hist-brewing: aerobic fermentation

ulfin at portup.com ulfin at portup.com
Mon Jun 4 16:37:18 PDT 2001


The famous Al K. writes:

>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...

When did I say he was wrong?  I was merely offering an
alternative diagnosis.  Did I misspell "may"? :)

>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.

Yes, I understand that.  And it has nothing to do with
what I wrote.  I specified a high pitching rate to
minimizes the need for the yeast to reproduce in the
fermenter.

>cell membranes (which keep the yeast guts in and the
>poisonous alcohol out)

That's a great phrase.

>As for aerobic fermentation, I suspect you thought Angus meant the oxygen
>would be around during fermentation.

No, I knew what Angus meant.  And given the assumption that
there wasn't enough active yeast, I agree with it.  However,
that assumption is not a given.

>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.

Unless the oxygen forces it to drop out the of Krebs cycle back into
respiration via the Pastuer Effect.  Distillers (who, btw, also
use varieties of S. Cerevisiae) depend on the Crabtree Effect in
making their beer and go to great lengths to exclude oxygen lest
it promote sugar-wasting catabolic respiration during the
fermentation.

But, I must concede, the wasted sugars probably wouldn't
even be noticable on the homebrewer's hydrometer.

Dan



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