X
biomass
V
variable
W
water
e
void fraction within the bed (dimensionless)
l
heat of vaporization of water (J kg –1 )
m
specific growth rate (h –1 )
m max maximum specific growth rate (h –1 )
m To
frequency factor for growth (h –1 )
r, Ç
density (kg m –3 )
1
Definition and Applications
Fermentation processes can be divided generally into submerged and solidstate fermentations. The major difference between these two bioprocesses is the
amount of free liquid in the substrate. Solid-state fermentation (SSF) involves
the growth of microorganisms on moist solid substrate particles in the absence
or near absence of visible liquid water between the particles. Of course
microbes need water for growth. In SSF systems they obtain water from the
moisture held within the substrate particles. The growth medium within a
bioreactor consists of a bed containing many moist solid substrate particles, the
water content of which could conceivably range anywhere from 12 wt%, below
which biological activity does not occur, up to the maximum water-holding
capacity of the solid, which in some cases is as high as 80 wt%, although typical
water contents are near the middle of this range [1]. In contrast, the nutrient
broth in a typical submerged liquid fermentation (SLF) might contain around
50 g l –1 of solutes, and therefore has a water content of 95 wt%, and appears and
behaves reasonably similarly to pure water. Figure 1 gives an overview of the
most obvious differences between SSF and SLF systems, while Table 1 gives a
more detailed comparison [2].
There is not a sharp boundary between SSF and SLF. First, the water content
at which liquid water appears between the substrate particles is a function of
the absorbency of the material, and this varies amongst the various solid
substrates used in SSF processes, such as wood chips, grains, and meals. Second,
as one increases the water content of the substrate past the maximum waterholding capacity, the system would first be considered a slurry, and later a
suspension of insoluble solids. There is no direct transition from SSF to SLF.
Many SSF processes involve the utilization of polymeric carbon and energy
sources, which might be present in the substrate bound within relatively complex structures. They may only become accessible to the microorganism after
the degradation or penetration of cell walls, for example. In contrast, most SLF
processes involve soluble monomeric nutrients, and even when polymers are
used they are usually either soluble or at least well dispersed. As a consequence,
in SLF the net amount of carbon and energy substrate accessible will typically
decline throughout the fermentation, but may either decline, increase, or
remain constant at different stages of growth with solid substrates [3].
Biochemical Engineering Aspects of Solid State Bioprocessing
65
biomass
V
variable
W
water
e
void fraction within the bed (dimensionless)
l
heat of vaporization of water (J kg –1 )
m
specific growth rate (h –1 )
m max maximum specific growth rate (h –1 )
m To
frequency factor for growth (h –1 )
r, Ç
density (kg m –3 )
1
Definition and Applications
Fermentation processes can be divided generally into submerged and solidstate fermentations. The major difference between these two bioprocesses is the
amount of free liquid in the substrate. Solid-state fermentation (SSF) involves
the growth of microorganisms on moist solid substrate particles in the absence
or near absence of visible liquid water between the particles. Of course
microbes need water for growth. In SSF systems they obtain water from the
moisture held within the substrate particles. The growth medium within a
bioreactor consists of a bed containing many moist solid substrate particles, the
water content of which could conceivably range anywhere from 12 wt%, below
which biological activity does not occur, up to the maximum water-holding
capacity of the solid, which in some cases is as high as 80 wt%, although typical
water contents are near the middle of this range [1]. In contrast, the nutrient
broth in a typical submerged liquid fermentation (SLF) might contain around
50 g l –1 of solutes, and therefore has a water content of 95 wt%, and appears and
behaves reasonably similarly to pure water. Figure 1 gives an overview of the
most obvious differences between SSF and SLF systems, while Table 1 gives a
more detailed comparison [2].
There is not a sharp boundary between SSF and SLF. First, the water content
at which liquid water appears between the substrate particles is a function of
the absorbency of the material, and this varies amongst the various solid
substrates used in SSF processes, such as wood chips, grains, and meals. Second,
as one increases the water content of the substrate past the maximum waterholding capacity, the system would first be considered a slurry, and later a
suspension of insoluble solids. There is no direct transition from SSF to SLF.
Many SSF processes involve the utilization of polymeric carbon and energy
sources, which might be present in the substrate bound within relatively complex structures. They may only become accessible to the microorganism after
the degradation or penetration of cell walls, for example. In contrast, most SLF
processes involve soluble monomeric nutrients, and even when polymers are
used they are usually either soluble or at least well dispersed. As a consequence,
in SLF the net amount of carbon and energy substrate accessible will typically
decline throughout the fermentation, but may either decline, increase, or
remain constant at different stages of growth with solid substrates [3].
Biochemical Engineering Aspects of Solid State Bioprocessing
65