In situ fluorescence cell mass measurements of Saccharomyces cerevisiae using cellular tryptophan

This work describes a new spectroscopic optical fiber/rod technique for in situ real time measurement of cell mass and product concentrations in bioreactors using intrinsic fluorescence. The variable excitation/emission wavelength capability of this sensor allows for species-selective measurement during fermentations. Cell mass (tryptophan) and product concentrations (pyridoxine) have been measured during fermentations of Saccharomyces cerevisiae. The effects of varying substrate concentration and oxygen concentration on the observed cell mass signals are eliminated by direct measurement of cell mass, as opposed to indirect measurement schemes such as those using NADH fluorescence. The sensor is robust and able to undergo many cycles of in situ steam sterilization without degradation, and its fluorescence signal is linear with concentration for all species studied in this work. Tryptophan fluorescence from yeast is shown to be a better measure of cell mass than NADH fluorescence.