat-setting protein
   generally show a very strong increase in modulus with filler particle
   concentration, as illustrated by curve CB in Figure 17.20. Microscopical
   observation then shows the particles to be aggregated; this would materially
   increase the effective particle volume fraction and also increase anisometry;
   see frame 4 of Figure 17.20. Whether these effects can fully explain the large
   discrepancy between theory and results, remains to be established.
   Large deformation propertiesof gels with various filler particles have
   also been studied, but the results vary widely and explanations are largely
   lacking.

Question

A food technologist wants to make heat-set protein gels filled with emulsion droplets,
using as little protein as possible and a high oil concentration. Solutions are made of
1.5 and 3%protein, which give gels on heating with shear moduliGof about 0.1 and
1 kPa, respectively. In each solution oil is emulsified at a volume fractionjof 0.4,