Trichoderma reesei could be increased by inoculating with Candida utilis after
72 h, at which time the fungus had ceased growth but had not utilized all the
sugars [68]. These excess sugars provided the substrate for growth of the yeast.
Adding C. utilis at the beginning of the fermentation was less effective because
it competed with the fungus for the sugars, retarding fungal growth and cellulase production. In other work, a novel method of sequential culture with five
different fungal strains cultured successively on rice straw and wheat bran gave
higher cellulase yields than monoculture [69].
In undefined mixed culture either the natural microflora of the substrate is
allowed to develop or the substrate is inoculated with a mixed inoculum
prepared by traditional methods. This type of process relies on the fermentation conditions to select for the desired microbial groups. Such processes are
commonly used for traditionally prepared fermented foods and for waste treatment.
1.3
A General SSF Process
Solid-state fermentation processes involve the same general steps as are involved in SLF, namely upstream processing of substrate and inoculum, fermentation, and downstream processing and waste disposal, although there are differences in how some of the steps are carried out (Fig. 2). A typical process has
the following features:
1. The substrate is usually of agricultural origin, and might be grains such as
rice, tubers such as cassava, processed materials such as meals (e.g., corn
meal) or byproducts such as straw. The carbon source is usually a macromolecule, such as starch or cellulose, although some solid substrates contain
significant amounts of soluble sugars.
2. The substrate may require preparation or pretreatment such as chopping or
grinding to reduce particle size, cracking to make the interior of the particles
more accessible, cooking or chemical hydrolysis to increase the susceptibility
of the macromolecules to degradation by microbial enzymes during the
fermentation, and sterilization or pasteurization to eliminate or reduce
contaminants present in the substrate.
3. The microorganism is usually a filamentous fungus, requiring aerobic conditions for growth and product formation. Spore inocula are commonly used
because they are easy to prepare and are reasonably stable. Several culturing
steps may be required to grow up sufficient amounts of inoculum from the
stock culture.
4. The inoculum is mixed into the substrate to initiate the fermentation.
5. The bioreactor may or may not allow for mixing and aeration. If air is supplied it needs to be pretreated before entering the bioreactor to control the
temperature and relative humidity, and treated upon leaving the bioreactor
to prevent indiscriminate spreading of the process organism into the
environment. One of the major challenges in operating the bioreactor is to
prevent undesirable temperature rises in the fermenting substrate bed due to
the production of waste metabolic heat by the microorganism.
Biochemical Engineering Aspects of Solid State Bioprocessing
73
72 h, at which time the fungus had ceased growth but had not utilized all the
sugars [68]. These excess sugars provided the substrate for growth of the yeast.
Adding C. utilis at the beginning of the fermentation was less effective because
it competed with the fungus for the sugars, retarding fungal growth and cellulase production. In other work, a novel method of sequential culture with five
different fungal strains cultured successively on rice straw and wheat bran gave
higher cellulase yields than monoculture [69].
In undefined mixed culture either the natural microflora of the substrate is
allowed to develop or the substrate is inoculated with a mixed inoculum
prepared by traditional methods. This type of process relies on the fermentation conditions to select for the desired microbial groups. Such processes are
commonly used for traditionally prepared fermented foods and for waste treatment.
1.3
A General SSF Process
Solid-state fermentation processes involve the same general steps as are involved in SLF, namely upstream processing of substrate and inoculum, fermentation, and downstream processing and waste disposal, although there are differences in how some of the steps are carried out (Fig. 2). A typical process has
the following features:
1. The substrate is usually of agricultural origin, and might be grains such as
rice, tubers such as cassava, processed materials such as meals (e.g., corn
meal) or byproducts such as straw. The carbon source is usually a macromolecule, such as starch or cellulose, although some solid substrates contain
significant amounts of soluble sugars.
2. The substrate may require preparation or pretreatment such as chopping or
grinding to reduce particle size, cracking to make the interior of the particles
more accessible, cooking or chemical hydrolysis to increase the susceptibility
of the macromolecules to degradation by microbial enzymes during the
fermentation, and sterilization or pasteurization to eliminate or reduce
contaminants present in the substrate.
3. The microorganism is usually a filamentous fungus, requiring aerobic conditions for growth and product formation. Spore inocula are commonly used
because they are easy to prepare and are reasonably stable. Several culturing
steps may be required to grow up sufficient amounts of inoculum from the
stock culture.
4. The inoculum is mixed into the substrate to initiate the fermentation.
5. The bioreactor may or may not allow for mixing and aeration. If air is supplied it needs to be pretreated before entering the bioreactor to control the
temperature and relative humidity, and treated upon leaving the bioreactor
to prevent indiscriminate spreading of the process organism into the
environment. One of the major challenges in operating the bioreactor is to
prevent undesirable temperature rises in the fermenting substrate bed due to
the production of waste metabolic heat by the microorganism.
Biochemical Engineering Aspects of Solid State Bioprocessing
73