resulting in the highest enzyme concentrations in the leachate. With four to five
contact stages, leaching efficiencies of 85% can be achieved using leaching
solution to dry solids ratios as low as 1 l of leaching solution per kg of dry solids
[130, 204]. These high efficiencies are obtained because the countercurrent
movement of solids and liquid through the process increases extraction efficiency: the liquid leaves in equilibrium with the fresh solids, rather than in
equilibrium with the spent solids, as occurs in the other methods. Pressing of
the solids between extraction stages can help to improve recovery [205].
Recently this method has been applied to the recovery of pectinolytic enzymes
using three extraction stages [206].
The disadvantage of counter current leaching is the number of contact
stages. For example similar extraction efficiencies can be obtained in one-step
pulsed-flow extraction, although the final leachate has a twofold lower enzyme
concentration [199].
In a variation of the counter current method called “semi-continuous
multiple contact forced percolation”, six-fold higher concentrations of fungal
rennet were claimed in comparison with five-stage counter-current extraction
[207]. However, it is difficult to reconcile a six-fold increase in concentration
with the fact that the leaching solution to solids ratios were similar, and the
percentage recoveries were similar.
8.4
Recovery by Direct Pressing
Enzymes can also be recovered by direct pressing. Hydraulic pressing without leachate addition was done by Roussos et al. [208]. They applied 220 bar
pressure for 1 min, then added a volume of water equal to the leachate volume
released in the first pressing, and repeated the pressing. They were able to
recover 80% of the water originally present in the solids on the first pressing, although the fraction of water retained will obviously depend on the
type of solid. After the second pressing, enzyme recoveries of 85–95% were
obtained. These recoveries are possible with the addition of relatively little
liquid. In total the ratio of final leachate to initial mass of fermented solids
(fresh weight) was only 1.2. Despite the apparent success of this method, and the
widespread use of industrial scale presses in the fruit processing industry, the
use of direct pressing to recover products from SSF has not received further
attention [208].
8.5
Recovery of Non-Volatile Small Organic Molecules
Small, non-volatile, organic molecules which have been produced in SSF include pigments, antibiotics, plant growth factors, and simple organic acids such
as lactic and citric acids. As for enzymes the only extraction methods available
are pressing and leaching. However, in contrast to the work done in the leaching
of enzymes, relatively little attention has been given to the extraction of these
small non-volatile organic molecules.
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D.A. Mitchell et al.
contact stages, leaching efficiencies of 85% can be achieved using leaching
solution to dry solids ratios as low as 1 l of leaching solution per kg of dry solids
[130, 204]. These high efficiencies are obtained because the countercurrent
movement of solids and liquid through the process increases extraction efficiency: the liquid leaves in equilibrium with the fresh solids, rather than in
equilibrium with the spent solids, as occurs in the other methods. Pressing of
the solids between extraction stages can help to improve recovery [205].
Recently this method has been applied to the recovery of pectinolytic enzymes
using three extraction stages [206].
The disadvantage of counter current leaching is the number of contact
stages. For example similar extraction efficiencies can be obtained in one-step
pulsed-flow extraction, although the final leachate has a twofold lower enzyme
concentration [199].
In a variation of the counter current method called “semi-continuous
multiple contact forced percolation”, six-fold higher concentrations of fungal
rennet were claimed in comparison with five-stage counter-current extraction
[207]. However, it is difficult to reconcile a six-fold increase in concentration
with the fact that the leaching solution to solids ratios were similar, and the
percentage recoveries were similar.
8.4
Recovery by Direct Pressing
Enzymes can also be recovered by direct pressing. Hydraulic pressing without leachate addition was done by Roussos et al. [208]. They applied 220 bar
pressure for 1 min, then added a volume of water equal to the leachate volume
released in the first pressing, and repeated the pressing. They were able to
recover 80% of the water originally present in the solids on the first pressing, although the fraction of water retained will obviously depend on the
type of solid. After the second pressing, enzyme recoveries of 85–95% were
obtained. These recoveries are possible with the addition of relatively little
liquid. In total the ratio of final leachate to initial mass of fermented solids
(fresh weight) was only 1.2. Despite the apparent success of this method, and the
widespread use of industrial scale presses in the fruit processing industry, the
use of direct pressing to recover products from SSF has not received further
attention [208].
8.5
Recovery of Non-Volatile Small Organic Molecules
Small, non-volatile, organic molecules which have been produced in SSF include pigments, antibiotics, plant growth factors, and simple organic acids such
as lactic and citric acids. As for enzymes the only extraction methods available
are pressing and leaching. However, in contrast to the work done in the leaching
of enzymes, relatively little attention has been given to the extraction of these
small non-volatile organic molecules.
128
D.A. Mitchell et al.