Agosin (1999)

YXC

Gibberella fujikuroi

0.19 kg-X kg-CO2-1

Perez-Correa and

YXO Aspergillus oryzae 1.06-1.16 Cmol-X mol O2- Nagel et al aIn this case biomass was measured indirectly on the basis of glucosamine. b Cmol stands for "carbon mol".

YXO Aspergillus oryzae 1.06-1.16 Cmol-X mol O2- Nagel et al aIn this case biomass was measured indirectly on the basis of glucosamine. b Cmol stands for "carbon mol".

17.2.4.1 Experimental Approach for Parameter Estimation

The first challenge is to measure the O2 uptake rate (OUR, mol-O2 h-1) experimentally. There are various possibilities. The most reliable method is to grow a culture within an enclosed headspace that is continuously aerated with a known flow rate of air and to measure the inlet and outlet O2 concentrations (Cin and Cout, respectively), as shown in Fig. 17.2. The OUR can then be calculated as:

The variables on the right hand side of Eq. (17.6) can have various different units, as long as the units used combine to give the correct units for the OUR. For example, F, the dry air flow rate, might originally be measured in L h-1, which will need to be converted to mol-dry-air h-1, taking into account the temperature and pressure of the air. Cin and Cout will typically be measured as volume percentages, in air that has been dried to prevent water from interfering with the measurement (which is important if a paramagnetic O2 analyzer is used). At the low pressures used in SSF processes, the air will behave as an ideal gas, and therefore the %(v/v) is also equal to the mol% of O2 in the gas. It is then simple to express Cin and Cout in terms of mol-O2 mol-dry-air-1 (i.e., dividing the %(v/v) by 100).

Another possibility is to remove a sample of a culture from a fermentation and place it in an enclosed headspace with an O2 electrode of the type that is capable of measuring O2 concentrations in a gas phase. It is also possible to undertake the culture in a sealed chamber and remove and analyze samples by gas chromatogra-phy, although in this case the O2 level in the chamber will decrease significantly during the growth cycle and this might influence growth.

17.2.4.2 Treatment of the Data for Parameter Estimation

If the experiments are done in a system that allows biomass measurement, it is possible to determine the values of YXO and mo. Once the growth kinetic equation has been determined from the biomass profile, both the integrated and differential forms of the kinetic equation can be substituted into Eq. (17.4) (the differential forms of various kinetic equations were presented in Table 16.1, while the integral forms of these equations were presented in Table 14.1). The resulting equation can be integrated to give an equation that gives the cumulative O2 uptake (the total mol of O2 consumed since growth commenced, called "COU') as a function of time. For example, if the organism shows logistic growth kinetics, then substitution of the differential form (Eq. 16.3) and the integral form (Eq. 14.6) into Eq. (17.4) and subsequent integration will give Ooijkaas et al. (2000):

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