the quality and composition of such substrates may differ from batch to batch
since each plant grows in a particular microenvironment and has a particular
genetic make-up. Plants obtained from different harvests have experienced
different conditions, and their compositions can vary. The time of harvest and
the length and conditions of storage can also affect substrate composition.
Substrates arising from agricultural raw materials typically consist of several
macromolecules. A structural macromolecule may provide an inert matrix
within which the carbon source, such as starch molecules or soluble sugars, are
located, or the structural macromolecule itself may be the carbon source. The
complexity of such substrates can lead to phenomena such as nutrient limitation, substrate inhibition, catabolite repression, and complex patterns of induction and repression of extracellular enzymes. This can have consequences for
growth and sporulation.
Supplementation of the raw substrate may be required in order to stimulate
growth, induce enzyme synthesis, or prolong secondary metabolite production.
Most traditional food fermentations do not require nutritional supplementation. In cellulosic media, supplements of 0.5% of glucose or cellobiose, 0.5%
peptone, asparagine, or yeast extract are in use [75].
Nitrogen is an important nutrient in SSF. Many solid substrates are supplemented with soluble sources of nitrogen during substrate preparation. The
nitrogen source can play an important role in affecting the pH changes in the
substrate during the fermentation. The ammonium ion is taken up as ammonia,
thereby releasing a proton into the medium and causing a decrease in pH, a proton is taken up from the medium when nitrate is transported into the cell, and
this causes the pH to increase, while pH increases also occur when urea is deaminated. A combination of these nitrogen sources can be used to reduce the
pH changes during the fermentation [76, 77].
Some attention has been given to C:N ratios. However, note that overall C:N
ratios do not necessarily reflect the relative availabilities of the carbon and
nitrogen sources because the nutrients present are not necessarily equally accessible to the organism. Therefore optimal C:N ratios in SSF can potentially be
quite different from those found in SLF. Also, even though the substrate may
contain high levels of carbon, the provision of sufficient levels of the nitrogen
source at the start of the fermentation to enable complete utilization of the
carbon source can potentially cause substrate inhibition [78].
Natural solid substrates generally need some kind of pretreatment to make
their chemical constituents more accessible and their physical structure more
susceptible to mycelial penetration. Physical pretreatments involve chopping or
grinding to reduce particle size, and cracking to make the interior of the
particles more accessible. Chemical pretreatments such as high temperature
cooking with acid or alkali can disrupt intraparticle barriers to diffusion, and
may also be important in hydrolyzing macromolecules to produce soluble
nutrients.
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D.A. Mitchell et al.
since each plant grows in a particular microenvironment and has a particular
genetic make-up. Plants obtained from different harvests have experienced
different conditions, and their compositions can vary. The time of harvest and
the length and conditions of storage can also affect substrate composition.
Substrates arising from agricultural raw materials typically consist of several
macromolecules. A structural macromolecule may provide an inert matrix
within which the carbon source, such as starch molecules or soluble sugars, are
located, or the structural macromolecule itself may be the carbon source. The
complexity of such substrates can lead to phenomena such as nutrient limitation, substrate inhibition, catabolite repression, and complex patterns of induction and repression of extracellular enzymes. This can have consequences for
growth and sporulation.
Supplementation of the raw substrate may be required in order to stimulate
growth, induce enzyme synthesis, or prolong secondary metabolite production.
Most traditional food fermentations do not require nutritional supplementation. In cellulosic media, supplements of 0.5% of glucose or cellobiose, 0.5%
peptone, asparagine, or yeast extract are in use [75].
Nitrogen is an important nutrient in SSF. Many solid substrates are supplemented with soluble sources of nitrogen during substrate preparation. The
nitrogen source can play an important role in affecting the pH changes in the
substrate during the fermentation. The ammonium ion is taken up as ammonia,
thereby releasing a proton into the medium and causing a decrease in pH, a proton is taken up from the medium when nitrate is transported into the cell, and
this causes the pH to increase, while pH increases also occur when urea is deaminated. A combination of these nitrogen sources can be used to reduce the
pH changes during the fermentation [76, 77].
Some attention has been given to C:N ratios. However, note that overall C:N
ratios do not necessarily reflect the relative availabilities of the carbon and
nitrogen sources because the nutrients present are not necessarily equally accessible to the organism. Therefore optimal C:N ratios in SSF can potentially be
quite different from those found in SLF. Also, even though the substrate may
contain high levels of carbon, the provision of sufficient levels of the nitrogen
source at the start of the fermentation to enable complete utilization of the
carbon source can potentially cause substrate inhibition [78].
Natural solid substrates generally need some kind of pretreatment to make
their chemical constituents more accessible and their physical structure more
susceptible to mycelial penetration. Physical pretreatments involve chopping or
grinding to reduce particle size, and cracking to make the interior of the
particles more accessible. Chemical pretreatments such as high temperature
cooking with acid or alkali can disrupt intraparticle barriers to diffusion, and
may also be important in hydrolyzing macromolecules to produce soluble
nutrients.
76
D.A. Mitchell et al.