which is a function of their conformation. Also the viscosity of the protein solutions which effects the film drainage depends on this conformation. In the
case of cultivation media, the protein conformation, and therefore the
foaminess and foam stability, is influenced by the medium composition as well,
which is often not well defined and changes during the cultivation. In the case
of microbial cell flotation, the process depends on the protein enrichment, i.e.
on the interaction of the proteins with the gas/liquid interface, as well as on the
reciprocal action of cells with the protein and gas/liquid interface.
Microorganisms have a complex cell envelope structure. Their surfaces
charge and their hydrophobicity cannot be predicted, only experimentally
determined [131]. Several microorganisms are not hydrophobic enough to be
floated. They need collectors, similar to ore flotation. In cultivation media
proteins which adsorb on the cell surface act as collectors. The interrelationship
between cell envelope and proteins cannot be predicted, only experimentally
evaluated. The accumulation of cells on the bubble surface depends not only on
the properties of the interface, proteins and cells, but on the bubble size and
velocity as well [132]. On account of this complex interrelationship between
several parameters, prediction of flotation performance of microbial cells
based on physicochemical fundamentals is not possible. Therefore, only
empirical relationships are known which cannot be generalized. Based on
the large amount of information collected in recent years, mathematical
models have been developed for the calculation of the behavior of protein
solutions and particular microbial cells. They hold true only for systems (e.g.
BSA solutions and particular yeast strains) which are used for their evaluation.
In spite of this, several recommendations for protein and microbial cell
flotation can be made.
Flotation of proteins and microbial cells is especially efficient at low concentrations, which are well below the values common in microbial cultivations
with complex media and high-performance strains used in industry. Therefore,
they are only suited for the recovery of proteins and cells from synthetic media
with low protein and cell concentrations. However, at low protein concentrations, the enzymes are deactivated. Dilution of the cultivation medium to
enhance enrichment of the cells is uneconomical, because the improvement in
the enrichment is fully compensated by the reduction in cell concentration in
the feed. The final cell concentration in the foam liquid remains the same. The
protein separation S can be increased by reducing the aeration rate and protein
concentration in the feed and increasing the bubble size, the temperature,
aerated liquid layer and foam layer heights, as long as the foam remains stable.
The foaminess of several cultivation media can be predicted on the basis of the
equilibrium surface tension.
Cell separation can be increased by reducing the feed rate, the aeration rate
and increasing the aerated liquid layer and the foam layer height, as long as the
foam remains stable. It is also increased with a larger column diameter.
Modeling of microbial cell flotation is still in the early stages, because the model
does not include microbial cell properties and therefore only holds true for e.g.
a particular strain of Saccharomyces carlsbergensis with a strong hydrophobic
cell envelope, which was used for the verification of the model.
Recovery of Proteins and Microorganisms from Cultivation Media by Foam Flotation
229
case of cultivation media, the protein conformation, and therefore the
foaminess and foam stability, is influenced by the medium composition as well,
which is often not well defined and changes during the cultivation. In the case
of microbial cell flotation, the process depends on the protein enrichment, i.e.
on the interaction of the proteins with the gas/liquid interface, as well as on the
reciprocal action of cells with the protein and gas/liquid interface.
Microorganisms have a complex cell envelope structure. Their surfaces
charge and their hydrophobicity cannot be predicted, only experimentally
determined [131]. Several microorganisms are not hydrophobic enough to be
floated. They need collectors, similar to ore flotation. In cultivation media
proteins which adsorb on the cell surface act as collectors. The interrelationship
between cell envelope and proteins cannot be predicted, only experimentally
evaluated. The accumulation of cells on the bubble surface depends not only on
the properties of the interface, proteins and cells, but on the bubble size and
velocity as well [132]. On account of this complex interrelationship between
several parameters, prediction of flotation performance of microbial cells
based on physicochemical fundamentals is not possible. Therefore, only
empirical relationships are known which cannot be generalized. Based on
the large amount of information collected in recent years, mathematical
models have been developed for the calculation of the behavior of protein
solutions and particular microbial cells. They hold true only for systems (e.g.
BSA solutions and particular yeast strains) which are used for their evaluation.
In spite of this, several recommendations for protein and microbial cell
flotation can be made.
Flotation of proteins and microbial cells is especially efficient at low concentrations, which are well below the values common in microbial cultivations
with complex media and high-performance strains used in industry. Therefore,
they are only suited for the recovery of proteins and cells from synthetic media
with low protein and cell concentrations. However, at low protein concentrations, the enzymes are deactivated. Dilution of the cultivation medium to
enhance enrichment of the cells is uneconomical, because the improvement in
the enrichment is fully compensated by the reduction in cell concentration in
the feed. The final cell concentration in the foam liquid remains the same. The
protein separation S can be increased by reducing the aeration rate and protein
concentration in the feed and increasing the bubble size, the temperature,
aerated liquid layer and foam layer heights, as long as the foam remains stable.
The foaminess of several cultivation media can be predicted on the basis of the
equilibrium surface tension.
Cell separation can be increased by reducing the feed rate, the aeration rate
and increasing the aerated liquid layer and the foam layer height, as long as the
foam remains stable. It is also increased with a larger column diameter.
Modeling of microbial cell flotation is still in the early stages, because the model
does not include microbial cell properties and therefore only holds true for e.g.
a particular strain of Saccharomyces carlsbergensis with a strong hydrophobic
cell envelope, which was used for the verification of the model.
Recovery of Proteins and Microorganisms from Cultivation Media by Foam Flotation
229