In the future, SSF may play an important role in feeding the world’s population, which reached six billion during the year 1999, and is expected to reach 8
to 12 billion people during the twenty-first century. At the present time the
majority of the people in the developing world are vegetarians for economic
reasons. Already meat substitutes are being produced by spinning soybean
protein into fibers to give a meat-like texture. Production of microbial protein
could be a strategic alternative for cheap future food production, and SSF has
the potential to play a role here [5]. However, SSF technology must be improved
before this potential can be fulfilled. This review shows that, although significant advances have been made in the development of SSF technology over the
last decade, many further improvements are required.
1.1
Microbial Types
One of the most important features of SSF is the low availability of water in the
system. Water activities, which are more important in controlling microbial
growth than water content, are typically lower than those encountered in SLF.
The water activity (a w ) of the moist solid substrate can easily be measured: it is
the ratio of the vapor pressure of water above the substrate in a closed system
to the vapor pressure of pure water at the same temperature. The water activity
of the substrate is the measured relative humidity divided by 100.Values around
0.95–0.98 might be considered typical for solid substrates. These water activities are ideal for the growth of many fungi, especially filamentous fungi, which
typically grow optimally at water activities of 0.96–0.98, and still grow reasonably well at water activities as low as 0.9, and, as a result, the majority of SSF
processes involve filamentous fungi. In contrast many bacteria and yeasts grow
best at water activities around 0.99, with growth rapidly decreasing as the water
activity falls, and growth being completely inhibited at values around 0.9 [64].
Filamentous fungi have other features which give them advantages for SSF
processes over unicellular organisms. The mycelial growth form is ideally suited
to rapid colonization of the whole of a solid surface, which can later be followed
by an increase in density. Fungal hyphae can also cross regions of low nutrient
availability in search of nutrients, although the capability to do this does vary
from species to species. The hyphal growth mode also allows fungi to penetrate
into substrate particles, which may play an important role in degrading the
particle structure and making nutrients available. Furthermore, many filamentous fungi can produce a range of hydrolytic enzymes to degrade the
macromolecules found in solid substrates. Amylases and cellulases are the most
important enzymes for growth on solid substrates of agricultural origin,
although proteases and lipases aid in particle degradation and penetration. In
fact, it is for these reasons that fungi are very commonly found growing
naturally on solid materials in nature such as pieces of wood, leaves, and roots.
There are also a number of features of filamentous fungi which can be taken
advantage of within SSF processes. A number of filamentous fungi grow well at
low pH values, enabling the use in some SSF systems of a combination of low
water activity and low pH to create an environment which is favorable for
Biochemical Engineering Aspects of Solid State Bioprocessing
71
to 12 billion people during the twenty-first century. At the present time the
majority of the people in the developing world are vegetarians for economic
reasons. Already meat substitutes are being produced by spinning soybean
protein into fibers to give a meat-like texture. Production of microbial protein
could be a strategic alternative for cheap future food production, and SSF has
the potential to play a role here [5]. However, SSF technology must be improved
before this potential can be fulfilled. This review shows that, although significant advances have been made in the development of SSF technology over the
last decade, many further improvements are required.
1.1
Microbial Types
One of the most important features of SSF is the low availability of water in the
system. Water activities, which are more important in controlling microbial
growth than water content, are typically lower than those encountered in SLF.
The water activity (a w ) of the moist solid substrate can easily be measured: it is
the ratio of the vapor pressure of water above the substrate in a closed system
to the vapor pressure of pure water at the same temperature. The water activity
of the substrate is the measured relative humidity divided by 100.Values around
0.95–0.98 might be considered typical for solid substrates. These water activities are ideal for the growth of many fungi, especially filamentous fungi, which
typically grow optimally at water activities of 0.96–0.98, and still grow reasonably well at water activities as low as 0.9, and, as a result, the majority of SSF
processes involve filamentous fungi. In contrast many bacteria and yeasts grow
best at water activities around 0.99, with growth rapidly decreasing as the water
activity falls, and growth being completely inhibited at values around 0.9 [64].
Filamentous fungi have other features which give them advantages for SSF
processes over unicellular organisms. The mycelial growth form is ideally suited
to rapid colonization of the whole of a solid surface, which can later be followed
by an increase in density. Fungal hyphae can also cross regions of low nutrient
availability in search of nutrients, although the capability to do this does vary
from species to species. The hyphal growth mode also allows fungi to penetrate
into substrate particles, which may play an important role in degrading the
particle structure and making nutrients available. Furthermore, many filamentous fungi can produce a range of hydrolytic enzymes to degrade the
macromolecules found in solid substrates. Amylases and cellulases are the most
important enzymes for growth on solid substrates of agricultural origin,
although proteases and lipases aid in particle degradation and penetration. In
fact, it is for these reasons that fungi are very commonly found growing
naturally on solid materials in nature such as pieces of wood, leaves, and roots.
There are also a number of features of filamentous fungi which can be taken
advantage of within SSF processes. A number of filamentous fungi grow well at
low pH values, enabling the use in some SSF systems of a combination of low
water activity and low pH to create an environment which is favorable for
Biochemical Engineering Aspects of Solid State Bioprocessing
71