62 1 Amino Acids, Peptides, Proteins
The water retention capacity of protein can be es-
timated with the following formula:
a=fc+ 0 .4fp+ 0 .2fn (1.98)
(a: g water/g protein; fc,fp,fn: fraction of
charged, polar, neutral amino acid residues).
1.4.3.4 FoamFormationandFoamStabilization
In several foods, proteins function as foam-
forming and foam-stabilizing components, for
example in baked goods, sweets, desserts and
beer. This varies from one protein to another.
Serum albumin foams very well, while egg albu-
min does not. Protein mixtures such as egg white
can be particularly well suited (cf. 11.4.2.2). In
that case, the globulins facilitate foam formation.
Ovomucin stabilizes the foam, egg albumin and
conalbumin allow its fixation through thermal
coagulation.
Foams are dispersions of gases in liquids. Pro-
teins stabilize by forming flexible, cohesive films
around the gas bubbles. During impact, the pro-
tein is adsorbed at the interface via hydropho-
bic areas; this is followed by partial unfolding
(surface denaturation). The reduction of surface
tension caused by protein adsorption facilitates
the formation of new interfaces and further gas
bubbles. The partially unfolded proteins associate
while forming stabilizing films.
The more quickly a protein molecule diffuses
into interfaces and the more easily it is dena-
tured there, the more it is able to foam. These
values in turn depend on the molecular mass, the
surface hydrophobicity, and the stability of the
conformation.
Foams collapse because large gas bubbles grow
at the expense of smaller bubbles (dispropor-
tionation). The protein films counteract this
disproportionation. That is why the stability of
a foam depends on the strength of the protein
film and its permeability for gases. Film strength
depends on the adsorbed amount of protein
and the ability of the adsorbed molecules to
associate. Surface denaturation generally releases
additional amino acid side chains which can enter
into intermolecular interactions. The stronger
the cross-linkage, the more stable the film.
Since the smallest possible net charge promotes
association, the pH of the system should lie in
the range of the isoelectric points of the proteins
that participate in film formation.
In summary, the ideal foam-forming and foam-
stabilizing protein is characterized by a low
molecular weight, high surface hydrophobicity,
good solubility, a small net charge in terms of the
pH of the food, and easy denaturability.
Foams are destroyed by lipids and organic sol-
vents such as higher alcohols, which due to their
hydrophobicity displace proteins from the gas
bubble surface without being able to form stable
films themselves. Even a low concentration of
egg yolk, for example, prevents the bursting of
egg white. This is attributed to a disturbance of
protein association by the lecithins.
The foam-forming and foam-stabilizing charac-
teristics of proteins can be improved by chemical
and physical modification. Thus a partial enzy-
matic hydrolysis leads to smaller, more quickly
diffusing molecules, better solubility, and the re-
lease of hydrophobic groups. Disadvantages are
the generally lower film stability and the loss of
thermal coagulability. The characteristics can also
be improved by introducing charged or neutral
groups (cf. 1.4.6.2) and by partial thermal denatu-
ration (e. g. of whey proteins). Recently, the addi-
tion of strongly alkaline proteins (e. g. clupeines)
is being tested, which apparently increases the as-
sociation of protein in the films and allows the
foaming of fatty systems.1.4.3.5 GelFormation..........................................
Gels are disperse systems of at least two com-
ponents in which the disperse phase in the dis-
persant forms a cohesive network. They are char-
acterized by the lack of fluidity and elastic de-
formability. Gels are placed between solutions,
in which repulsive forces between molecules and
the disperse phase predominate, and precipitates,
where strong intermolecular interactions predom-
inate. We differentiate between two types of gel,
thepolymeric networksand theaggregated dis-
persions, although intermediate forms are found
as well.
Examples of polymeric networksare the gels
formed by gelatin (cf. 12.3.2.3.1) and polysac-
charides such as agarose (cf. 4.4.4.1.2) and