62
C.B. Elias and J.B. Joshi
Hoare [65], studied the precipitation of casein by ammonium sulfate salting
out, with the view to optimization of its subsequent recovery in industrial
centrifuges. He observed that the precipitate particle size distribution varied
with the conditions of precipitation and ageing. Virkar et al. [66] examined the
acid precipitation of soya proteins. This is an important step in the entire
process of protein production from soya extract where separation of soluble
protein from carbohydrates is carried out. A study of the effect of precipitation
conditions was done by carrying out experiments in two systems i.e. in a stirred
vessel in batch mode and in a continuous flow tubular reactor. The batch
reactor consisted of a 250 ml glass beaker stirred with a magnetic stirrer. The
acid addition was done either dropwise or at one time to see the effect of the
contacting procedure on the precipitation. In the tubular reactor the protein and
acid solutions were fed through two separate streams at controlled flow rates.
The fluid in the reactor was turbulent (NRe greater than 20 000). The acid flow
was always maintained two orders of magnitude lower than the protein feed
rate, thus it contributed little to the overall flow in the reactor.
In the batch reactor, a small increase in the particle size was seen with an
increase in the protein concentration. It was also seen that poor mixing resulted
in an increase in the particle size with a broadening of the distribution curve.
The rate of growth of the mean particle size was higher at high protein
concentration. In the tubular reactor macromixing was complete at a distance of
less than 0.3 m from the point of acid injection. The precipitation was studied at
two flow rates and conditions were turbulent at both the flow rates. The initial
growth rates were extremely rapid. The final mean particle sizes showed that the
mean particle diameter was smaller in the high flow rates. This is due to an
increase in the shear stress resulting in greater breakup rates. Thus the final
particle size distribution is due to an equilibrium between the formation of
particles and the breakup of particles by the shear stress.
An improvement in the aggregate strength making it more resistance to
breakup by shear has been found to occur due to aging in the stirred vessel. Bell
and Dunnill 1-67] studied this aspect of protein precipitates by examining the
shear disruption of soya protein precipitate particles. This is important in the
centrifugal recovery of protein precipitates as these are subjected to high shear
fields in the entry zones of centrifugal feed streams. The protein precipitates were
treated to capillary shear by passing them through capillary tubes at shear rates
between 104 and 105 s- 1 with exposure times between 0.004-0.2 s. These conditions are similar to those in the entry zones of centrifuges. The influence of the
conditions of aging on the resistance of aggregates to shear breakup was
evaluated by aging the dispersion in the reactor at a constant speed and then
subjecting an aliquot to capillary shear. The changes in particle size were
monitored using a Coulter counter. The stirrer speed used for agitation and
precipitation were the same at 150-650 rpm. At these speeds the mean particle
diameter was 53.5-8.8 gm, respectively.
Aging the precipitated protein solution by stirring leads to a state of dynamic
equilibrium where the rate of formation of aggregate equals the rate of breakup.
C.B. Elias and J.B. Joshi
Hoare [65], studied the precipitation of casein by ammonium sulfate salting
out, with the view to optimization of its subsequent recovery in industrial
centrifuges. He observed that the precipitate particle size distribution varied
with the conditions of precipitation and ageing. Virkar et al. [66] examined the
acid precipitation of soya proteins. This is an important step in the entire
process of protein production from soya extract where separation of soluble
protein from carbohydrates is carried out. A study of the effect of precipitation
conditions was done by carrying out experiments in two systems i.e. in a stirred
vessel in batch mode and in a continuous flow tubular reactor. The batch
reactor consisted of a 250 ml glass beaker stirred with a magnetic stirrer. The
acid addition was done either dropwise or at one time to see the effect of the
contacting procedure on the precipitation. In the tubular reactor the protein and
acid solutions were fed through two separate streams at controlled flow rates.
The fluid in the reactor was turbulent (NRe greater than 20 000). The acid flow
was always maintained two orders of magnitude lower than the protein feed
rate, thus it contributed little to the overall flow in the reactor.
In the batch reactor, a small increase in the particle size was seen with an
increase in the protein concentration. It was also seen that poor mixing resulted
in an increase in the particle size with a broadening of the distribution curve.
The rate of growth of the mean particle size was higher at high protein
concentration. In the tubular reactor macromixing was complete at a distance of
less than 0.3 m from the point of acid injection. The precipitation was studied at
two flow rates and conditions were turbulent at both the flow rates. The initial
growth rates were extremely rapid. The final mean particle sizes showed that the
mean particle diameter was smaller in the high flow rates. This is due to an
increase in the shear stress resulting in greater breakup rates. Thus the final
particle size distribution is due to an equilibrium between the formation of
particles and the breakup of particles by the shear stress.
An improvement in the aggregate strength making it more resistance to
breakup by shear has been found to occur due to aging in the stirred vessel. Bell
and Dunnill 1-67] studied this aspect of protein precipitates by examining the
shear disruption of soya protein precipitate particles. This is important in the
centrifugal recovery of protein precipitates as these are subjected to high shear
fields in the entry zones of centrifugal feed streams. The protein precipitates were
treated to capillary shear by passing them through capillary tubes at shear rates
between 104 and 105 s- 1 with exposure times between 0.004-0.2 s. These conditions are similar to those in the entry zones of centrifuges. The influence of the
conditions of aging on the resistance of aggregates to shear breakup was
evaluated by aging the dispersion in the reactor at a constant speed and then
subjecting an aliquot to capillary shear. The changes in particle size were
monitored using a Coulter counter. The stirrer speed used for agitation and
precipitation were the same at 150-650 rpm. At these speeds the mean particle
diameter was 53.5-8.8 gm, respectively.
Aging the precipitated protein solution by stirring leads to a state of dynamic
equilibrium where the rate of formation of aggregate equals the rate of breakup.