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Author Topic: Oxygenating Wort  (Read 2630 times)

S. cerevisiae

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Re: Oxygenating Wort
« Reply #15 on: May 12, 2015, 01:49:45 pm »
I worry about oxygen when I'm planning a brew, and until now, have always opted to put my faith in healthy yeast, often in the form of a single sachet of activated yeast.

If you primarily use dry yeast, then there's a reason why you can get away with not aerating your beer.  Dry yeast is not propagated the same way as liquid yeast.  Dry yeast is propagated continuously in a device known as a bioreactor.   The use of a bioreactor allows a dry yeast manufacturer to hold the medium in a steady state below the Crabtree threshold while continuously adding O2 and removing yeast cells. This type of propagation is known as aerobic propagation because biomass growth is accomplished via the aerobic metabolic pathway.  Aerobic propagation produces yeast cells with fully charged ergosterol and unsaturated fatty acid (UFA) reserves.  The downside of aerobic propagation is that it produces yeast cells that have experienced little if any ethanol stress.  It also places selective pressure on the culture that favors cells that reproduce well via the aerobic metabolic pathway, which is not the way that yeast is propagated in a batch of wort.

On the other, liquid yeast is propagated anaerobically in batches. The medium starts out above the Crabtree threshold, which causes biomass growth to be handled via the anaerobic metabolic pathway.  Dissolved O2 is shunted to the aerobic metabolic pathway for the production of ergosterol and UFAs at the beginning of the process.   These stores are partially or fully consumed during propagation, resulting in cells that place a higher initial O2 load on a batch of wort/starter wort when pitched.  However, anaerobic propagation has the advantage that it goes through all of the phases and experiences the stresses that are experienced during fermentation;  hence, liquid yeast has less of tendency to produce funky flavor and aroma compounds.
« Last Edit: May 12, 2015, 08:44:21 pm by S. cerevisiae »

Offline Frankenbrew

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Re: Oxygenating Wort
« Reply #16 on: May 12, 2015, 02:49:12 pm »
I worry about oxygen when I'm planning a brew, and until now, have always opted to put my faith in healthy yeast, often in the form of a single sachet of activated yeast.

If you primarily use dry yeast, then there's a reason why you can get away with not aerating your beer.  Dry yeast is not propagated the same way as liquid yeast.  Dry yeast is propagated continuously in a device known as a bioreactor.   The use of a bioreactor allows a dry yeast manufacturer to hold the medium in a steady state below the Crabtree threshold while continuously adding O2 and removing yeast cells. This type of propagation is known as aerobic propagation because biomass growth is accomplished via aerobic metabolic pathway.  Aerobic propagation produces yeast cells with fully charged ergosterol and unsaturated fatty acid (UFA) reserves.  The downside of aerobic propagation is that it produces yeast cells that have experienced little if any ethanol stress.  It also places selective pressure on the culture that favors cells that reproduce well via the aerobic metabolic pathway, which is not the way that yeast is propagated in a batch of wort.

On the other, liquid yeast is propagated anaerobically in batches. The medium starts out above the Crabtree threshold, which causes biomass growth to be handled via the anaerobic metabolic pathway.  Dissolved O2 is shunted to the aerobic metabolic pathway for the production of ergosterol and UFAs at the beginning of the process.   These stores are partially or fully consumed during propagation, resulting in cells that place a higher initial O2 load on a batch of wort/starter wort when pitched.  However, anaerobic propagation has the advantage that it goes through all of the phases and experiences the stresses that are experienced during fermentation;  hence, liquid yeast has less of tendency to produce funky flavor and aroma compounds.

Mark, Excellent explanation for us laymen. Thank you.

I use mostly dry yeast, which I treat the same as I do liquid yeast, which I use when I am brewing a beer in which its character is yeast dependent, Belgian, Kolsch, Biere de Garde, etc. I shake the fermenter violently for 45 seconds with proper posture for back support. I have never had a stuck fermentation in 21+ years. I have brewed a few lagers, but have always used either S23 or 34/70, which I think is a great dry lager yeast. Again, I use the shimmy shake method. Works for me, and others seem to like my beer.

My 2 cents.
Frank C.

And thereof comes the proverb: 'Blessing of your
heart, you brew good ale.'

Offline unclebrazzie

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Re: Oxygenating Wort
« Reply #17 on: May 13, 2015, 12:50:03 am »
If you primarily use dry yeast, ...aroma compounds.

Thanks Mark! Excellent run-down!
All truth is fiction.
--Don Quichote