hist-brewing: oxygen at packaging

George de Piro george at evansale.com
Tue Nov 27 19:57:53 PST 2001


Edward recently wrote:

"When you bottle you want the yeast to reactivate in
order to use the new sugars added to create
carbonation.so.it seems to me that a little O2 should
not be a problem."

While it is true that pitched wort needs copious amounts of oxygen, there is
nothing more damaging to fermented beer than oxygen.  The yeast do not
require oxygen to be "reactivated."  A brief summary of yeast metabolism
will clarify the issue.

The only reason yeast require oxygen is to create sterols, fatty compounds
that make up an important part of their cell membranes.  Without enough
sterol, yeast cells cannot divide.  Oxygen provided to the yeast at pitching
is important because it enables the yeast to make healthy cell membranes and
divide.  Division of the yeast cells is important because brewers do not
pitch enough yeast cells to complete a fermentation in a reasonable amount
of time; we rely on the yeast to grow to adequate numbers to get the job

Once fermentation is complete, we do not need yeast in the beer other than
to carbonate it (if you are bottle of cask conditioning).  It does not
require a lot of yeast to accomplish this conditioning.  We do not want to
encourage new yeast growth at bottling because too much yeast can lead to
off-flavors and heading problems in the brew (due to yeast autolysis).

Oxygen quickly stales finished beer, yielding papery-tasting compounds,
reducing malt character, speeding diacetyl formation, dulling the hops, etc.
While yeast are good antioxidants, they cannot overcome the large amount of
oxygen that sloppy racking can introduce into the beer.  It is important to
minimize air contact with the finished beer.

Studies have shown that beer exposed to air will taste stale within 6 DAYS
if stored warm.  If kept refrigerated it may last up to 6 weeks.  That's not
a terribly long time.

None of this would have mattered much to medieval brewers, so if you want to
recreate the beers of old, perhaps some sloppy brewing techniques are
needed, including using microbiologically contaminated yeast.  On the other
hand, many of us don't like sour, oxidized beer, regardless of its
historical accuracy, and work to avoid it.  The decision is the brewers.

Have fun!

George de Piro

Head Brewer
C.H. Evans Brewing Company
at the Albany Pump Station

Malted Barley Appreciation Society
Homebrew Club

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