Some comments on respiratory quotient (RQ) determination from the analysis of exit gas from a fermentor

Recently, the respiratory quotient (RQ) of microbes measured in situ in a fermentor by exit-gas analysis has been used successfully, for instance, in a fed-batch culture of baker's yeast as a criterion to control the feeding rate.(1-3) It is significant here to keep RQ values close to unity throughout; any deviations of RQ from unity give rise to deterioration of the cell growth yield.However easy it might be to keep RQ values around unity by controlling the feeding rate, the question of whether or not RQ values determined by gas analysis at the fermentor exit could generally represent those in vivo deserves attention. Indeed, for a fermentation carried out at an alkaline side, gas analysis would give RQ values that differ remarkably from true values because of the medium's "storage" of CO(2) released from microbes. The purpose of this communication is to make clear those factors that would affect true RQ values in the analysis of exit gas from a fermentor.