applied pressure will, of course, speed up the process. Sincepsynis quite small—of the
order of 1 Pa—external pressures can have a very strong effect. Finally, a look at
Figure 17.16b will show that one should not wait too long before starting syneresis
(i.e., before cutting the gel into pieces), since thenpsynrapidly decreases.17.2.4 Heat-Set Protein Gels
When a protein solution is heated, a gel may be formed. This occurs with
well-solubleglobular proteins, if the protein concentration is above a critical
valuec 0. The gel is only formed after at least part of the protein has been
heat denatured—e.g., as inferred from a change in the DSC curve—and the
gel formation is irreversible upon cooling; see Figure 17.14d. Gel formation
is a relatively slow process, taking at least several minutes and possibly
hours.
These observations are almost generally valid, but further details
about formation and properties of the heat-set gels are widely variable
among proteins. This is because globular proteins vary greatly in molecular
structure and conformational stability, as was discussed in Chapter 7;
especially Section 7.2 gives useful information in relation to denaturation.
The situation is even more complex because heat-set gels are generally made
of protein mixtures, such as isolates of whey proteins or soya proteins,
where the various species react in a different way during heat treatment and
often mutually react as well. The bonds keeping the denatured protein
molecules together in the gel network are of various natures. Presumably,
22 S 22 S 22 bridges and hydrophobic interactions often play major roles.
Moreover, H-bonds can be involved in intermolecular junctions between
b-strands.
Gel Structure. The gels come in two main types, although the
difference is rather gradual.- Fine-stranded. These are clear gels, consisting of relatively fine
strands, that are branched to form a network. Strand thickness generally is
between 20 and 50 nm, i.e., clearly larger than molecular size (4–8 nm).
These gels typically form at a pH far away from the isoelectric point (pI) at
low ionic strength. - Particle. These are turbid gels, with a coarse microstructure. The
building blocks are roughly spherical particles, diameter generally between
0.1 and 4mm; the larger ones would contain well over a million protein
molecules. These gels typically form at a pH close to the isoelectric point at
high ionic strength.