[Iliff Ave]

Is there anything special I need to know? I was recently informed that when repitching slurry (from dry yeast?) that the oxygen is highly depleted so you have to ensure proper wort oxygenation. Other than amount differences is there anything else I should be aware of specifically with lager yeast vs. ale yeast? My plan is to use Mr. Malty to calculate the necessary amount. 

[tommymorris]

I have done just the same many times without issue. I pitch slurry that originated from dry yeast according to Mr Malty. I don't measure the slurry. I just approximate and err on the high side. I do try to aerate extra well when pitching slurry. I use the mix stir method for aeration.

If you are worried about depleted oxygen I think this would a good time to use the shaken not stirred starter method and pitch at high krausen.

[beersk]

That's all bupkis. It's yeast just the same. I've repitched from an initial pitch of dry yeast many times and it produces fantastic beer.

[tommymorris]

I think the oxygen depletion reference is likely related to the fact that the dry yeast process bonds oxygen to the dry yeast and therefore you don't need to aerate when pitching dry yeast. After fermentation that oxygen is gone. So when pitching slurry you will need to aerate (just like any other slurry).

I pitch straight slurry pretty often. I just aerate well and have had good success. PS. I usually only pitch slurry 1 generation and very rarely 2. I usually want to move on to another yeast by then and I don't keep old slurry.  It's too cheap to buy new yeast.

I mentioned the shaken not stirred starter method as extra insurance. It's also useful to reinvigorate older slurry or slurry that's more than a few generations old.

[majorvices]

I think the oxygen depletion reference is likely related to the fact that the dry yeast process bonds oxygen to the dry yeast and therefore you don't need to aerate when pitching dry yeast. After fermentation that oxygen is gone. So when pitching slurry you will need to aerate (just like any other slurry).

I pitch straight slurry pretty often. I just aerate well and have had good success. PS. I usually only repurchase 1 generation and very rarely 2. I usually want to move to another yeast by then and I don't keep old slurry.  It's too cheap to buy new yeast.

I mentioned the shaken not stirred starter method as extra insurance. It's also useful to reinvigorate older slurry or slurry that's more than a few generations old.

Yes, o2 is less important to dry yeast because the yeast has its glycogen already reserved, which is the reason you aerate in the first place, so that the yeast have strong cell walls to help in budding. It doesn't hurt to aerate dry yeast, but it may not be necessary. However, it's always a good idea to aerate slurry since by the time fermentation is finished their reserves are depleted, regardless if the yeast was dry or liquid to begin with.

[S. cerevisiae]

The information that you received is oversimplified.  Yeast cells do not store O₂, they store ergosterol and unsaturated fatty acids (UFAs).

Dry yeast does not require much in the way of dissolved O₂ on the initial pitch because it is propagated aerobically below the Crabtree threshold in a bioreactor.   All reproduction is via the aerobic metabolic pathway.  Ergosterol and unsaturated fatty acids (UFAs) are synthesized in the aerobic metabolic pathway; therefore, the cells go into a fermentation with fully-charged ergosterol and UFA reserves.

Liquid is yeast propagated above the Crabtree threshold, which means that all reproduction is via the anaerobic metabolic pathway.  Dissolved O₂ along with a small amount of carbon is shunted to the aerobic metabolic pathway for ergosterol and UFA biosynthesis during the lag phase.  One of the reasons why we want to pitch a starter at high krausen is to preserve ergosterol and UFAs stores because all replication beyond this point is for replacement only, and mother cells share their ergosterol and UFA reserves with their daughters.

With that said, dry yeast is just like liquid yeast at the end of fermentation.  O₂ is needed on a repitch to rebuild the ergosterol and UFA reserves that were depleted during replication and fermentation.

[S. cerevisiae]

Yes, o2 is less important to dry yeast because the yeast has its glycogen already reserved, which is the reason you aerate in the first place

The compounds that are biosynthesized using O₂ are ergosterol and unsaturated fatty acids.  Glycogen and trehalose are stored at the end of fermentation as a survival response. The cell wall also thickens when carbon becomes limiting.  The cells enter a phase known as quiescence.  This survival response is much like that of a bear preparing for hibernation.

[majorvices]

Yes, o2 is less important to dry yeast because the yeast has its glycogen already reserved, which is the reason you aerate in the first place

The compounds that are biosynthesized using O₂ are ergosterol and unsaturated fatty acids.  Glycogen and trehalose are stored at the end of fermentation as a survival response. The cell wall also thickens when carbon becomes limiting.  The cells enter a phase known as quiescence.  This survival response is much like that of a bear preparing for hibernation.

I was going to say sterols and then changed it. Oh well, I'm not a microbiologist just a brewer relying on remembering stuff he has read that other people wrote. Thanks for the clarification.