However, the interparticle processes can be influenced by the manner in which
the substrate particles are prepared. For example, the smaller the particle size
the smaller the distance over which intraparticle mass transfer processes must
occur. Unfortunately, small particle sizes can decrease the efficiency of the
interparticle transfer processes.
Despite our very limited ability to influence these processes in the way we
operate the bioreactor, it is still essential to understand their influence on the
system. Understanding how and when microscale processes control process
performance can prevent unfruitful attempts to improve performance by manipulating the operational variables of the bioreactor. Such understanding might
point to more useful strategies. For example, for a process controlled by intraparticle mass transfer, it might be possible to disrupt barriers to diffusion
within the substrate particle, such as plant cell walls. Independently of these
reasons, characterization of at least some of the microscale phenomena is
necessary for the construction of appropriate expressions to include in macroscale material and energy balances.
Although the various microscale processes are all interrelated, as shown in
the diagram, they will be discussed one by one.
4.1
Microbial Growth in Response to its Environment
The important environmental variables which influence growth, and other
growth related activities such as release of hydrolytic enzymes and of products,
were described in detail by Prior et al. [76]. These variables include the concentrations of carbon and nitrogen sources, the oxygen concentration, product
concentrations, temperature, pH and water activity. This section shows approaches which have been used to quantify the effects of these key environmental conditions on growth.
The basic equation for growth is
dX
5 = mX
(1)
dt
where the question is essentially as to the effect of the environment on the value
of m.
4.1.1
Effects of Nutrients, Oxygen, and Biomass Concentrations on Growth
In mechanistic descriptions of the effect of nutrient concentrations on growth
in SSF, it is not valid to relate the specific growth rate with the nutrient concentrations obtained by mashing the substrate particles prior to analysis. The
mass transfer limitations within SSF particles cause concentration gradients of
nutrients to arise and therefore the actual nutrient concentrations experienced
by the organism are not equal to the average concentrations given by the
mashing procedure (Fig. 4) [83–85]. In fact, microbial cells or hyphal tips
82
D.A. Mitchell et al.
the substrate particles are prepared. For example, the smaller the particle size
the smaller the distance over which intraparticle mass transfer processes must
occur. Unfortunately, small particle sizes can decrease the efficiency of the
interparticle transfer processes.
Despite our very limited ability to influence these processes in the way we
operate the bioreactor, it is still essential to understand their influence on the
system. Understanding how and when microscale processes control process
performance can prevent unfruitful attempts to improve performance by manipulating the operational variables of the bioreactor. Such understanding might
point to more useful strategies. For example, for a process controlled by intraparticle mass transfer, it might be possible to disrupt barriers to diffusion
within the substrate particle, such as plant cell walls. Independently of these
reasons, characterization of at least some of the microscale phenomena is
necessary for the construction of appropriate expressions to include in macroscale material and energy balances.
Although the various microscale processes are all interrelated, as shown in
the diagram, they will be discussed one by one.
4.1
Microbial Growth in Response to its Environment
The important environmental variables which influence growth, and other
growth related activities such as release of hydrolytic enzymes and of products,
were described in detail by Prior et al. [76]. These variables include the concentrations of carbon and nitrogen sources, the oxygen concentration, product
concentrations, temperature, pH and water activity. This section shows approaches which have been used to quantify the effects of these key environmental conditions on growth.
The basic equation for growth is
dX
5 = mX
(1)
dt
where the question is essentially as to the effect of the environment on the value
of m.
4.1.1
Effects of Nutrients, Oxygen, and Biomass Concentrations on Growth
In mechanistic descriptions of the effect of nutrient concentrations on growth
in SSF, it is not valid to relate the specific growth rate with the nutrient concentrations obtained by mashing the substrate particles prior to analysis. The
mass transfer limitations within SSF particles cause concentration gradients of
nutrients to arise and therefore the actual nutrient concentrations experienced
by the organism are not equal to the average concentrations given by the
mashing procedure (Fig. 4) [83–85]. In fact, microbial cells or hyphal tips
82
D.A. Mitchell et al.