servation that the cell wall of larger cells contains more polysaccharides and
less proteins. Therefore, they are more hydrophilic. There was no difference in
the fatty acid composition and amount of the cell wall of the two strains. DSM
2155 yeast cells differ in their morphology considerably. LGB H620 cells formed
only single or budding cells, strain DSM 2155 formed chain-like cell aggregates
with size depending on the cultivation conditions: in the presence of adequate
substrate, concentration cell aggregates consisting of up to eight cells were
formed, and during substrate limitation single cells dominated [123]. The difference of flotation between the two strains is related to their surface properties.
The cells of the LBG strain are less hydrophobic (water contact angle 27°) and
the cells of the DSM strain are very hydrophobic (water contact angle 70°). With
prolonged cultivation time, the hydrophobicity of the cells diminished and,
consequently, their flotation as well. The difference in surface hydrophobicity
between the two strains originates from variations in their surface chemical
composition. In batch-culture strain DSM 2155 had a lower oxygen and hydroxide content on its surface, and a higher proportion of hydrocarbon C, as
compared with strain LBG H620. Another property that seems to be related to
the behavior of the yeast is the surface charge. Strain LBG H620 is more
negatively charged than DSM 2155 (electrophoretic mobility at pH 4.0 was
–1.85 ¥ 10 –8 m 2 V –1 and –1.35, respectively). The latter strain has a greater
tendency to aggregate due to lower electrostatic repulsion and stronger hydrophobic interactions. According to Armory et al. [129] and Moses et al. [130] the
hydrophobicity of the surface of yeasts and bacteria can be related to the O/C
atomic ratio and the electrokinetic potential to the P/C atomic ratio. The Plimitation caused, as expected, a decrease in the surface P/C atomic ratio. The
investigations by Tybussek et al. [122] indicate that the differences in the P/C
atomic ratio correlate well with changes in the electrophoretic mobility for
strain LBG H620, but not for strain DSM 2155, which does not change with the
P/C ratio. More investigations are necessary to obtain quantitative relationships
between physicochemical surface properties and floatability of yeast cells.
6
Conclusions
There have been many investigations on surfactant foams, foam films, Plateau
border, foam drainage and their physical chemistry. The application of these
results to protein foams is only partly possible. Proteins are macromolecules,
their adsorption is coupled with a change in conformation at the gas/liquid
interface which is a slow process. It takes 15 to 20 h to obtain the equilibrium
surface tension. The residence time of the protein molecules in a flotation
column is too short to attain the equilibrium surface tension during this time.
Therefore, the transport of proteins to the interface and their adsorption at the
interface are dynamical processes which are far from equilibrium. Surfactants
have well-defined hydrophilic and hydrophobic domains. Thus it is relatively
easy to calculate their interaction with the interface. Protein molecules have
several hydrophilic and hydrophobic domains, and their interaction with the interface depends on the hydrophilic/hydrophobic character of their surface
228
K. Schügerl
less proteins. Therefore, they are more hydrophilic. There was no difference in
the fatty acid composition and amount of the cell wall of the two strains. DSM
2155 yeast cells differ in their morphology considerably. LGB H620 cells formed
only single or budding cells, strain DSM 2155 formed chain-like cell aggregates
with size depending on the cultivation conditions: in the presence of adequate
substrate, concentration cell aggregates consisting of up to eight cells were
formed, and during substrate limitation single cells dominated [123]. The difference of flotation between the two strains is related to their surface properties.
The cells of the LBG strain are less hydrophobic (water contact angle 27°) and
the cells of the DSM strain are very hydrophobic (water contact angle 70°). With
prolonged cultivation time, the hydrophobicity of the cells diminished and,
consequently, their flotation as well. The difference in surface hydrophobicity
between the two strains originates from variations in their surface chemical
composition. In batch-culture strain DSM 2155 had a lower oxygen and hydroxide content on its surface, and a higher proportion of hydrocarbon C, as
compared with strain LBG H620. Another property that seems to be related to
the behavior of the yeast is the surface charge. Strain LBG H620 is more
negatively charged than DSM 2155 (electrophoretic mobility at pH 4.0 was
–1.85 ¥ 10 –8 m 2 V –1 and –1.35, respectively). The latter strain has a greater
tendency to aggregate due to lower electrostatic repulsion and stronger hydrophobic interactions. According to Armory et al. [129] and Moses et al. [130] the
hydrophobicity of the surface of yeasts and bacteria can be related to the O/C
atomic ratio and the electrokinetic potential to the P/C atomic ratio. The Plimitation caused, as expected, a decrease in the surface P/C atomic ratio. The
investigations by Tybussek et al. [122] indicate that the differences in the P/C
atomic ratio correlate well with changes in the electrophoretic mobility for
strain LBG H620, but not for strain DSM 2155, which does not change with the
P/C ratio. More investigations are necessary to obtain quantitative relationships
between physicochemical surface properties and floatability of yeast cells.
6
Conclusions
There have been many investigations on surfactant foams, foam films, Plateau
border, foam drainage and their physical chemistry. The application of these
results to protein foams is only partly possible. Proteins are macromolecules,
their adsorption is coupled with a change in conformation at the gas/liquid
interface which is a slow process. It takes 15 to 20 h to obtain the equilibrium
surface tension. The residence time of the protein molecules in a flotation
column is too short to attain the equilibrium surface tension during this time.
Therefore, the transport of proteins to the interface and their adsorption at the
interface are dynamical processes which are far from equilibrium. Surfactants
have well-defined hydrophilic and hydrophobic domains. Thus it is relatively
easy to calculate their interaction with the interface. Protein molecules have
several hydrophilic and hydrophobic domains, and their interaction with the interface depends on the hydrophilic/hydrophobic character of their surface
228
K. Schügerl