Transient behaviour of baker's yeast during enforced periodical variation of dissolved oxygen concentration

The aim of the investigation was to find out the influence of the variation of the dissolved oxygen concentration in the microenvironment of yeast cells on their physiological behaviour in small laboratory reactors and estimate their behaviour in large industrial reactors. Since the morphology of the laboratory and industrial yeasts differed considerably, their transient behaviour was investigated and compared. For this purpose, the strain Saccharomyces cerevisiae H620 and an industrial strain were cultivated on synthetic as well as on complex medium in batch operation during periodical variation of the dissolved oxygen concentration, monitoring the most important key parameters. Also the yeast was cultivated in batch as well as in continuous operation, the cell-containing culture medium was recirculated through a nonaerated loop at different recirculation rates (residence times of the cells in the loop), and the key operation variables were monitored. It was found that the transient behaviour of laboratory and industrial yeasts differed slightly. Since cells growing in batch culture are more sensitive to dissolved oxygen concentration variation than cells growing in continuous culture, the transient behavior of cells cultivated in batch operation varied from those in continuous operation. If the anaerobic phase was longer than 1 min, ethanol was produced. However, it was consumed during the aerobiosis again, provided that phase was considerably longer than the anaerobic phase. This means that the yeast cultivation was not influenced by the periodic operation of the dissolved oxygen. Judging from the measurements in large stirred tank and airlift tower loop reactors, in general, the cells would spend more time in the aerobic than in the anaerobic flow region, and they would spend less than 1 min in the anaerobic flow region. Therefore, no considerable effect of the periodically varied dissolved oxygen concentration on the cell cultivation can be expected in large-scale reactors. In the stirred tank-loop-system at high pumping rates/high frequencies of periodically varied dissolved oxygen concentration, unexpectedly, the formation of ethanol was observed, which might be caused by stress imposed on the cells.