"step up cultures" should not be necessary if there is dissolved oxygen (DO) available to the cells. Yeast cells need molecular oxygen to synthesize ergosterol for their membrane, which is the equivalent of cholesterol in ours. The biochemical pathway for ergosterol synthesis is separate from that for respiration. Therefore, even if growing by fermentation because of the Crabtree effect, there should be more than enough DO to supply the needs of ergosterol synthesis. In fact, there is usually sufficient ergosterol in aerobically-grown starter cultures to last for several generations before it becomes too dilute following partitioning into daughter cell membranes. As there is some dissolved oxygen in most growth media, there may be enough to supply cells growing under anaerobic conditions for several generations more, so long as it is not stripped by e.g. gas sparging (e.g. N2, CO2) or excessive agitation.

I too agree with Michael, that step up cultures are not necessary. Oxygen can be provided sufficiently during addition of medium or inoculum. That DO is sufficient for sterol/membrane synthesis. I am quite sure that there is no need for stripping of DO by any means. Even when the organism is growing under anaerobic conditions it makes no difference. The organism will never be under stress due to lack of ergosterol. It may be under stress due to accumulation of some other metabolite and the growth might slow down due to this factor.

You have to differentiate between fermentation caused by the Crabtree effect and that caused by lack of dissolved oxygen. So, if the yeast is growing without aeration, as in alcoholic beverage production, the CO2 production will normally prevent the entry of air and can even strip dissolved oxygen. Therefore, after several generations without dissolved oxygen, and thus without ergosterol synthesis, the daughter cells can end up with insufficient ergosterol in the cytoplasmic membrane. They will be stressed and may die.

Thanks for the informative discussion! Much appreciated...

Dear Michael:

In case of alcoholic beverage fermentation, such a phenomenon is pretty rare. This is primarily because these are invariably batch fermentation. This might hold true for SCP production like in bakers yeast fermentation, where one looks for healthy yeast cells capable of producing good amount of carbon dioxide. But then it is not only a fed batch system, but also an aerobic process. So in either way such a stres on daughter cells of the yeast culture is very rare, unless the person incharge of the fermentation is truly inexperienced.

any batch fermentation in flask after some period of time results in oxygen limitation and slow down growth rate of the cells, it is easy to see by taking a sample and measuring OD (Optical Density), after inoculation you will see exponential growth and when growth becomes linear that the time of oxygen limitation, the best chart to see it is Ln (OD) vs time, the exponential growth will be linear in this chart and the linear growth will start after that. Typically if you know the final OD then the exponential growth is around 30-40% of the maximum OD.