In an interesting variation of leaching, more recently an aqueous two-phase
system was used for recovery of amyloglucosidase produced by SSF of wheat
bran by Aspergillus niger [203]. The two-phase system consisted of a PEG 6000
phase and a potassium phosphate phase. Here 95% of the glucoamylase was
recovered in the potassium phosphate phase, while all the visible particles of
bran collected at the interface. This combined extraction and separation step
gave a purification factor of 11.
8.3.2
Retention of Leaching Solution in the Substrate Particles
Normally SSF processes are carried out with moist substrates at less than their
maximum water holding capacity. During the leaching process these substrates
will tend to retain sufficient leaching solution to reach their maximum waterholding capacity. Any leaching solution retained in the solids will contain
enzyme, which represents a loss during the recovery process. Increasing the
amount of leaching solution would decrease the fractional loss of enzyme,
assuming that there is equilibrium between the retained and extraparticle
leaching solution, because the fraction of overall liquid retained would be lower.
However, this is not a feasible option because it leads to more diluted leachate.
In fact, the advantage that SSF processes have in producing high product concentrations compared to SLF would be lost if large dilutions occurred in the
extraction step. Alternatively, the solids can be pressed to minimize the amount
of liquid retained in the product.
8.3.3
Leaching Mode
Various types of leaching can be undertaken – direct extraction, percolation,
pulsed flow extraction, and countercurrent extraction.
In direct extraction leaching solution is added to the solids in a vessel which
is then agitated. This type of extraction is common in the laboratory, but it
requires large solvent to solids ratios. In percolation the solids are placed in a
column and the leaching solution is added to the top. A valve in the outlet at the
bottom is opened and the liquid slowly drains through the solids bed. In pulsedflow extraction the solids are placed in a column and a dosing pump is used to
add leaching solution at the top at a controlled rate [199]. A four-stage countercurrent extraction involves four vessels which contain solids at different stages
of leaching. Fresh leaching solution is added to the solids which have already
been leached three times. After this leaching the solids are thrown out and
replaced by a new batch of solids which have already been leached three times
and the leachate is fed into the vessel containing solids which have already been
leached twice. The leaching process is repeated until liquid which has passed
through three batches of solids is finally used to leach fresh solids. The leachate
from this step represents the final leachate product.
Of these methods, multiple-stage countercurrent extraction enables the
highest efficiencies and lowest ratios of leaching solution to solid, thereby
Biochemical Engineering Aspects of Solid State Bioprocessing
127
system was used for recovery of amyloglucosidase produced by SSF of wheat
bran by Aspergillus niger [203]. The two-phase system consisted of a PEG 6000
phase and a potassium phosphate phase. Here 95% of the glucoamylase was
recovered in the potassium phosphate phase, while all the visible particles of
bran collected at the interface. This combined extraction and separation step
gave a purification factor of 11.
8.3.2
Retention of Leaching Solution in the Substrate Particles
Normally SSF processes are carried out with moist substrates at less than their
maximum water holding capacity. During the leaching process these substrates
will tend to retain sufficient leaching solution to reach their maximum waterholding capacity. Any leaching solution retained in the solids will contain
enzyme, which represents a loss during the recovery process. Increasing the
amount of leaching solution would decrease the fractional loss of enzyme,
assuming that there is equilibrium between the retained and extraparticle
leaching solution, because the fraction of overall liquid retained would be lower.
However, this is not a feasible option because it leads to more diluted leachate.
In fact, the advantage that SSF processes have in producing high product concentrations compared to SLF would be lost if large dilutions occurred in the
extraction step. Alternatively, the solids can be pressed to minimize the amount
of liquid retained in the product.
8.3.3
Leaching Mode
Various types of leaching can be undertaken – direct extraction, percolation,
pulsed flow extraction, and countercurrent extraction.
In direct extraction leaching solution is added to the solids in a vessel which
is then agitated. This type of extraction is common in the laboratory, but it
requires large solvent to solids ratios. In percolation the solids are placed in a
column and the leaching solution is added to the top. A valve in the outlet at the
bottom is opened and the liquid slowly drains through the solids bed. In pulsedflow extraction the solids are placed in a column and a dosing pump is used to
add leaching solution at the top at a controlled rate [199]. A four-stage countercurrent extraction involves four vessels which contain solids at different stages
of leaching. Fresh leaching solution is added to the solids which have already
been leached three times. After this leaching the solids are thrown out and
replaced by a new batch of solids which have already been leached three times
and the leachate is fed into the vessel containing solids which have already been
leached twice. The leaching process is repeated until liquid which has passed
through three batches of solids is finally used to leach fresh solids. The leachate
from this step represents the final leachate product.
Of these methods, multiple-stage countercurrent extraction enables the
highest efficiencies and lowest ratios of leaching solution to solid, thereby
Biochemical Engineering Aspects of Solid State Bioprocessing
127