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Agreed. Less space means less compressibility of air in the head space.Ich think this is not about pressure. The additional pressure caused by an air lock equals the height of water inside it, in other words, nearly no additional pressure.
I guess it is about the quick exit that co2 can take due to lower co2 concentration in the surrounding air caused by good exchange of air, or about additional oxygen intake or both.
Any way, an open air lock or a nearly closed lid wouldn't do it in this scenario as the air would be still kind of trapped within the fermenter.
Having the CO2 spill over the sides and to not be concentrated is what allows the vinyl-4-guaiacol
ester formation. CO2 in high concentration inhibits the esters. That's pretty much the common statement by the experts and most publications advocating for open fermatation.
That said, you need to do the 113F ferulic acid rest so this will happen.