Originally Posted by Loup
According to the guy at my LHBS, having a lot of head space leaves a lot of air that bacteria could be hanging out in and the bacteria could possibly overpower the yeast, but he really didn't seem too concerned about it himself.
This is one of those brewing "myths" or errors that keeps getting perpetuated over and over as canon, with little basis of fact, because;
Originally Posted by beerthirty
as soon as fermentation begins so does production of CO2. CO2 being heavier than O2 will form a protective cap over your brew. As to airs other major component nitrogen, it is an inert gas and will have no effect either way.
And, if the CO2 has built up enough to be coming out of our airlock (or the lid seal or the grommet) then the heavier CO2 would have effectively vented or voided out any air in the headspace. It's the same reason we fill from the bottom to the top with our bottle fillers, and leave the cap on loosely before crimping it, to allow the CO2 to push out any air in the bottles....
Or that's what I've always believed.
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