7.2
Application of Off-Line Measurement Techniques
Off-line measurements of state variables are more difficult to integrate into control schemes due to the typically long delays in sample analysis. However, they
are still crucial in post-fermentation analysis of the performance of the system,
and can be used in the validation of both predictive and interpretive models.
They may also be important to provide a check on on-line measurements, since
off-line methods typically suffer from less noise than on-line methods. The
most important off-line measurements are discussed below.
7.2.1
Measuring Water Content and Water Activity
Water content is much easier to measure than water activity. Most often the
sample is dried at 100–105 °C for 24 h or until constant weight is attained. Some
other methods as vacuum-oven drying, moisture evolution analysis, Karl
Fischer titration, gas chromatography, near infrared analysis, and nuclear
magnetic resonance could be also applied. Oven drying is the simplest and least
expensive.
A wide range of methods can be used to determine the water activity of
samples of solids removed from the bed, although electric hygrometers, in
which the solids are placed in an enclosed chamber with a sensor that measures
the equilibrium relative humidity, are the most convenient. Several commercial
electric hygrometers are available, each of which works on a different principle.
Hygrometers are usually accurate to within 2%, but different hygrometers have
different operating ranges.
7.2.2
Measuring pH
Flat-ended electrodes have been used to make off-line measurements directly
on the surfaces of solid substrates [174, 175]. However, off-line measurements
are usually made by measuring the pH of aqueous suspensions or extracts of
the solid sample.
7.2.3
Estimation of Biomass
Growth of biomass is typically of crucial importance in SSF processes, even
if the biomass itself is not the final product. The processes of nutrient consumption, oxygen consumption, carbon dioxide evolution, waste heat release,
and product formation are intimately related to the growth process and
therefore it is usually essential to characterize the growth curve. Unfortunately,
on-line biomass measurement is still impossible in SSF, despite efforts to
develop on-line sensors based on Fourier transform infrared spectroscopy
[176].
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D.A. Mitchell et al.
Application of Off-Line Measurement Techniques
Off-line measurements of state variables are more difficult to integrate into control schemes due to the typically long delays in sample analysis. However, they
are still crucial in post-fermentation analysis of the performance of the system,
and can be used in the validation of both predictive and interpretive models.
They may also be important to provide a check on on-line measurements, since
off-line methods typically suffer from less noise than on-line methods. The
most important off-line measurements are discussed below.
7.2.1
Measuring Water Content and Water Activity
Water content is much easier to measure than water activity. Most often the
sample is dried at 100–105 °C for 24 h or until constant weight is attained. Some
other methods as vacuum-oven drying, moisture evolution analysis, Karl
Fischer titration, gas chromatography, near infrared analysis, and nuclear
magnetic resonance could be also applied. Oven drying is the simplest and least
expensive.
A wide range of methods can be used to determine the water activity of
samples of solids removed from the bed, although electric hygrometers, in
which the solids are placed in an enclosed chamber with a sensor that measures
the equilibrium relative humidity, are the most convenient. Several commercial
electric hygrometers are available, each of which works on a different principle.
Hygrometers are usually accurate to within 2%, but different hygrometers have
different operating ranges.
7.2.2
Measuring pH
Flat-ended electrodes have been used to make off-line measurements directly
on the surfaces of solid substrates [174, 175]. However, off-line measurements
are usually made by measuring the pH of aqueous suspensions or extracts of
the solid sample.
7.2.3
Estimation of Biomass
Growth of biomass is typically of crucial importance in SSF processes, even
if the biomass itself is not the final product. The processes of nutrient consumption, oxygen consumption, carbon dioxide evolution, waste heat release,
and product formation are intimately related to the growth process and
therefore it is usually essential to characterize the growth curve. Unfortunately,
on-line biomass measurement is still impossible in SSF, despite efforts to
develop on-line sensors based on Fourier transform infrared spectroscopy
[176].
122
D.A. Mitchell et al.