[Guenther]

Mark- are you saying there's nothing to be gained from oxygenating wort when pitching dry yeast? if so, are there any negatives if you do?

While it will not hurt, aeration is not as critical with dry yeast as it is with liquid yeast because dry yeast is propagated aerobically; hence, dry yeast cells never go through a period of ergosterol and unsaturated fatty acid (UFA) depletion like their batch grown liquid counterparts.  The purpose of aerating wort is to allow yeast cells to rebuild their ergosterol and UFA stores in preparation for budding and fermentation.

Hi Mark,

Could you share the literature references where I could get detailed info regarding the dry yeast aerobic propagation? I've read articles about it... but not so 'scientific' ones, and I'd like to read more about it (in Brazil we use a lot of dry yeast).. this is a subject I really enjoy.

When you say they propagate aerobically, you mean they maintain sugar levels down enough to avoid the crabtree effect?

I thought dry yeast had high levels of sterols and UFA (as well as trehalose) just because how they cold condition the yeast before lyophilization, since it would stimulate a pre-hibernation behavior (which would get the sterols and UFA reserves up).

Thanks,

Guenther

[S. cerevisiae]

Could you share the literature references where I could get detailed info regarding the dry yeast aerobic propagation? I've read articles about it... but not so 'scientific' ones, and I'd like to read more about it (in Brazil we use a lot of dry yeast).. this is a subject I really enjoy.

You will not find many research publications that apply specifically to brewer's yeast.  However, you will will find a plethora of publications that apply to the Saccharomyces cerevisiae species.  You just need to use the search terms "chemostat," "aerobic," "Crabtree," and "saccharomyces."  You will have to weed through a lot of chaff to find the wheat.

When you say they propagate aerobically, you mean they maintain sugar levels down enough to avoid the crabtree effect?

Yes

I thought dry yeast had high levels of sterols and UFA (as well as trehalose) just because how they cold condition the yeast before lyophilization, since it would stimulate a pre-hibernation behavior (which would get the sterols and UFA reserves up).

No, cold conditioning has nothing to do with it.  High levels of sterols and UFAs are the result of propagation via respiration (i.e., aerobic propagation).  Sterols and UFAs are synthesized in the respirative metabolic pathway.

While the paper linked below compares lipid compositional differences between aerobically and anaerobically propagated yeast, the author answers your question in the introductory paragraph.
 
http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1982.tb04123.x/pdf

"Aerobically grown yeast cells contain sterols and unsaturated fatty acids in excess amounts. When aerobic pitching yeast is used, aeration of wort is of no advantage, because no more sterols and unsaturated fatty acids are synthesized by enriched yeast cells."

Additionally, unlike dried yeast that is propagated in a lab, industrially produced dry yeast is not dried in a lyophilizer.   It is dried using the same technology that is used for drying active baker's yeast; namely,  fluidized-bed dryers.