Role of Hydrodynamic Shear on Activity and Structure of Proteins
65
Including terms for aggregate growth via collision and breakup via hydrodynamic shear they obtained an empirical expression for the rate of change of D,
the size of the aggregate, as it approached the equilibrium size De, where growth
and breakup are in balance as
dD/dt = ~- Kd(D -- De)
(13)
where -Ka is the protein aggregate breakup rate constant. Here again although the kinetics of breakup have been included, the distribution of sizes has
not been considered. Brown and Glatz [72], reviewed the above models as well
as the model elucidated by Glasgow and Luecke [73] for a change in aggregate
numbers at a particular size as a result of breakup via both hydrodynamic and
collisional forces. They carried out experiments with isoelectric precipitates of
soya protein and studied its breakup in an agitated vessel driven by a six-bladed
turbine impeller in order to evaluate these models. The mean shear rates used in
their study ranged from 1030-1340 s -1. They concluded that the breakup of
protein aggregates smaller than the Kolmogoroff microscale, occurs predominantly due to collision fragmentation. Although the mechanism of aggregate
growth cannot be completely described for all situations, it can be generalized
that the size of the final precipitate depends upon a balance between growth
and fragmentation. Fragmentation by different mechanisms includes deformation and rupture of particles due to fluctuations in hydrodynamic pressures,
erosion of the aggregate and rupture of the primary particles by shear or
collision. As further recovery of precipitates is strongly dependent on their size
and strength it is imperative to describe the behaviour of these systems under
different flow conditions comprehensively, incorporating particle strength, aging parameters and kinetics of the precipitation process besides the mean
velocity gradient.
4.4 Membrane Filtration
Membrane filtration processes are being increasingly used for concentration,
separation or purification of biological material in laboratory as well as industrial scale applications. Ultrafiltration is an especially popular method for the
concentration and purification as it is versatile and selection of membrane
allows fractionation of proteins based on their molecular weights with relative
ease as compared to other traditional purification techniques. This method
involves recirculation of enzymes through ultrafiltration modules which may be
in tube like configuration or they may be used in fiat membrane type devices. In
either case the protein solution is subjected to fluid forces and it is important to
characterize the performance of these devices not only in terms of their separation efficiencies but also to examine whether this operation causes any damage
to the protein molecules.
One of the problems with membrane filtration systems is renewing of the
surface to prevent reduction in the flux due to concentration polarization
65
Including terms for aggregate growth via collision and breakup via hydrodynamic shear they obtained an empirical expression for the rate of change of D,
the size of the aggregate, as it approached the equilibrium size De, where growth
and breakup are in balance as
dD/dt = ~- Kd(D -- De)
(13)
where -Ka is the protein aggregate breakup rate constant. Here again although the kinetics of breakup have been included, the distribution of sizes has
not been considered. Brown and Glatz [72], reviewed the above models as well
as the model elucidated by Glasgow and Luecke [73] for a change in aggregate
numbers at a particular size as a result of breakup via both hydrodynamic and
collisional forces. They carried out experiments with isoelectric precipitates of
soya protein and studied its breakup in an agitated vessel driven by a six-bladed
turbine impeller in order to evaluate these models. The mean shear rates used in
their study ranged from 1030-1340 s -1. They concluded that the breakup of
protein aggregates smaller than the Kolmogoroff microscale, occurs predominantly due to collision fragmentation. Although the mechanism of aggregate
growth cannot be completely described for all situations, it can be generalized
that the size of the final precipitate depends upon a balance between growth
and fragmentation. Fragmentation by different mechanisms includes deformation and rupture of particles due to fluctuations in hydrodynamic pressures,
erosion of the aggregate and rupture of the primary particles by shear or
collision. As further recovery of precipitates is strongly dependent on their size
and strength it is imperative to describe the behaviour of these systems under
different flow conditions comprehensively, incorporating particle strength, aging parameters and kinetics of the precipitation process besides the mean
velocity gradient.
4.4 Membrane Filtration
Membrane filtration processes are being increasingly used for concentration,
separation or purification of biological material in laboratory as well as industrial scale applications. Ultrafiltration is an especially popular method for the
concentration and purification as it is versatile and selection of membrane
allows fractionation of proteins based on their molecular weights with relative
ease as compared to other traditional purification techniques. This method
involves recirculation of enzymes through ultrafiltration modules which may be
in tube like configuration or they may be used in fiat membrane type devices. In
either case the protein solution is subjected to fluid forces and it is important to
characterize the performance of these devices not only in terms of their separation efficiencies but also to examine whether this operation causes any damage
to the protein molecules.
One of the problems with membrane filtration systems is renewing of the
surface to prevent reduction in the flux due to concentration polarization