system, glucose and oxygen are only available in the 100-mm layer at the surface
of the particle.
The importance of oxygen transfer in limiting growth has led several authors
to consider the measurement of the parameter K L a in solid state fermentation
systems. Durand et al. [123], adapting a method used in liquid culture, used sulfite oxidation rates to estimate K L a in a packed bed bioreactor. Gowthaman et
al. [124] estimated K L a simply on the basis of the inlet and outlet oxygen concentrations, although their results are questionable because they assumed that
the decrease in oxygen concentration within the liquid film was always 10% of
the saturation concentration, which will not be the case. More recently, Thibault
et al. [125] suggest that this direct application of K L a concepts from submerged
fermentation to SSF is conceptually incorrect. They point out the differences in
the geometries of SLF and SSF systems: in liquid culture there are thin static gas
and liquid films at the air-water interfaces of bubbles, there is assumed to be no
consumption of oxygen within this thin static liquid film, and there is convective flow on the water side, whereas in SSF there is no convection on the
liquid side, rather; as soon as the oxygen is transferred from the solid to the
liquid phase, the oxygen diffuses through and is consumed within a static biofilm. As a result, rather than using K L a to try to describe oxygen limitations
within SSF systems it is better to use a new term, the average biofilm conductance, K F a, where K F is the ratio of the dissolved oxygen diffusivity to the
thickness of the aerobic portion of the biofilm [125]. Note that this is slightly
different from K L a, which in SLF represents the oxygen diffusivity divided by
the thickness of the static liquid layer, where the major mass transfer resistance
is assumed to occur.
For mycelial growth in SSF, it is probable that those fungal hyphae exposed
directly to the air can take up oxygen directly from the air. In systems with
forced aeration it is unlikely that transfer of oxygen to this aerial biomass will
be limiting [116]. In this case nutrient movement within the aerial hyphae layer
may be the factor controlling growth. Diffusion is probably the mechanism for
translocation of at least some nutrients, such as glucose and orthophosphate,
within the hyphae of some fungi, including Rhizopus nigricans [126].
Nopharatana et al. [116] showed that the diffusion of glucose within the aerial
hyphae of a fungus has the potential to control the shape of the biomass
concentration profile against height above the surface and how this develops
over time. Of course, even for systems in which there is a significant amount of
aerial mycelium, there will be hyphae in the liquid film at the substrate particle
surface, and hyphae penetrating into the substrate. This region of the biomass
can quickly suffer from oxygen limitation, so at least some of the biomass in SSF
will always be under oxygen limitation.
In summary, intraparticle oxygen limitation is an intrinsic property of SSF
systems, even if operation of a bioreactor enables high concentrations to be
maintained in the interparticle air spaces [122]. At least some of the biomass
will have little oxygen available for the majority of the fermentation. Of course
this is one of the reasons why the density of penetrative hyphae quickly falls
with depth below the particle surface [109].
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D.A. Mitchell et al.
of the particle.
The importance of oxygen transfer in limiting growth has led several authors
to consider the measurement of the parameter K L a in solid state fermentation
systems. Durand et al. [123], adapting a method used in liquid culture, used sulfite oxidation rates to estimate K L a in a packed bed bioreactor. Gowthaman et
al. [124] estimated K L a simply on the basis of the inlet and outlet oxygen concentrations, although their results are questionable because they assumed that
the decrease in oxygen concentration within the liquid film was always 10% of
the saturation concentration, which will not be the case. More recently, Thibault
et al. [125] suggest that this direct application of K L a concepts from submerged
fermentation to SSF is conceptually incorrect. They point out the differences in
the geometries of SLF and SSF systems: in liquid culture there are thin static gas
and liquid films at the air-water interfaces of bubbles, there is assumed to be no
consumption of oxygen within this thin static liquid film, and there is convective flow on the water side, whereas in SSF there is no convection on the
liquid side, rather; as soon as the oxygen is transferred from the solid to the
liquid phase, the oxygen diffuses through and is consumed within a static biofilm. As a result, rather than using K L a to try to describe oxygen limitations
within SSF systems it is better to use a new term, the average biofilm conductance, K F a, where K F is the ratio of the dissolved oxygen diffusivity to the
thickness of the aerobic portion of the biofilm [125]. Note that this is slightly
different from K L a, which in SLF represents the oxygen diffusivity divided by
the thickness of the static liquid layer, where the major mass transfer resistance
is assumed to occur.
For mycelial growth in SSF, it is probable that those fungal hyphae exposed
directly to the air can take up oxygen directly from the air. In systems with
forced aeration it is unlikely that transfer of oxygen to this aerial biomass will
be limiting [116]. In this case nutrient movement within the aerial hyphae layer
may be the factor controlling growth. Diffusion is probably the mechanism for
translocation of at least some nutrients, such as glucose and orthophosphate,
within the hyphae of some fungi, including Rhizopus nigricans [126].
Nopharatana et al. [116] showed that the diffusion of glucose within the aerial
hyphae of a fungus has the potential to control the shape of the biomass
concentration profile against height above the surface and how this develops
over time. Of course, even for systems in which there is a significant amount of
aerial mycelium, there will be hyphae in the liquid film at the substrate particle
surface, and hyphae penetrating into the substrate. This region of the biomass
can quickly suffer from oxygen limitation, so at least some of the biomass in SSF
will always be under oxygen limitation.
In summary, intraparticle oxygen limitation is an intrinsic property of SSF
systems, even if operation of a bioreactor enables high concentrations to be
maintained in the interparticle air spaces [122]. At least some of the biomass
will have little oxygen available for the majority of the fermentation. Of course
this is one of the reasons why the density of penetrative hyphae quickly falls
with depth below the particle surface [109].
94
D.A. Mitchell et al.