2.2.1
Size and Shape of the Particles
Particle size and shape are extremely important. They affect the surface area to
volume ratio of the particle, the packing density within the substrate mass, and
the size and shape of void spaces between the particles. Small particles, or
particles with large flat surfaces, tend to pack together closely, making it difficult to aerate the substrate mass. If the microorganism can penetrate into the
particle this increases the directly accessible substrate and decreases the
distances over which diffusion needs to occur. In this case the optimal particle
size will be influenced by the depth of penetration. The optimal particle size
often represents a compromise between the accessibility of nutrients and the
availability of oxygen. Particle sizes from less than 1 mm to almost 1 cm have
often been used in SSF [79]. Many raw substrates, such as tubers or stalks,
require processing in order to achieve particle sizes appropriate for the
fermentation, which can be a costly operation. Methods for achieving this differ
depending on the substrate, but may include chopping, grinding, or rasping.
Particle size may be difficult to characterize exactly, especially if the particles
have irregular shapes. Possibly the best way to characterize particles is to
determine the characteristic length (the ratio of volume to surface area) since this
can be used to extrapolate between different shapes. Particle size distributions
can be done by passing the substrate through a series of meshes with different
aperture sizes, although this can be difficult with moist or sticky substrates, and
in any case will work most effectively for roughly cubic or spherical particles.
Consistency and strength are also important characteristics since particles
may deform due to the forces exerted by the agitator, or simply under the pressure of overlying substrate, causing the interparticle voids to disappear [79].
However, this aspect has received no attention, other than passing comments to
the effect that if agitation during the fermentation was too frequent the substrate became compacted.
2.2.2
Substrate Types
Regarding the main carbon source used by the microorganism, substrates can
be divided into three main groups: those that have starch, those which consist
mainly of cellulose or lignocellulose, and those with soluble sugars as the main
carbon source.
Starchy substrates that have been used in SSF processes include rice, cassava,
wheat bran, rice bran, buckwheat seeds, rice bran, corn meal, sweet potato
residue, and banana meal. Some of these substrates, such as rice, provide a
nutritionally complete medium for microbial growth whereas others require
addition of nutrients. For example, a nitrogen source must be added to cassava.
Starchy substrates may need pretreatment such as:
1. Boiling or steaming to gelatinize the starch, unless the organism has a good
potential to attack raw starch. On the large scale steaming is usually more
practical.
Biochemical Engineering Aspects of Solid State Bioprocessing
77
Size and Shape of the Particles
Particle size and shape are extremely important. They affect the surface area to
volume ratio of the particle, the packing density within the substrate mass, and
the size and shape of void spaces between the particles. Small particles, or
particles with large flat surfaces, tend to pack together closely, making it difficult to aerate the substrate mass. If the microorganism can penetrate into the
particle this increases the directly accessible substrate and decreases the
distances over which diffusion needs to occur. In this case the optimal particle
size will be influenced by the depth of penetration. The optimal particle size
often represents a compromise between the accessibility of nutrients and the
availability of oxygen. Particle sizes from less than 1 mm to almost 1 cm have
often been used in SSF [79]. Many raw substrates, such as tubers or stalks,
require processing in order to achieve particle sizes appropriate for the
fermentation, which can be a costly operation. Methods for achieving this differ
depending on the substrate, but may include chopping, grinding, or rasping.
Particle size may be difficult to characterize exactly, especially if the particles
have irregular shapes. Possibly the best way to characterize particles is to
determine the characteristic length (the ratio of volume to surface area) since this
can be used to extrapolate between different shapes. Particle size distributions
can be done by passing the substrate through a series of meshes with different
aperture sizes, although this can be difficult with moist or sticky substrates, and
in any case will work most effectively for roughly cubic or spherical particles.
Consistency and strength are also important characteristics since particles
may deform due to the forces exerted by the agitator, or simply under the pressure of overlying substrate, causing the interparticle voids to disappear [79].
However, this aspect has received no attention, other than passing comments to
the effect that if agitation during the fermentation was too frequent the substrate became compacted.
2.2.2
Substrate Types
Regarding the main carbon source used by the microorganism, substrates can
be divided into three main groups: those that have starch, those which consist
mainly of cellulose or lignocellulose, and those with soluble sugars as the main
carbon source.
Starchy substrates that have been used in SSF processes include rice, cassava,
wheat bran, rice bran, buckwheat seeds, rice bran, corn meal, sweet potato
residue, and banana meal. Some of these substrates, such as rice, provide a
nutritionally complete medium for microbial growth whereas others require
addition of nutrients. For example, a nitrogen source must be added to cassava.
Starchy substrates may need pretreatment such as:
1. Boiling or steaming to gelatinize the starch, unless the organism has a good
potential to attack raw starch. On the large scale steaming is usually more
practical.
Biochemical Engineering Aspects of Solid State Bioprocessing
77