[toby]

All kidding aside, it's commonly known that one who refers to himself in the third person is the expert.

Toby does not agree with this criteria.

Help! Help! I'm being repressed!

Now we see the violence inherent in the system!

[Beerbecue]

And the dichotomy of repression and obsession.

[S. cerevisiae]

It's actually due to the cell count.

Actually, it's due to how dry yeast is propagated.  It takes approximately 3.8 trillion yeast cells to reach saturation (a.k.a. maximum cell density) in a 19L (5-gallon) batch; therefore, the difference between 100, 200, or even 400 billion cells is in the noise.  What matters is yeast health, and dry yeast cells are is healthier after rehydration than liquid yeast cells that are propagated the way that most home brewers propagate yeast.


Dry yeast is propagated aerobically below the Crabtree threshold in a bioreactor (propagating below the Crabtree threshold results in the carbon source being consumed via the respirative metabolic pathway).  Aerobic propagation produces yeast cells with fully-charged ergosterol and unsaturated fatty acid (UFA) reserves. The reason why we add O₂ to a batch of wort is so that the cells that have been pitched can rebuild their ergosterol and UFA reserves.  Ergosterol and UFAs make yeast cell membranes more pliable, which, in turn, makes it easier for yeast cells to pass nutrients and waste products through the cell walls.  Dry yeast cells can skip this step.


Liquid yeast is propagated above the Crabtree threshold; therefore, all reproduction is fermentative.  Only the initial mother cells have fully-charged ergosterol and UFA reserves in fermentative reproduction. These mother cells share their ergosterol and UFA reserves with their daughters during each budding event.  The reason why we pitch at high krausen instead of allowing a starter to ferment out is because all replication after high krausen has been reached is for replacement only, which means that we are wasting ergosterol and UFA reserves.  In essence, the health of a culture decreases as a fermentation proceeds towards quiescence.

[denny]

Thanks, Mark.

[S. cerevisiae]

For those who are interested in understanding how the Crabtree affects Saccharomyces genus yeast strains, The link below points to an interesting paper that I recently read that compares how Saccharomyces (Crabtree positive) and Kluyveromyces (Crabtree negative) yeast strains respond to changes in glucose concentration in a chemostat. A chemostat is a type of bioreactor.   It's the type of bioreactor that is used to propagate dry yeast.  It's called a chemostat because the bioreactor can maintain the solution at a steady (chemically static) state by continuously adding medium and O₂ while continuously drawing off yeast.  That's how Lallemand and Fermentis can propagate below the Crabtree threshold.

www.researchgate.net/profile/Gianni_Liti/publication/41322073_Physiological_responses_of_Crabtree_positive_and_Crabtree_negative_yeasts_to_glucose_upshifts_in_a_chemostat/links/00b7d52697ccb18184000000.pdf?disableCoverPage=true

[dunngood]

No expert here but this is the way I understand it. When liquid yeast is first pitched they need  oxygen to build sterols and other needs for full formation.
The Danstar report says they add enough Lipids to not need this phase and pretty much go to the formation phase where both liquid and dry yeast are wanted to expand 2 to 3 times to finish.
Unless you are brewing a high gravity beer there is enough cell count in a dry pack.