Effect of changes in the pH and carbon dioxide evolution rate on the measured respiratory quotient of fermentations

Abstract

The total concentration of dissolved carbon dioxide in fermentation broths is one to two orders of magnitude greater than that of oxygen for pH > 6.5. The rate of change in this total concentration can be sufficiently large to produce a discrepancy between the carbon dioxide transfer rate (CTR) across the gas-liquid interface, available from gas analyses, and the carbon dioxide evolution rate (CER) of biomass in the fermentor. The CER is the variable of most interest to fermentation technologists but cannot be measured directly. The CTR is commonly used to yield the measured respiratory quotient (called here the TQ, or transfer quotient). Evaluation of the real underlying respiratory quotient (RQ), however, requiures the unmeasureable CER. Equations defining the problem are presented and are found to accurately predict the discrepancy between the TQ and the RQ in fed-batch fermentations of Escherichia coli. During the exponential growth phase, the TQ is less than the RQ. A changing pH can cause the TQ to be bigger or smaller than the RQ, while pH fluctuations associated with on-off pH controller action make the CTR and hence the TQ noisy. The RQ is estimated on-line during an E. coli fermentation and is shown to be constant during the fermentation, even though the TQ varies greatly.