2009_Book_FoodChemistry

62 1 Amino Acids, Peptides, Proteins
The water retention capacity of protein can be estimated
with the following formula:
a = f c + 0.4f p + 0.2f n (1.98)
(a: g water/g protein; f c , f p , f n : fraction of
charged, polar, neutral amino acid residues).
1.4.3.4 Foam Formation and Foam
Stabilization
In several foods, proteins function as foamforming
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 albumin
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. Proteins
stabilize by forming flexible, cohesive films
around the gas bubbles. During impact, the protein
is adsorbed at the interface via hydrophobic
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 denatured
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 (disproportionation).
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 foamstabilizing
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 solvents
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 characteristics
of proteins can be improved by chemical
and physical modification. Thus a partial enzymatic
hydrolysis leads to smaller, more quickly
diffusing molecules, better solubility, and the release
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 denaturation
(e. g. of whey proteins). Recently, the addition
of strongly alkaline proteins (e. g. clupeines)
is being tested, which apparently increases the association
of protein in the films and allows the
foaming of fatty systems.
1.4.3.5 Gel Formation
Gels are disperse systems of at least two components
in which the disperse phase in the dispersant
forms a cohesive network. They are characterized
by the lack of fluidity and elastic deformability.
Gels are placed between solutions,
in which repulsive forces between molecules and
the disperse phase predominate, and precipitates,
where strong intermolecular interactions predominate.
We differentiate between two types of gel,
the polymeric networks and the aggregated dispersions,
although intermediate forms are found
as well.
Examples of polymeric networks are the gels
formed by gelatin (cf. 12.3.2.3.1) and polysaccharides
such as agarose (cf. 4.4.4.1.2) and