4.1.3
Effect of pH on Growth
Microbial growth is usually significantly affected by the local pH. This has important consequences for SSF processes since pH gradients arise within the
substrate particles, and although pH correcting solutions can be added to the
substrate in mist form with some bioreactor designs, fine pH control in SSF is
impracticable [84]. Models have been proposed which relate growth rate to pH
based on empirical equations describing experimental data for the effect of pH
on growth rate [102, 103]. However, the diffusion of protons within the substrate
particles was not included in the equations, which raises questions as to the
validity of such models, since the microbial growth response will depend on
local pH and not some global average.
4.1.4
Effect of Water Activity on Growth
Relatively little effort has been made to incorporate the effects of water activity within bioreactor models, and it is done by using empirical expressions
to obtain an expression for m w . Pitt [102] assumed that the specific growth
rate fell linearly between an a w of 1 and the minimum a w for growth while
Muck et al. [103] broke the curve into several linear segments. Data for the
effect of water activity on the growth of fungi is commonly obtained by
measuring the effect on the radial extension rate of colonies. Assuming that
the width of the peripheral growth zone of the colonies is constant despite
variations in water activity, then the radial extension rate will be directly
proportional to the specific growth rate [104], and therefore such data can be
adapted for SSF.
In colony extension rate data there is typically an optimum water activity at
a value somewhat less than a w =1, with the radial extension rate falling off
exponentially as the water activity decreases below this optimum [105, 106].
The following equation describes the exponential part of the curve:
a w
r = A ln 51 + r m
(11)
a w0
where A is a constant, and r m is the radial extension rate at the optimal water
activity for growth, a w0 .
4.1.5
Combining Environmental Effects
The simultaneous variation of various environmental variables within SSF
systems raises the question of how their effects on growth should be combined.
The best idea is to identify a value for m max , the specific growth rate at the
optimum combination of T, pH, water activity, and with saturating nutrient
concentrations. The relative effects of the different environmental variables can
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D.A. Mitchell et al.
Effect of pH on Growth
Microbial growth is usually significantly affected by the local pH. This has important consequences for SSF processes since pH gradients arise within the
substrate particles, and although pH correcting solutions can be added to the
substrate in mist form with some bioreactor designs, fine pH control in SSF is
impracticable [84]. Models have been proposed which relate growth rate to pH
based on empirical equations describing experimental data for the effect of pH
on growth rate [102, 103]. However, the diffusion of protons within the substrate
particles was not included in the equations, which raises questions as to the
validity of such models, since the microbial growth response will depend on
local pH and not some global average.
4.1.4
Effect of Water Activity on Growth
Relatively little effort has been made to incorporate the effects of water activity within bioreactor models, and it is done by using empirical expressions
to obtain an expression for m w . Pitt [102] assumed that the specific growth
rate fell linearly between an a w of 1 and the minimum a w for growth while
Muck et al. [103] broke the curve into several linear segments. Data for the
effect of water activity on the growth of fungi is commonly obtained by
measuring the effect on the radial extension rate of colonies. Assuming that
the width of the peripheral growth zone of the colonies is constant despite
variations in water activity, then the radial extension rate will be directly
proportional to the specific growth rate [104], and therefore such data can be
adapted for SSF.
In colony extension rate data there is typically an optimum water activity at
a value somewhat less than a w =1, with the radial extension rate falling off
exponentially as the water activity decreases below this optimum [105, 106].
The following equation describes the exponential part of the curve:
a w
r = A ln 51 + r m
(11)
a w0
where A is a constant, and r m is the radial extension rate at the optimal water
activity for growth, a w0 .
4.1.5
Combining Environmental Effects
The simultaneous variation of various environmental variables within SSF
systems raises the question of how their effects on growth should be combined.
The best idea is to identify a value for m max , the specific growth rate at the
optimum combination of T, pH, water activity, and with saturating nutrient
concentrations. The relative effects of the different environmental variables can
88
D.A. Mitchell et al.