NADH and flavin fluorescence responses of starved yeast cultures to substrate additions

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Abstract
Model experiments were performed with starved yeast (Saccharomyces cerevisiae) cultures in a batch reactor in order to develop a better understanding of NAD(P)H and flavin culture fluorescence. Fluorescence was monitored during aerobic–anaerobic–aerobic transitions and ethanol and glucose substrate addition experiments. Interpretations of the fluorescence responses obtained are provided, with consideration given to redox compartmentation and the formation of ethanol shortly after a glucose addition. An analytical spectrofluorophotometer was interfaced to a personal computer and adapted to measure fluorescence in a bioreactor. This was achieved by the use of quartz fiber‐optic waveguides to convert the right‐angle cuvette geometry of the analytical spectrofluorophotometer to an open‐ended fluorescence probe geometry, resulting in a flexible culture fluorescence apparatus. Features of the apparatus include variable excitation and emission wavelengths, allowing for detection of NAD(P)H or flavin fluorescence, as well as small slit widths, a variable sampling rate, excitation and emission scanning capabilities, and good sensitivity.