Presumably, about the same happens during the formation of most
heat-set protein gels, although often at a faster rate. (In the example given,
the temperature was such that denaturation was quite slow, allowing us to
distinguish the various stages.) However, the rates of each of the processes
will greatly vary, according to the protein(s) involved and depending on
such conditions as protein concentration, pH, ionic strength, and especially
heating temperature or rate of temperature increase.
Fractality. Thepermeabilityof heat-set protein gels ranges for the
most part between 10^15 and 10^12 m^2 ; the main variables are protein
concentration and size of the particles. This allows determination of the
relation between log B and log c, while keeping the physicochemical
conditions constant. If, moreover, the concentration is taken as that on the
onset of gelation, as in Figure 17.19, and a linear relation results, the initial
fractal dimensionality can be established. For systems as given in the figure,
a value ofD& 2 :4 resulted, well in agreement with the values obtained for
the truly fractal acid casein gels (Section 17.2.3). However, the ongoing
aggregation will upset the fractal properties of the final gel.
For soya protein gels made at a low pH (3.8), relations between logB
and logc, as well as between logG^0 and logc(fora¼2, i.e., stretched
strands),Dvalues of 2.3 resulted. In many cases, however, the relations do
not agree well with those for fractal particle gels. In conclusion, fractal
aggregation is generally essential in obtaining a heat-set protein gel, but the
final structure is mostly not fractal any more.
Rheological Properties. Some relations between the dynamic
shear modulus G^0 (at a frequency of about 1 Hz) and protein concentration
care given in Figure 17.13d. At highercvalues,G^0 is often proportional to
c^2 ; assuming the gel to be fractal and thata¼2 [see Eq. (17.18)], this would
implyD¼2.0. This is not realistic, sinceDshould be higher, andaprobably
as well; moreover, these are incompatible. The value ofG^0 also depends on
the time elapsed after incipient gelation (Figure 17.19), pH, ionic strength,
etc.G^0 values are rarely over 100 kPa.
Generally, curves ofG^0 (or its logarithm) versuscare vertical at some
lowcvalue. In other words, there is acritical concentration c 0 , below which
no modulus can be measured. For example,b-lactoglobulin solutions gave
c 0 values ranging from 0.4%(high salt, pH close to pI) to 8%(high salt, pH
far from pI). However, for other heating rates or waiting times, otherc 0
values can be observed. The value that the modulus assumes depends on
many conditions, and for each protein system studied, relations between
parameters likeB,G^0 ,c 0 , and process variables have to be established and if
possible explained.