Headspace Logic?

[dan46nbrew]

Older concepts seem to indicate that too much headspace could affect the taste of beer for various reasons such as exposure to O2 and fermenter pressure. I read Brulosophys tests which appear to show that headspace doesn't really matter, especially with the latest test done.

Is there any logical explanation as to why older dogma thought too much headspace will affect the taste of beer? If you have more headspace, during fermentation CO2 will eventually fill all of it, and when pressure is high enough escape into the liquid the blowoff tube is submerged in. The beer should never be exposed to O2 once that happens.

Also the pressure in a small vs larger fermenter should be the same regardless of headspace once CO2 equilibrium is reached. Any flaws in this logic?

 

[Vale71]

Pressure in an open fermenter is always equal to atmospheric pressure plus 1-2 millibar if an airlock or blowoff is used.

As for O2 you seem to believe that CO2 will push oxygen out of the headspace until only CO2 is left. This is not how it works, gases mix readily and you'll have a mixture of O2 and CO2 in the headspace with an ever decreasing ratio of O2 to CO2. If the headspace is disproportionately large compared to the actual beer volume you might end up with residual O2 in the headspace but since fermentation really produces a huge amount of CO2 you'd need a ridiculously underfilled fermenter for that to become an issue.

 

[dan46nbrew]

Pressure in an open fermenter is always equal to atmospheric pressure plus 1-2 millibar if an airlock or blowoff is used.

As for O2 you seem to believe that CO2 will push oxygen out of the headspace until only CO2 is left. This is not how it works, gases mix readily and you'll have a mixture of O2 and CO2 in the headspace with an ever decreasing ratio of O2 to CO2. If the headspace is disproportionately large compared to the actual beer volume you might end up with residual O2 in the headspace but since fermentation really produces a huge amount of CO2 you'd need a ridiculously underfilled fermenter for that to become an issue.

So the reason I'm asking is I'm planning on buying a CF15 fermenter but would like to make 5 gallon batches in it at times. What if you run your CO2 in the fermenter after the wort is in to flush out the O2 right after you put the wort in. Will this work?

 

[day_trippr]

^That^ sounds a bit hinky, as reproducing yeast benefit from O2 during the lag phase.
Faced with the same proposition I'd seal up the fermentor and put an uber low pressure (0.4 psi, for instance) CO2 feed on it once FG is hit. Which is what I do now...

Cheers!

 

[Vale71]

So the reason I'm asking is I'm planning on buying a CF15 fermenter but would like to make 5 gallon batches in it at times. What if you run your CO2 in the fermenter after the wort is in to flush out the O2 right after you put the wort in. Will this work?

It will but you'll use up a ton of CO2, considering you have over 10 gal of empty space...

 

[dan46nbrew]

^That^ sounds a bit hinky, as reproducing yeast benefit from O2 during the lag phase.
Faced with the same proposition I'd seal up the fermentor and put an uber low pressure (0.4 psi, for instance) CO2 feed on it once FG is hit. Which is what I do now...

Cheers!

So this brings up another question... If you are oxygenating the wort won't some of that escape into the fermenter? It kind of seems like even if there is some residual oxygen left after a few days of fermentation producing CO2 isn't this beneficial for the yeast as you mentioned? I'm still trying to understand why even with say 10 gallons of headspace this would be detrimental to the beer?

 

[day_trippr]

I had access to a DO meter for a month and found all kinds of neat stuff with it. For instance, yeast can knock the hell out of the dissolved O2 content in a surprisingly short period - they literally sucked all the measurable O2 out of a 5 gallon batch within 24 hours (that I had oxygenated using .5 lpm for four minutes while stirring the O2 wand, immediately after pitching). And it might have been even quicker than that; I waited 24 hours to take the successive reading.

So I wouldn't be concerned with what would normally be perceived as excessive head space - on the front end. Yes, trying to cram oxygen into wort will likely result in quite a bit of it ending up in the head space at the beginning, but presumably as the yeast strip the wort O2 content the head space O2 will find its way through the wort to the yeast. So far, all good.

It's the back end - post fermentation - that issues might arise. Nature and the gas laws are not easily denied and will conspire to bring that fermentor head space O2 content up - and given oxygen makes up around 200,000 ppm of our air that's a pretty big army at the gates, so to speak. I'm not sure, for instance, how fast O2 will "migrate" through a fluid filled air-lock but for sure it's trying to do just that - it's the nature of things to seek equilibrium.

All that rambling said, time matters, and avoiding prolonged existence in this theoretically over-sized fermentor should minimize any potential oxidation issue. Ie: I would favor quick turn-around versus forgetting there was a big ass fermentor filled with a hoppy pale ale, fading away...

Cheers!