Leaching solutions are likely to be similar to those used for extraction for the
purpose of assaying the product concentrations. Water has been used for
extraction of oxytetracycline, cephamycin C, citric acid, and lactic acid; 95%
ethanol has been used for extraction of Monascus pigment; both phosphate buffer
and ethyl acetate have been used for extraction of penicillin [209–215]. However,
since high concentrations in the extract are usually not crucial in the assays,
direct extraction is used, and therefore high leaching solution to solids ratios of
5:1 to 10:1 have been used. These are not appropriate for recovery during downstream processing since they lead to low product concentrations in the leachate.
Within this group of products the exception is gibberellic acid, for which
downstream processing has received some attention. Kumar and Lonsane [130]
pointed out that one of the reasons for exploring gibberellic acid production in
SSF is the low concentrations achieved within fermentation broths in SLF.
However, SSF processes would lose any advantages of high concentrations in the
fermented mass if large liquid volumes had to be used for extraction. Therefore
they used a four-stage countercurrent leaching system, which allows low
leaching solution to solids ratios. Before extraction, the fermented bran was
dried at 50 °C. The loss of gibberellic acid during this drying was negligible, not
exceeding 5% of the total. Extraction was best at low pH values, being optimal
at the lowest value tested, pH 2.5. Aqueous solutions of methanol, ethanol,
acetone, and ethyl acetate were tested. Water containing 10% ethanol gave the
highest degree of extraction. The use of four contact stages in a countercurrent
leaching system led to 87% recovery and a gibberellic acid concentration of
0.9 mg ml
–1 .
Supercritical fluid extraction has also been investigated for the removal of
undesirable co-products after production of gibberellic acid on wheat bran by
Gibberella fujikuroi in SSF [216]. In this case the gibberellic acid remained
within the solid mass, although the method can potentially be used for extraction of products. However, although supercritical fluid extraction has high
leaching efficiencies, it involves high capital and operating costs and therefore
will only be feasible for high value products [184].
8.6
Recovery of Volatile Products by Stripping
Volatile products such as ethanol and aroma compounds can potentially be
recovered during the fermentation by stripping them out of the solids with a
flowing gas stream.
In the case of ethanol, extraction with water is undesirable due to the dilution of the ethanol, whereas direct pressing gives poor yields [184]. Direct
distillation from the fermented solids performs relatively poorly, although
distillation is economically feasible if combined with animal feed production
from the solid wastes [217, 218]. As an alternative, forced gas circulation can be
used to strip ethanol from the substrate. This has been shown for a gas-solid
fluidized bed and for a stirred bed [90, 219, 220]. A further advantage is that
continuous stripping during the fermentation, rather than simply recovering
the ethanol at the end of the fermentation, avoids the product inhibition
Biochemical Engineering Aspects of Solid State Bioprocessing
129
purpose of assaying the product concentrations. Water has been used for
extraction of oxytetracycline, cephamycin C, citric acid, and lactic acid; 95%
ethanol has been used for extraction of Monascus pigment; both phosphate buffer
and ethyl acetate have been used for extraction of penicillin [209–215]. However,
since high concentrations in the extract are usually not crucial in the assays,
direct extraction is used, and therefore high leaching solution to solids ratios of
5:1 to 10:1 have been used. These are not appropriate for recovery during downstream processing since they lead to low product concentrations in the leachate.
Within this group of products the exception is gibberellic acid, for which
downstream processing has received some attention. Kumar and Lonsane [130]
pointed out that one of the reasons for exploring gibberellic acid production in
SSF is the low concentrations achieved within fermentation broths in SLF.
However, SSF processes would lose any advantages of high concentrations in the
fermented mass if large liquid volumes had to be used for extraction. Therefore
they used a four-stage countercurrent leaching system, which allows low
leaching solution to solids ratios. Before extraction, the fermented bran was
dried at 50 °C. The loss of gibberellic acid during this drying was negligible, not
exceeding 5% of the total. Extraction was best at low pH values, being optimal
at the lowest value tested, pH 2.5. Aqueous solutions of methanol, ethanol,
acetone, and ethyl acetate were tested. Water containing 10% ethanol gave the
highest degree of extraction. The use of four contact stages in a countercurrent
leaching system led to 87% recovery and a gibberellic acid concentration of
0.9 mg ml
–1 .
Supercritical fluid extraction has also been investigated for the removal of
undesirable co-products after production of gibberellic acid on wheat bran by
Gibberella fujikuroi in SSF [216]. In this case the gibberellic acid remained
within the solid mass, although the method can potentially be used for extraction of products. However, although supercritical fluid extraction has high
leaching efficiencies, it involves high capital and operating costs and therefore
will only be feasible for high value products [184].
8.6
Recovery of Volatile Products by Stripping
Volatile products such as ethanol and aroma compounds can potentially be
recovered during the fermentation by stripping them out of the solids with a
flowing gas stream.
In the case of ethanol, extraction with water is undesirable due to the dilution of the ethanol, whereas direct pressing gives poor yields [184]. Direct
distillation from the fermented solids performs relatively poorly, although
distillation is economically feasible if combined with animal feed production
from the solid wastes [217, 218]. As an alternative, forced gas circulation can be
used to strip ethanol from the substrate. This has been shown for a gas-solid
fluidized bed and for a stirred bed [90, 219, 220]. A further advantage is that
continuous stripping during the fermentation, rather than simply recovering
the ethanol at the end of the fermentation, avoids the product inhibition
Biochemical Engineering Aspects of Solid State Bioprocessing
129