Here is a topic I have been pondering...
I have read that (i) membrane sterols are a rate limiting factor for yeast growth/division/budding, (ii) the production of sterols by the yeast require oxygen, and thus sterols are produced prior to fermentation. The rationale for doing step up starter cultures is to maximize sterol production (or minimize the limitation of insufficient sterols), and thus generate healthier yeast. At the same time, final cell number has a peculiar relationship to inoculation rate: when fewer yeast are pitched, the final number is greater (cf growth charts).
I know there are lengthy threads on aeration of starters on stir plates, etc. It seems to me that sterols should only be limiting if an airlock is used for a starter culture (i.e. sterols will be made from the dissolved oxygen & the yeast will begin to ferment in an anaerobic environment). In a non-airlocked starter, the yeast will be growing under aerobic conditions (not to re-open the previous debate, but there should be adequate oxygen diffusion from the atmosphere to provide oxygen for sterol production with a simple foil covered or foam stoppered Erlenmeyer...).
So taking all of this together, are step up starters a relic of using airlocks, where oxygen would be limiting? Given that smaller pitching rates yield greater final cell counts, it seems to me that if you are not using an airlock, it would be more advantageous to start with a small starter pitch to get a better yield. If this is the case, why bother with the step ups?
I have read that (i) membrane sterols are a rate limiting factor for yeast growth/division/budding, (ii) the production of sterols by the yeast require oxygen, and thus sterols are produced prior to fermentation. The rationale for doing step up starter cultures is to maximize sterol production (or minimize the limitation of insufficient sterols), and thus generate healthier yeast. At the same time, final cell number has a peculiar relationship to inoculation rate: when fewer yeast are pitched, the final number is greater (cf growth charts).
I know there are lengthy threads on aeration of starters on stir plates, etc. It seems to me that sterols should only be limiting if an airlock is used for a starter culture (i.e. sterols will be made from the dissolved oxygen & the yeast will begin to ferment in an anaerobic environment). In a non-airlocked starter, the yeast will be growing under aerobic conditions (not to re-open the previous debate, but there should be adequate oxygen diffusion from the atmosphere to provide oxygen for sterol production with a simple foil covered or foam stoppered Erlenmeyer...).
So taking all of this together, are step up starters a relic of using airlocks, where oxygen would be limiting? Given that smaller pitching rates yield greater final cell counts, it seems to me that if you are not using an airlock, it would be more advantageous to start with a small starter pitch to get a better yield. If this is the case, why bother with the step ups?