Head space

I've been searching for a while and I couldn't find a lot of information on this. How exactly does head space affect fermentation? I understand there needs to be some to contain the krausen or you need a blowoff tube, but is the optimum amount of head space just enough to contain the krausen and leave as little space as possible? I've also read that no matter how much head space your yeast will produce a layer of CO2 that will protect it from oxygen. So does it really matter?

jakeman777 said:

I've been searching for a while and I couldn't find a lot of information on this. How exactly does head space affect fermentation? I understand there needs to be some to contain the krausen or you need a blowoff tube, but is the optimum amount of head space just enough to contain the krausen and leave as little space as possible? I've also read that no matter how much head space your yeast will produce a layer of CO2 that will protect it from oxygen. So does it really matter?

Click to expand...

30% is recommended but still does not guarantee that a blow off wont be required because every batch will behave differently.

Yes, during fermentation CO2 will create a protective blanket over the beer

Head space is a larger concern if bulk aging in secondary after fermentation is complete. You want as little head space as possible to prevent oxidation from the lack of a CO2 blanket

Thanks for the reply. So just to be clear is there a limit of headspace the CO2 can protect from? Say you put 5 gallons in a 20 gallon fermenter or something with a large difference, that won't matter at all?

Also, what is the cutoff time where you want to reduce headspace? If I were using a conical and didnt need to secondary could there still be about 30% head space and oxidation won't occur since the CO2 won't be removed?

A CO2 blanket is a myth. In a relatively short time, whatever CO2 is produce will mix with the gasses already in the headspace to reach equilibrium. During active fermentation there is enough CO2 produced so that the concentration of other gasses is negated, but you still don’t want to overdo it. Personally I wouldn’t go over 50%. Even 30% is pretty high. That’s saying that you could only ferment 4.5 gallons in a 6.5 gallon carboy. You’d be fine at that amount, but it’s not necessary. Most brewers go more like 5.25 to 5.5 in a carboy that size and only use a blow-off tube for security.

Oh okay. So you want to try and reduce the headspace to as little as possible after primary fermentation is done and the gases equilibriate? The reason I ask is because I was low on ingredients and decided to brew a 3 gallon batch and ferment it in a 5 gallon carboy and I wasn't sure how that extra head space would affect fermentation.

jakeman777 said:

Oh okay. So you want to try and reduce the headspace to as little as possible after primary fermentation is done and the gases equilibriate? The reason I ask is because I was low on ingredients and decided to brew a 3 gallon batch and ferment it in a 5 gallon carboy and I wasn't sure how that extra head space would affect fermentation.

Click to expand...

For your situation the headspace is fine as long as you are not continuously removing the lid and only planning on keeping the beer in there for 3 weeks or so. You should be good.

I would not however rack 3 gallons of beer I to a 5 gallon secondary as oxidation would become a bigger concern then

Yeah I figured it wouldn't really affect me but I was feeling scientifically inclined and decided to figure out what would happen on an industrial scale! Thanks for all the help!

jakeman777 said:

. . . brew a 3 gallon batch and ferment it in a 5 gallon carboy . . .

Click to expand...

My standard batch is around the 7 gallon size, but divided between two 5 gallon carboys, so right in the range you're describing. If I'm going to secondary, I top off a single 6 1/2 gallon carboy or use two 3 gallon carboys if I want to play with different dry hops.

So what do you do with your extra half gallon?

What isn't lost to trub goes into a PET bottle w/ Carbonator Cap.
Carb and chill for a couple of days gives a feel for how the batch is progressing.

jakeman777 said:

I've been searching for a while and I couldn't find a lot of information on this. How exactly does head space affect fermentation? I understand there needs to be some to contain the krausen or you need a blowoff tube, but is the optimum amount of head space just enough to contain the krausen and leave as little space as possible? I've also read that no matter how much head space your yeast will produce a layer of CO2 that will protect it from oxygen. So does it really matter?

Click to expand...

I think this was proably answered by the thread, which is in a closed system, it doesn't matter if you have excessive head space during fermentation, but once racked you want as little as possible.

someone figured out (and I don't remember where I read it on line) that the average beer makes 60 gallons of CO2 at STP per gallon of wort (actually it was 300 gallons on 5 gallons, but it is the same thing). If you have 2 gallons of air mixed with 180 gallons of CO2, you pretty much have 2 gallons of CO2 in your fermentor- ok about 98% CO2 and 2% N2 and most O2 will have probably gone into solution our out by finish time.

If air can flow in, then the CO2 will mix out and negate the 'blanket' that everyone talks about.

Anyhow if you are only making a 1 gallon test batch and only have a 5 gallon bucket, by all means you can primary, but as soon as you want to rack, find a closely sized container.

One reason for the 30% rule was the 'do I need a blowoff'... more head space = less need for a blow off. But at some point, there are deminishing space returns, I mean if 30% is good, why not 70%... Wait I've got a 10 gallon bucket with 2 gallons of beer taking up space in my house/garage!! heck no, fill that sucker up!. Again your specific situation will require you address this as you need.