- Add another substance that gives a weak gel, such as xanthan. This seems
to be too expensive. - Add a suitable low-methoxyl pectin, ensure that the pH is not too low, and
add some CaCl 2. This gives a sufficiently stiff, but somewhat brittle, consistency.
17.2.3 Fractal Particle Gels
As discussed in Chapter 13, aggregating small particles form fractal
aggregates or clusters. These become ever more tenuous, and at some stage
the clusters fill the whole system, forming a gel, provided that no
disturbance (agitation, sedimentation) occurs. There is no lower limit to
the concentration, but gels have not been observed for a particle volume
fractionj<0.001; in practicej&¼ 0 :01 is a minimum, since the gel
would otherwise be very weak. The gel network consists of strands of
particles that are linked to other strands, leaving pores of various sizes;
often, the strands have an average thickness corresponding to a few particle
diameters. Here we will further discuss the properties of particle gels,
proceeding on the treatment of fractal aggregation in Section 13.2.3.
The gels obtained are often called ‘‘fractal,’’ but this is to some extent
misleading. Figure 17.15 gives an example of the number of particles and the
fractal dimensionality Das a function of length scale, i.e., the distance from a
central particle. Inside the building block of the gel, i.e., at a scale smaller
thanaeff,D¼3 or nearly so. Also at macroscopic scales, which means
clearly larger thanRg,Dmust equal 3: if we take two cubes of gel with edges
of 1 and 2 cm, the latter cube will, of course, contain 2^3 ¼8 times as much
gelling material (particles) as the former. BetweenaeffandRg¼aeffj1/(D3),
the dimensionality is<3 and hence fractal. This means that the fractal
range will decrease with increasing value ofj. For instance, forD¼2.34,j
¼0.03 givesRg¼ 200 aeff, and forj¼0.3 this is reduced to 6aeff. Because
of the transition zones between the regimes forD, this means that abovej
& 0 :3, the fractal nature of the gel is lost.
Fractal aggregation can occur in many food dispersions; some
examples are discussed in Sections 17.2.4 and 17.3.1. However, the
formation of truly fractal gels is not common. Most casein gels are fractal,
and they have been well studied. Consequently, we will concentrate on these
systems in this section. We will first briefly describe the particles.
Milk contains proteinaceous particles, calledcasein micelles. These are
roughly spherical, volume-surface average diameter about 90 nm; they
contain besides the various caseins (aS1,aS2,b, andk) some undissolved
calcium phosphate (about 8% of the dry matter), and are swollen