22
J. Gomes and A.S. Menawat
feed concentration of 125 g 1- l, the carbon resource supply is below the quantity
need to sustain high spectinomycin productivity.
In the diametrically opposite case, for glucose feed concentrations of
175 g 1-1 and 200 g 1-~, the residual glucose concentrations are, 1.16 g 1-1 and
1.58 g1-1, respectively, whereas, the corresponding glucose uptake rates are
5.0 g h-1 and 5.17 g h-x. An examination of the residual glucose concentration
profiles show that an increasing trend towards the later part of the bioproduction contributes to the higher residual glucose concentrations. Although the
glucose uptake rate increased significantly, the residual glucose concentration
remained nearly the same. Therefore, it appears that a glucose feed concentration in excess of that required for the production of spectinomycin increases the
flux of the carbon resource in alternative pathways. In this process the microorganism would utilize more energy in alternate pathways thereby resulting in
a lower yield of spectinomycin.
An analysis of the profiles indicates that the increase in the residual glucose
concentration towards the end of production contributes significantly to the
value of 31.3 g1-1 for the 400g1-1 glucose feed concentration. Table 1 also
shows that the dual saturation profile in spectinomycin synthesis observed for
other glucose feed concentrations are not apparent in this case. However, the
glucose uptake rate is as high as 9.3 g h- 1. Since the yield of spectinomycin in
this case is only 173 units, the carbon could be channelized towards the
synthesizing of cell material or towards the accumulation of intermediates. We
speculate that a distinctly different metabolic behavior occur for high glucose
feed concentrations.
Therefore excluding the results for the 400 g 1-1 glucose feed concentration,
the highest time-weighted average residual glucose concentration of 1.67 g 1-1
occurs for the optimum glucose feed concentration of 150 g 1-1. However, the
absolute value of 3-3.5 gl-1 is more important. Menawat 1-48] found that the
residual glucose concentration of 2.1 g 1-1 gives the highest productivity of
spectinomycin. From the residual glucose profile it appears that if the glucose
concentration in the bioreactor is maintained at the higher level throughout the
production phase, the highest yield of spectinomycin will be obtained. To
achieve this it may be necessary to implement a variable glucose feeding strategy
during the production phase.
4.4 Variations in Air Flow Rate
The air flow rate is continuously manipulated to meet the changing oxygen
needs of the microorganism. Consequently, the air flow rate profile reflects the
variations in oxygen demand during the course of bioproduction. The oxygen
requirement is the highest during the exponential growth phase. During the
transition from growth to production of spectinomycin, the air flow rate gradually decreases. It reaches a new value which remains nearly constant during the
production phase. Compressed air was supplied to the bioreactor at a constant
J. Gomes and A.S. Menawat
feed concentration of 125 g 1- l, the carbon resource supply is below the quantity
need to sustain high spectinomycin productivity.
In the diametrically opposite case, for glucose feed concentrations of
175 g 1-1 and 200 g 1-~, the residual glucose concentrations are, 1.16 g 1-1 and
1.58 g1-1, respectively, whereas, the corresponding glucose uptake rates are
5.0 g h-1 and 5.17 g h-x. An examination of the residual glucose concentration
profiles show that an increasing trend towards the later part of the bioproduction contributes to the higher residual glucose concentrations. Although the
glucose uptake rate increased significantly, the residual glucose concentration
remained nearly the same. Therefore, it appears that a glucose feed concentration in excess of that required for the production of spectinomycin increases the
flux of the carbon resource in alternative pathways. In this process the microorganism would utilize more energy in alternate pathways thereby resulting in
a lower yield of spectinomycin.
An analysis of the profiles indicates that the increase in the residual glucose
concentration towards the end of production contributes significantly to the
value of 31.3 g1-1 for the 400g1-1 glucose feed concentration. Table 1 also
shows that the dual saturation profile in spectinomycin synthesis observed for
other glucose feed concentrations are not apparent in this case. However, the
glucose uptake rate is as high as 9.3 g h- 1. Since the yield of spectinomycin in
this case is only 173 units, the carbon could be channelized towards the
synthesizing of cell material or towards the accumulation of intermediates. We
speculate that a distinctly different metabolic behavior occur for high glucose
feed concentrations.
Therefore excluding the results for the 400 g 1-1 glucose feed concentration,
the highest time-weighted average residual glucose concentration of 1.67 g 1-1
occurs for the optimum glucose feed concentration of 150 g 1-1. However, the
absolute value of 3-3.5 gl-1 is more important. Menawat 1-48] found that the
residual glucose concentration of 2.1 g 1-1 gives the highest productivity of
spectinomycin. From the residual glucose profile it appears that if the glucose
concentration in the bioreactor is maintained at the higher level throughout the
production phase, the highest yield of spectinomycin will be obtained. To
achieve this it may be necessary to implement a variable glucose feeding strategy
during the production phase.
4.4 Variations in Air Flow Rate
The air flow rate is continuously manipulated to meet the changing oxygen
needs of the microorganism. Consequently, the air flow rate profile reflects the
variations in oxygen demand during the course of bioproduction. The oxygen
requirement is the highest during the exponential growth phase. During the
transition from growth to production of spectinomycin, the air flow rate gradually decreases. It reaches a new value which remains nearly constant during the
production phase. Compressed air was supplied to the bioreactor at a constant