2. Grinding, rasping, or chopping to reduce particle size.
3. Granulation to convert a fine meal into particles several millimeters in diameter to avoid the tight packing which occurs with powders.
4. Cracking to damage the surface of rice or wheat grains to make the interior
more accessible.
Starchy substrates can suffer from problems of stickiness, which can cause substrate particles to agglomerate during the SSF process, especially if the substrate
bed is mixed. This excludes air and is detrimental to growth.
Various different processes have been investigated for the utilization of
lignocellulosic solid substrates such as wheat straw, corn and rice stover, wheat
bran, sugar beet pulp, and wood. Some processes have the aim of degrading
cellulose, and the microorganisms involved must be able to produce cellulolytic
enzymes. Effective cellulose hydrolysis requires the synergistic action of several
cellulases. Most studies emphasizing cellulose utilization have either aimed to
increase the protein content of the substrate for feeding to ruminants, or to
produce cellulases and other hydrolytic enzymes. Other processes aim to
degrade the lignin preferentially to increase the ability of ruminants to digest
the materials. Lignin peroxidase is the major enzyme involved in lignin
degradation. Lignin is unable to act as a sole carbon and energy source, so
lignolytic fungi also degrade some cellulose and hemicellulose.
Lignocellulosic substrates usually require significant pretreatment to disrupt
the structure of cellulose and lignin molecules within the substrate. Substrates
are often ground to particle sizes of 1–2 mm to increase the surface area for
attack and to disrupt cell walls. In addition, the crystalline structure of cellulose
may be disrupted by steaming under pressure. Sometimes the steaming is
carried out in conjunction with chemical pretreatment with acid or alkali.
Solid substrates containing significant amounts of soluble sugars include
grape pomace, sweet sorghum, sugar beet, pineapple waste, carob pods, and
coffee pulp. Another strategy is to impregnate inert solid materials such as
bagasse or hemp with soluble sugars in order to provide an SSF environment for
growth [37].
2.2.3
Substrate Sterilization
If the process conditions are highly selective for the process organism or the
process organism is extremely fast growing, then it may be possible to avoid a
sterilization step or simply to pasteurize the substrate. However, for many SSF
processes it is essential to sterilize the substrate.
During sterilization by heat there are two steps in the heat transfer process –
heat transfer to the particle surface and intraparticle heat transfer. Due to the
poor mixing characteristics of solid beds and the fact that intraparticle heat
transfer is limited to conduction, it is more problematic to ensure sterility of a
solid substrate than it is to ensure sterility of a liquid medium. In unmixed beds
it is highly likely that the effectiveness of the sterilization process will vary with
position.
78
D.A. Mitchell et al.
3. Granulation to convert a fine meal into particles several millimeters in diameter to avoid the tight packing which occurs with powders.
4. Cracking to damage the surface of rice or wheat grains to make the interior
more accessible.
Starchy substrates can suffer from problems of stickiness, which can cause substrate particles to agglomerate during the SSF process, especially if the substrate
bed is mixed. This excludes air and is detrimental to growth.
Various different processes have been investigated for the utilization of
lignocellulosic solid substrates such as wheat straw, corn and rice stover, wheat
bran, sugar beet pulp, and wood. Some processes have the aim of degrading
cellulose, and the microorganisms involved must be able to produce cellulolytic
enzymes. Effective cellulose hydrolysis requires the synergistic action of several
cellulases. Most studies emphasizing cellulose utilization have either aimed to
increase the protein content of the substrate for feeding to ruminants, or to
produce cellulases and other hydrolytic enzymes. Other processes aim to
degrade the lignin preferentially to increase the ability of ruminants to digest
the materials. Lignin peroxidase is the major enzyme involved in lignin
degradation. Lignin is unable to act as a sole carbon and energy source, so
lignolytic fungi also degrade some cellulose and hemicellulose.
Lignocellulosic substrates usually require significant pretreatment to disrupt
the structure of cellulose and lignin molecules within the substrate. Substrates
are often ground to particle sizes of 1–2 mm to increase the surface area for
attack and to disrupt cell walls. In addition, the crystalline structure of cellulose
may be disrupted by steaming under pressure. Sometimes the steaming is
carried out in conjunction with chemical pretreatment with acid or alkali.
Solid substrates containing significant amounts of soluble sugars include
grape pomace, sweet sorghum, sugar beet, pineapple waste, carob pods, and
coffee pulp. Another strategy is to impregnate inert solid materials such as
bagasse or hemp with soluble sugars in order to provide an SSF environment for
growth [37].
2.2.3
Substrate Sterilization
If the process conditions are highly selective for the process organism or the
process organism is extremely fast growing, then it may be possible to avoid a
sterilization step or simply to pasteurize the substrate. However, for many SSF
processes it is essential to sterilize the substrate.
During sterilization by heat there are two steps in the heat transfer process –
heat transfer to the particle surface and intraparticle heat transfer. Due to the
poor mixing characteristics of solid beds and the fact that intraparticle heat
transfer is limited to conduction, it is more problematic to ensure sterility of a
solid substrate than it is to ensure sterility of a liquid medium. In unmixed beds
it is highly likely that the effectiveness of the sterilization process will vary with
position.
78
D.A. Mitchell et al.