The impact of fluid mechanical stress on Saccharomyces cerevisiae cells during continuous cultivation in an agitated, aerated bioreactor; its implication for mixing in he brewing process and aerobic fermentations

The introduction of mechanical agitation or re-circulation of CO2 from the headspace to facilitate the natural CO2-driven mixing in anaerobic brewing fermentations has the potential to cause damage to yeast cells through increased hydrodynamic stress. In addition, such damage has been reported in aerobic yeast fermentations. Here, a reproducible, duplicate series of aerobic, carbon-limited, continuous culture yeast fermentations have been carried out up to approximately 340 h. In the first series, mean specific energy dissipation rates, epsilon(T) from similar to4.5 x 10(-2) to similar to6.0 W kg(-1) were used, the former mimicking for approximately 20 h the maximum level found in anaerobic fermentations at the large scale and the latter a higher value than that usually found in aerobic fermentations. In the second, 4 was held at similar to4.5 x 10(-2) W kg(-1) and two aeration rates (1 and 3 vvm) were used. Both values are very high for brewing, even if headspace CO2 recirculation is used. The latter value is also high for aerobic fermentations. In all cases, the dissolved oxygen level was held constant by gas blending at 40% of saturation. At epsilon(T) > similar to4.5 W kg(-1), a transient effect on the cells as measured by multi-parameter flow cytometry was found, but all other measured parameters, including mass spectrometry and classical microbiological data, showed that, statistically, there was no effect on cellular morphology or physiology. For all other conditions of agitation and aeration, all measured parameters remained constant. This study indicates that the potentially deleterious effects of agitation or CO2 gas recirculation can be largely discounted with I respect to brewing fermentations and for more intense aerobic fermentations up to similar to6 W kg(-1) and 3 vvm.