Physical Chemistry of Foods

Many proteins form a coarse precipitate rather than a gel if the pH is
very close to the isoelectric point, combined with a high ionic strength.
Under these conditions the net charge on the protein is quite small and the
thickness of the electric double layer is also small, allowing rapid
aggregation. Presumably, some electrostatic repulsion between the (dena-
tured) protein molecules is needed for a gel, rather than a precipitate, to be
formed. This may in part explain the slowness of gel formation.
The following factors affect the coarseness of the gel:
Type of protein. For example, fine-stranded gels are far more readily
formed from myosin (an elongated molecule) ora-lactalbumin (a
small molecule that does not form 22 S 22 S 22 bridges) than fromb-
lactoglobulin or soya protein.
The effects ofpH and ionic strengthhave already been discussed.
Divalent cations, especially Ca^2 þ, are very effective in causing coarse
gels to be formed (at a pH above pI), because they strongly promote
protein aggregation. This may be due to the strong binding of Ca
ions to several proteins, thereby lowering electric charge, or to the
formation of intermolecular Ca bridges.
For a higherprotein concentration, the particles making up the gel
often are larger.
Therate of heatingof the protein solution can have a large effect. For
example, heating a 10%solution ofb-lactoglobulin at pH 5.3 at a
rate of 5 K?min
^1 or faster gave particles of about 3mm, whereas a
rate of 0.1 K?min
^1 resulted in particles of about 15mm diameter.
Themechanismby which the relatively large particles are formed is still
a matter of debate. It has been proposed that they are the result of phase
separation, presumably between unfolded and native molecules, which leads
to small volume elements of a high concentration of denatured molecules,
which subsequently form roughly spherical gel particles. Anyway, it has
become clear that the particles themselves are not quite rigid and do not
(always) consist of closely packed protein molecules. However, other
mechanisms cannot be ruled out and the question is far from settled. Nor is
it clear why the strands in clear gels can be relatively long and of almost
constant diameter.
Gel formationinvolves a number of consecutive reactions: (1) protein
molecules become denatured; (2) denatured molecules aggregate to form
(roughly spherical, or elongated) particles; and (3) these particles then
aggregate further to form a space-filling network. After a little while, all of
these reactions proceed at the same time, unless the temperature is much
higher than the denaturation temperature of the protein, when denaturation
can be a very fast reaction.