Role of Hydrodynamic Shear on Activity and Structure of Proteins
57
4 Deactivation in Unit Operations
In the above section the study of shear effects on proteins had been carried out
by treating the proteins to defined shear conditions in model contractors.
However, these systems do not completely represent the conditions which
proteins encounter in real operations. In routine use, proteins are subjected to
conditions of shear in different unit operations. These include fermentation such
as either production of protein in stirred, agitated reactors or also during their
use in enzyme catalysed reactions. Proteins also undergo conditions of shear in
other stages such as filtration and cell disruption. Some typical unit operations
have been considered below.
4.1 Fermentation
It has been widely reported that the production of proteins can be correlated to
the operating conditions existing in the bioreactor which includes the shear rate.
In this context the term fermentation has been used in its widest possible
meaning to include bioprocesses involving catalysts of biological origin such as
cells or enzymes. Specifically the use of proteins is being considered here. The
effects of shear on microbial, plant and animal cells was considered by the
authors in a preceding review [41]. In the case of filamentous organisms such as
actinomycetes and fungi it is known that the shear rate affects protein production indirectly by affecting the morphology. The formation of diffuse mycelia or
spherical pellets is controlled by the agitation rate. The protein production is in
turn influenced by the morphology of the organisms [42, 43].
Early studies by Basu and Pal [44] showed that fungal cellulase were
deactivated due to shaking in flasks during the enzymatic hydrolysis of cellulose
carried out in shaker flasks. This implied that the production or use of enzymes
in stirred reactors would also be potentially damaging to the proteins. Virkar et
al. [40] investigated the deactivation of the enzyme alcohol dehydrogenase
(ADH) in a rotating disk reactor. This was a cylindrical reactor which had
a 66 mm diameter disk placed at approximately half the vessel depth. They
ensured complete filling of the vessel in order to eliminate entrapped air. The
disk was rotated at a constant speed of 3600 rpm. At this speed the flow
conditions in the reactor were turbulent and the average shear stress was
estimated at about 70 N m - 2. Under these conditions no loss of activity was
reported over a period of 5 h. In the cases, wherein the vessel was deliberately
incompletely filled there was a linear decrease in the activity of the enzyme with
time. This loss was accompanied by the formation of a very fine precipitate.
After a period of 5 h the activity decreased to about 60% of the initial activity.
In another study, Jones and Lee [45] carried out a kinetic analysis of
enzymatic hydrolysis of cellulose using a dried commercial preparation of
57
4 Deactivation in Unit Operations
In the above section the study of shear effects on proteins had been carried out
by treating the proteins to defined shear conditions in model contractors.
However, these systems do not completely represent the conditions which
proteins encounter in real operations. In routine use, proteins are subjected to
conditions of shear in different unit operations. These include fermentation such
as either production of protein in stirred, agitated reactors or also during their
use in enzyme catalysed reactions. Proteins also undergo conditions of shear in
other stages such as filtration and cell disruption. Some typical unit operations
have been considered below.
4.1 Fermentation
It has been widely reported that the production of proteins can be correlated to
the operating conditions existing in the bioreactor which includes the shear rate.
In this context the term fermentation has been used in its widest possible
meaning to include bioprocesses involving catalysts of biological origin such as
cells or enzymes. Specifically the use of proteins is being considered here. The
effects of shear on microbial, plant and animal cells was considered by the
authors in a preceding review [41]. In the case of filamentous organisms such as
actinomycetes and fungi it is known that the shear rate affects protein production indirectly by affecting the morphology. The formation of diffuse mycelia or
spherical pellets is controlled by the agitation rate. The protein production is in
turn influenced by the morphology of the organisms [42, 43].
Early studies by Basu and Pal [44] showed that fungal cellulase were
deactivated due to shaking in flasks during the enzymatic hydrolysis of cellulose
carried out in shaker flasks. This implied that the production or use of enzymes
in stirred reactors would also be potentially damaging to the proteins. Virkar et
al. [40] investigated the deactivation of the enzyme alcohol dehydrogenase
(ADH) in a rotating disk reactor. This was a cylindrical reactor which had
a 66 mm diameter disk placed at approximately half the vessel depth. They
ensured complete filling of the vessel in order to eliminate entrapped air. The
disk was rotated at a constant speed of 3600 rpm. At this speed the flow
conditions in the reactor were turbulent and the average shear stress was
estimated at about 70 N m - 2. Under these conditions no loss of activity was
reported over a period of 5 h. In the cases, wherein the vessel was deliberately
incompletely filled there was a linear decrease in the activity of the enzyme with
time. This loss was accompanied by the formation of a very fine precipitate.
After a period of 5 h the activity decreased to about 60% of the initial activity.
In another study, Jones and Lee [45] carried out a kinetic analysis of
enzymatic hydrolysis of cellulose using a dried commercial preparation of