Gel

A gel consists of polymers linked through covalent and/or non-covalent
interactions, to create a three-dimensional network. In whole egg as well as in
white and yolk, proteins are responsible for the gelling properties. Gelation
occurs when the protein stability in solution is modified, i.e., when the
equilibrium between attractive (Van der Waals) and repulsive (electrostatic,
steric) interactions is disrupted. The electrostatic repulsions vary with the net
charge of the proteins, that means with the ionisable protein groups and with the
physico-chemical characteristics of the solvent (pH, ionic strength). The
treatments that decrease the repulsive interactions, such as adjustment of pH
at proteins pI or addition of salts, induce destabilisation and thus can result in the
formation of aggregates or gels.
   Moreover, some treatments can modify the protein structure, with
consequences for the repulsive and attractive interactions mentioned above.
This is especially the case during heat treatments, which are the major techno-
logical treatments used in the food industry for egg white and yolk gelation.
Heat-induced gelation of egg conforms completely with the model of heat
gelation of globular proteins. It is a two-step phenomenon: in the first stage,
unfolding of native proteins occurs, disrupting the well-defined secondary and
tertiary structures and producing denatured proteins exposing their inner
hydrophobic regions; following unfolding, the denatured proteins interact to
form high molecular weight aggregates that can further interact with each other
to result in a three-dimensional gel . The unfolding and
aggregation steps depend on many factors (protein concentration, ionic strength,
pH, presence of sucrose, etc.) that can modify the number and/or the kind of
interactions, with final consequences on the gel rheology. In the heat-induced
gels of egg proteins, the interactions involved are predominantly hydrophobic
and electrostatic, but some highly energetic interactions can be observed
(disulfide bridges); thiol and amine groups are indeed very reactive, especially
in alkaline conditions.