2009_Book_FoodChemistry

1.4 Proteins 63
carrageenan (4.4.4.3.2). Formation of a threedimensional
network takes place through the
aggregation of unordered fibrous molecules
via partly ordered structures, e. g. while double
helices are formed (cf. 4.4.4.3.2, Fig. 4.14,
Fig. 12.21). Characteristic for gels of this type is
the low polymer concentration (∼1%) as well as
transparency and fine texture. Gel formation is
caused by setting a certain pH, by adding certain
ions, or by heating/cooling. Since aggregation
takes place mostly via intermolecular hydrogen
bonds which easily break when heated, polymeric
networks are thermo-reversible, i. e. the
gels are formed when a solution cools, and they
melt again when it is heated.
Examples of aggregated dispersions are the
gels formed by globular proteins after heating
and denaturation. The thermal unfolding of the
protein leads to the release of amino acid side
chains which may enter into intermolecular
interactions. The subsequent association occurs
while small spherical aggregates form which
combine into linear strands whose interaction
establishes the gel network. Before gel can be
formed in the unordered type of aggregation,
a relatively high protein concentration (5–10%)
is necessary. The aggregation rate should also be
slower than the unfolding rate, since otherwise
coarse and fairly unstructured gels are formed,
such as in the area of the iso-electric point.
The degree of denaturation necessary to start
aggregation seems to depend on the protein.
Since partial denaturation releases primarily
hydrophobic groups, intermolecular hydrophobic
bonds generally predominate, which results in
the thermoplastic (thermo-irreversible) character
of this gel type, in contrast to the thermoreversible
gel type stabilized by hydrogen bonds.
Thermoplastic gels do not liquefy when heated,
but they can soften or shrink. In addition to
hydrophobic bonds, disulfide bonds formed
from released thiol groups can also contribute to
cross-linkage, as can intermolecular ionic bonds
between proteins with different isoelectric points
in heterogeneous systems (e. g. egg white).
Gel formation can be improved by adding salt.
The moderate increase in ionic strength increases
interaction between charged macro-molecules or
molecule aggregates through charge shielding
without precipitation occurring. An example
is the heat coagulation of soybean curd (tofu,
cf. 16.3.1.2.3) which is promoted by calcium
ions.
1.4.3.6 Emulsifying Effect
Emulsions are disperse systems of one or
more immiscible liquids. They are stabilized
by emulsifiers – compounds which form
interface films and thus prevent the disperse
phases from flowing together (cf. 8.15).
Due to their amphipathic nature, proteins
can stabilize o/w emulsions such as milk
(cf. 10.1.2.3). This property is made use of
on a large scale in the production of food
preparations.
The adsorption of a protein at the interface of
an oil droplet is thermodynamically favored
because the hydrophobic amino acid residues
can then escape the hydrogen bridge network
of the surrounding water molecules. In addition,
contact of the protein with the oil droplet results
in the displacement of water molecules from the
hydrophobic regions of the oil-water boundary
layer. Therefore, the suitability of a protein as
an emulsifier depends on the rate at which it
diffuses into the interface and on the deformability
of its conformation under the influence
of interfacial tension (surface denaturation).
The diffusion rate depends on the temperature
and the molecular weight, which in turn can
be influenced by the pH and the ionic strength.
The adsorbability depends on the exposure of
hydrophilic and hydrophobic groups and thus
on the amino acid profile, as well as on the
pH, the ion strength and the temperature. The
conformative stability depends in the amino
acid composition, the molecular weight and
the intramolecular disulfide bonds. Therefore,
a protein with ideal qualities as an emulsifier
for an oil-in-water emulsion would have a relatively
low molecular weight, a balanced amino
acid composition in terms of charged, polar
and nonpolar residues, good water solubility,
well-developed surface hydrophobicity, and
a relatively stable conformation. The β-casein
molecule meets these requirements because
of less pronounced secondary structures and
no crosslinks due to the lack of SH groups
(cf. 10.1.2.1.1). The apolar “tail” of this flexible
molecule is adsorbed by the oil phase of the