Answer
The aggregates formed will be most likely of a fractal nature. The main question is
whether sedimentation (creaming) of clusters will occur before gelation. We thus
need to knowRsed[Eq. (13.17)] in relation toRg[Eq. (13.15)]. From Sections 13.2.1
and 2 it will be clear that the aggregation will be perikinetic, and because of the slow
decrease of pH, the stability factorWwill be high. This implies that the fractal
dimensionality will be about 2.3. From Table 6.1 we derive thatDr¼70 kg?m^3.
Equation (13.17) would then reduce to Rsed¼ 72? 10 ^9 =a^0 :^21 in SI units; since
a¼ 5? 10 ^7 , we obtainRsed¼1.5mm, or 3a. Equation (13.15) then readsRg¼a
j1.42¼ 70 a forj¼0.05. In other words, sedimentation will occur well before
gelation. (Even ifjis increased to 0.2, stillRg¼ 10 a; moreover, the value ofacould
be decreased to 0.2mm by strongly increasing the homogenization pressure, leading
toRsed¼1.8mm, which then may be just sufficient.)
The answer would thus be no, but in practice a perfect, though weak, gel is
formed. The following is the most likely explanation. The emulsion droplets have a
specific surface areaA¼ 6 j/d 32 ¼0.3 m^2 /ml. The surface load would be about 3 mg
of casein per m^2 (see Section 11.4.3). This makes up about 1 mg of adsorbed casein
per ml. Total casein concentration was 10 mg/ml, implying that 9 mg per ml is left in
solution. Na-caseinate tends to associate in solution into particles of about 400 kDa.
The number concentration of these particles would then beN¼ð 9 = 400 Þ?NAv& 1022
per m^3. These particles will also aggregate at low pH. The number of emulsion
droplets will be 6j/pd^3 , which equals about 10^17 per m^3. From Eq. (13.3) we see that
the aggregation rate will be proportional to 1/N. This means that the small caseinate
particles will aggregate 10^22 /10^17 ¼ 105 times faster than the emulsion droplets. (The
casein particles are far too small to show significant sedimentation.) What will thus
happen is that a space-filling network (a gel) of casein particles is formed that entraps
and binds to the emulsion droplets before the latter can sediment.
Note A1% Na-caseinate solution does indeed give a gel upon slow
acidification.13.2.4 Aggregation Times
In practice, technologists are often not so much interested in the aggregation
rate as in the time it will take before a perceptible change occurs in the
dispersion. This may be
- The emergence ofvisible particles
- The formation of agel
- Separation intolayers, i.e., a large-scale inhomogeneity.
Figure 13.9 illustrates an important aspect. Ift 25 t 1 , particles will
immediately coalesce upon aggregation. If so, particles will grow in size and