hist-brewing: aerobic fermentation

BrewInfo brewinfo at xnet.com
Mon Jun 4 11:02:39 PDT 2001

Dan writes:
>Angus says:
>>And you've never/seldom had a batch go up to and above 7%? Perhaps you
>>need more oxygen in your batches then.
>Er, that may be too much oxygen, not too little.  Oxygen promotes aerobic
>fermentation which produces more yeast and some alcohol, rather than
>just alcohol. Anaerobic fermentation is a more sugar-efficient method of
>alcohol production, but it requires a high pitching rate (of freshly active
>yeast) to skip the propogation stage.

No, Angus is right... not enough oxygen at pitching time.  Yeast need oxygen
to produce cell membranes.  If they don't get enough oxygen, they are
limited to the number of times they can divide and thus you can have not
enough yeast to completely ferment the batch.  Furthermore, insufficient
oxygen can mean weak cell membranes (which keep the yeast guts in and the
poisonous alcohol out) and the yeast expire before fermentation is complete.

As for aerobic fermentation, I suspect you thought Angus meant the oxygen
would be around during fermentation.  I'm certain Angus meant (as I did in
a post from a few days ago) that the oxygen is needed in the wort/must
*initially* when the yeast is being pitched.  The yeast consume all this
oxygen within a few hours, long before fermentation begins.  Furthermore,
Saccharomyces cerevisiae exhibit what's known as the Crabtree Effect.  This
is where the yeast forgo aerobic fermentation in favour of anaerobic
fermentation even in the presence of oxygen.  In most strains of S. cerevisiae
it takes less than 1% sugar in the solution for the Crabtree Effect to
take place.  In other words, aerobic metabolism of sugars (known to
biologists as "respiration") is virtually nil in typical brewing situations.


Al Korzonas, Lockport, Illinois, USA
korz at brewinfo.org

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