2.1.2
Investigations with Bovine Serum Albumin
Bovine serum albumin (BSA), a globular protein, is often applied as a model
protein for foam formation. The surface tension of BSA solutions as a function of
time indicates that, depending on the BSA concentration, it can take 15 to 20 h to
attain an equilibrium surface tension which is independent of the BSA concentration. The coagulation rates are slight and the loss of native protein in the surface
film due to adsorption and denaturation is compensated by the quick and continuous diffusive transport of native protein to the surface. Therefore, the critical
micelle concentration (CMC) and s CMC can be evaluated from these measurements.
2.1.3
Influence of Protein Structure
A comparison of foams formed by various proteins indicates that the foaminess
is related to the rate of decrease in the surface tension of the air/water interface
by protein molecules whereas foam stability is related to the structure of
adsorbed protein films. Thus flexible protein molecules, which can rapidly
reduce the surface tension of the air/water interface, give good foaminess
whereas highly ordered global molecules with slow surface denaturation rates
give poor foaminess. Rapid build-up of film pressure by proteins tends to lead
to formation of a coarse foam (with large bubbles) whereas a slow increase
favors small air bubbles, i.e. a creamy foam. Globular protein foams are more
stable than foams prepared with proteins of flexible structure. Therefore, the
foaminess of proteins with more-or-less random coil molecules (e.g. b-casein)
differs from that of globular proteins (e.g. BSA) [8].
2.1.4
Influence of Protein Environment
The solubility of proteins as well as the protein type influences the foaminess. The
solubility of proteins is lowest at their isoelectric point (IEP) and, therefore, their
foaminess is the highest at their IEP, if the proteins do not precipitate. However,
BSA has an anomalous behavior. In the acidic pH range BSA exhibits a non-folded configuration, the size of the albumin molecule changes. In the pH range between 4.5 and 10.5 the protein molecule has a compact shape. In this range no
change in configuration or molecular weight occurs [9]. Below and above this
range the protein molecule expands. This expansion is accompanied by a change
in secondary and tertiary structures. In accordance with this behavior maximum
foaminess is found at the beginning of these changes. Why the foaminess exhibits
a minimum at pH 3 and below 3 increases again is not yet clear [10].
It is well known that inorganic salts influence protein solubility [11]. The
efficiency of various salts is characterized by means of the position of the salts
in the lyotrophic or Hofmeister series [12]. By means of the turbidity temperature, measured with poly(ethylene oxide) NP-10 (PEO) solution, the influence of the water structure variation on the protein solubility can be deter196
K. Schügerl
Investigations with Bovine Serum Albumin
Bovine serum albumin (BSA), a globular protein, is often applied as a model
protein for foam formation. The surface tension of BSA solutions as a function of
time indicates that, depending on the BSA concentration, it can take 15 to 20 h to
attain an equilibrium surface tension which is independent of the BSA concentration. The coagulation rates are slight and the loss of native protein in the surface
film due to adsorption and denaturation is compensated by the quick and continuous diffusive transport of native protein to the surface. Therefore, the critical
micelle concentration (CMC) and s CMC can be evaluated from these measurements.
2.1.3
Influence of Protein Structure
A comparison of foams formed by various proteins indicates that the foaminess
is related to the rate of decrease in the surface tension of the air/water interface
by protein molecules whereas foam stability is related to the structure of
adsorbed protein films. Thus flexible protein molecules, which can rapidly
reduce the surface tension of the air/water interface, give good foaminess
whereas highly ordered global molecules with slow surface denaturation rates
give poor foaminess. Rapid build-up of film pressure by proteins tends to lead
to formation of a coarse foam (with large bubbles) whereas a slow increase
favors small air bubbles, i.e. a creamy foam. Globular protein foams are more
stable than foams prepared with proteins of flexible structure. Therefore, the
foaminess of proteins with more-or-less random coil molecules (e.g. b-casein)
differs from that of globular proteins (e.g. BSA) [8].
2.1.4
Influence of Protein Environment
The solubility of proteins as well as the protein type influences the foaminess. The
solubility of proteins is lowest at their isoelectric point (IEP) and, therefore, their
foaminess is the highest at their IEP, if the proteins do not precipitate. However,
BSA has an anomalous behavior. In the acidic pH range BSA exhibits a non-folded configuration, the size of the albumin molecule changes. In the pH range between 4.5 and 10.5 the protein molecule has a compact shape. In this range no
change in configuration or molecular weight occurs [9]. Below and above this
range the protein molecule expands. This expansion is accompanied by a change
in secondary and tertiary structures. In accordance with this behavior maximum
foaminess is found at the beginning of these changes. Why the foaminess exhibits
a minimum at pH 3 and below 3 increases again is not yet clear [10].
It is well known that inorganic salts influence protein solubility [11]. The
efficiency of various salts is characterized by means of the position of the salts
in the lyotrophic or Hofmeister series [12]. By means of the turbidity temperature, measured with poly(ethylene oxide) NP-10 (PEO) solution, the influence of the water structure variation on the protein solubility can be deter196
K. Schügerl