How much to ferment?


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May 29, 2013
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I've recently had a discussion with another brewer that concerned me. He was talking about how you can get off flavors from fermenting with too much head space in the primary. For instance, only filling the 6.5 gallon carboy with 3 gallons of wort. He said it has to do with negative pressure affecting the beer. Do anyone know anything about this or is it just the rantings of a mad man? :D

I'm curious as to this because I usually just make 3 gallon batches and had used a 6.5 gallon plastic bucket for fermentation. There seems to be an odd taste to it that I had been trying to figure out.
Define "negative pressure": By definition, pressure can never be less than zero.... Sounds like BS to me. What you might be worried about when you talk about head room is oxygen but as soon as the beer starts to ferment, it starts producing carbon dioxide which, heavier than air, lies on your beer like a snug little blanket, keeping oxygen out. But negative pressure, a fermenting beer will always be at or slightly above ambient pressure (if it weren't slightly above, no bubbles!).
If you are racking and leaving a lot of head space, then you are subjecting the beer to oxidation.

You could get a 5 gallon carboy or better bottle, and see if that changes anything.
Its not actual negative pressure but the pressure outside ends out greater than inside. He just used the term negative pressure. I think there's only one solution to this problem. Brew bigger batches! :D

Actually I've tried to brew a 5 gallon batch with what I have and it doesn't work out too well unless I use extract to supplement some of the malt. Very soon I think I"m going to find myself with a 10gal Rubbermaid mash/lauter tun. My stove top BIAB just isn't cutting it anymore.