VincentGR said:
I also have the same issue and do not understand what limits the growth of yeast in a starter other then the absence of needed nutrient or the presence of to much by-products. Can you explain?
In a way, this is what stops further growth in starters or in full batches. In an underpitched batch, yeast will update O2 and bud as far as it can, then once it senses proper pH, convert to fermentation; but may not produce a result desired by the brewer. The same is true in starters. So, in both cases, optimal wort volumes for the desired result is important. This is easily observed in simply doing side by side fermentations.
The why is more complex. Yes, nutrients, pH and sugar availability contribute a great deal to growth and a healthy fermentation. However, it begins with the sterols stored by the mother yeast cell, before she buds daughter cells. Sterol storage happens during the late phases of fermentation in preparation for dormancy.
Yeast under constant growth has a sterol issue because eventually enough is spent in budding that the mother cell can no longer metabolize correctly. In the meantime, the early daughter cells have started strong, but later daughter cells have weaker cell walls.
Again, why? Yeast cant do much more than one process at a time, internally. They store byproducts outside the cell and reuptake what's needed, when needed. The sterols have to go through an esterification process (not to be confused with later esters in beer aromatics), and then yeast make an enzyme to convert these to ergosterol. It's the ergosterol that's vital to forming healthy cell walls that will process maltose, not just glucose; plus keep the important trace nutrients inside the cell. Imperfect cell walls limit the transfer of molecules and allow important molecules to leak out. The result is dead yeast and/or early dormancy.
In underpitched situations, additional O2 will help sterol synthesis, but only up to the point that cells can produce the enzymes. Then the cells convert to fermentation and dormancy in order to rebuild their sterol levels. At that point, the brewer needs to decant the yeast and place it in a fresh starter to continue cell growth.
A healthy starter goes through most of fermentation (you should detect low acetaldehyde). At that point, sterol synthesis is complete and uptake of sterols has begun. Constant aeration speeds and aids this process. The resulting slurry can be used for another starter if more volume is needed, or used to make a batch.