[euge]

MY understanding is that one doesn't need to account for "growth rate" with dry yeast because there are already enough cells in the pitch. In an average beer.

That being said, if I remember/have time- the wort get aerated before the pitch anyway. Not much of a difference IMO.

[johnnyb]

MY understanding is that one doesn't need to account for "growth rate" with dry yeast because there are already enough cells in the pitch. In an average beer.

That being said, if I remember/have time- the wort get aerated before the pitch anyway. Not much of a difference IMO.

Yup, that's why I was wondering if it made a difference between 1 pack and 2 packs when not rehydrating and not aerating.
 

[S. cerevisiae]

Yes, yeast require O2 through the growth phase

Actually, dry yeast needs nowhere near as much dissolved O₂ as liquid yeast. The reason being is that dry yeast is propagated aerobically in a bioreactor.  Propagation in bioreactor allows the glucose content in the medium to be held in a steady state below the Crabtree threshold of 0.3% glucose weigh by volume.   The result is that all growth is via the respirative metabolic pathway.  Propagation via the respirative metabolic pathway results in yeast cells with fully charged ergosterol and unsaturated fatty acid (UFA) reserves (it is also over an order of magnitude more efficient).

All reproduction in batch-based liquid yeast propagation and beer fermentation is fermentative because the glucose level is above the Crabtree threshold.  What yeast cells do when O₂ is available is shunt it along with a small amount of carbon (sugar is carbon bound to water) to the respirative metabolic pathway for the production of ergosterol and UFAs.

With a batch propagated liquid culture, initial O₂ demands are based the point in the process where the cells are harvested.  Pitching a culture when it reaches high krausen places a lower O₂ load on the wort because the cells still have ergosterol and UFA reserves.  Waiting until a culture reaches quiescence (i.e., waiting until it ferments out), increases initial O₂ requirements because the mother cells that were alive while O₂ was still in solution share their ergosterol and UFA reserves with all of their daughter cells.  Allowing culture to proceed beyond high krausen results unnecessary ergosterol and UFA depletion. Reproduction after the end of the exponential phase is for replacement only.

[duboman]

Yes, yeast require O2 through the growth phase

Actually, dry yeast needs nowhere near as much dissolved O₂ as liquid yeast. The reason being is that dry yeast is propagated aerobically in a bioreactor.  Propagation in bioreactor allows the glucose content in the medium to be held in a steady state below the Crabtree threshold of 0.3% glucose weigh by volume.   The result is that all growth is via the respirative metabolic pathway.  Propagation via the respirative metabolic pathway results in yeast cells with fully charged ergosterol and unsaturated fatty acid (UFA) reserves (it is also over an order of magnitude more efficient).

All reproduction in batch-based liquid yeast propagation and beer fermentation is fermentative because the glucose level is above the Crabtree threshold.  What yeast cells do when O₂ is available is shunt it along with a small amount of carbon (sugar is carbon bound to water) to the respirative metabolic pathway for the production of ergosterol and UFAs.

With a batch propagated liquid culture, initial O₂ demands are based the point in the process where the cells are harvested.  Pitching a culture when it reaches high krausen places a lower O₂ load on the wort because the cells still have ergosterol and UFA reserves.  Waiting until a culture reaches quiescence (i.e., waiting until it ferments out), increases initial O₂ requirements because the mother cells that were alive while O₂ was still in solution share their ergosterol and UFA reserves with all of their daughter cells.  Allowing culture to proceed beyond high krausen results unnecessary ergosterol and UFA depletion. Reproduction after the end of the exponential phase is for replacement only.

Thanks for the explanation, so in my process I rehydrate my dry yeast and aerate the wort with an aquarium pump for about 15 minutes while the CFC fills the primary. Are you saying that the aeration prior to pitching the rehydrated yeast is not necessary and counter productive?

[S. cerevisiae]

Thanks for the explanation, so in my process I rehydrate my dry yeast and aerate the wort with an aquarium pump for about 15 minutes while the CFC fills the primary. Are you saying that the aeration prior to pitching the rehydrated yeast is not necessary and counter productive?

Aeration when pitching dry yeast is not as critical as when pitching a liquid culture.  Whether or not it is detrimental depends on if the culture scrubs the O2 from the wort, which it more than likely does during the exponential phase.  What are your apparent attenuation levels?

[duboman]

Thanks for the explanation, so in my process I rehydrate my dry yeast and aerate the wort with an aquarium pump for about 15 minutes while the CFC fills the primary. Are you saying that the aeration prior to pitching the rehydrated yeast is not necessary and counter productive?

Aeration when pitching dry yeast is not as critical as when pitching a liquid culture.  Whether or not it is detrimental depends on if the culture scrubs the O2 from the wort, which it more than likely does during the exponential phase.  What are your apparent attenuation levels?

Off the top of my head I can't be exact but I can tell you that I've not had any issues achieving my final gravities in the process I use so I'll assume I'm in good shape:)