Advanced Control of Bioprocesses

If an aerobic metabolism is dominant in a fermentor, the RQ value becomes 1.0 according to the following chemical equation. 

However, if an anaerobic metabolism such as ethanol fermentation occurs in a fermentor in the presence of high concentration of glucose, the RQ value becomes larger than 1.0, known as the Crabtree effect. This is because CO2 evolves without O2 consumption as follows. 

the RQ value can be used to estimate the metabolic status in an aerobic bioreactor. In practice, the RQ value can be determined based on the analysis of CO2 and O2 in the off-gas from a bioreactor. There may be difficulty in calculating the RQ due to the noise of measured data when the COj content in the off-gas is relatively low compared with the O2 content in the supply of air (20.91%). 

The RQ control, where the sugar-feeding rate is controlled so as to maintain the RQ value at approximately 1.0, is the distinct control method in aerobic fed-batch cultivation such as with baker s yeast production. By keeping the RQ at 1.0 during the fed-batch operation, cell production with a high yield will be achieved. 

When an NHg solution is used as a nitrogen source of fermentation, the consumption of NHg decreases the pH of the culture broth. A pH stat control utilizes the pH change as a process variable where the NHg addition is manipulated so as to keep the pH value constant during fermentation. The pH stat is often employed in a fed-batch cultivation of industrial glutamic acid production (high NHg consuming fermentation) using molasses as a feedstock. 

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