Subject: dissolved gases
Date: 1990-04-09 21:55:23 GMT
>From: firstname.lastname@example.org (John Mellby)
>Subject: Oxygen in solution necessary for Fermentation?
>Recently I was rereading Papazian's section in the Brewing Mead book
>and he said that the initial 12-14 hours of fermentation require lots
>of oxygen. He said this was typically ok, since there was lots of
>oxygen available in solution in cold tap water.
It's the usage of "require" that we've got to explore.
>But if this is the case, then when you boil the wort, wouldn't that drive
>the O2 out of the water, and leave the primary fermentation short of
>oxygen? In my case, since I don't have a wort chiller, I usually
Yes, absolutely. This makes it vital to take extra steps to get air into
the wort prior to pitching.
>boil 2-3 gallons of water and put it in the refridgerator to cool ahead of
>time (this usually reduces the wort temperature to where I can add the
>yeast). However, would this not reduce the supply of oxygen as well?
>Is there really that much O2 in solution, or is the fermentation stealing
>O from the H2O itself? O.K. so I'm not a chemist. I'm still puzzled?
No, the only source is going to be what is dissolved in the wort.
>Where does the O2 come from for the fermentation, if so much oxygen
In this circumstance, although it is needed, it probably isn't present.
The yeast "require" oxygen during respiration the same way people require
good nutrition while growing up. Yeast can get along without oxygen in
the sense that in many cases the fermentation will take place, it will
finish OK and drinkable beer will be made. However there are hazards
and sometimes the hazards can catch up with us.
Without adequate oxygen yeast respiration takes place with some alternative
biochemical mechanisms (boring technical terms omitted). These alternatives
are tradeoffs and carry penalties. For instance, without adequate oxygen
the yeast cannot reproduce beyond a small number of generations. This means
that in some situations the cell count may be too low to completely ferment
the beer in a reasonable time. A low cell count creates an infection risk too.
The yeast that are "born" without proper oxygen during respiration have poor
alcohol tolerance and low cell energy reserves. This means they will tend
to conk out before the job is done, especially when trying to make strong beer.
The parent cells are also weakened by having to donate scarce cell materials
to daughter cells.
Keep in mind that all the oxygen splashed or otherwise gotten into the
wort prior to pitching will be used up by the yeast. In fact it is the
depletion of available oxygen, among other things, that triggers the
transition to the anerobic phase. It is only after the anerobic phase of
fermentation has started that the presence of oxygen becomes harmful.
(But don't aerate wort while it is hot, since that does cause other problems).
For those of you getting one of the new Foxx counterpressure bottle fillers:
Pressure check everything, especially the valves, before you try to use it.
I can state with authority that although three leaky joints can be fixed with
hasty action and a bit of teflon tape, a defective, leaking valve will make you
wish you had a different hobby.
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