So obviously I'm reading all o the advice floating around to skip racking to secondary. And it makes perfect sense. But if I primary a 5 gal batch in a 6.5 gal bucket, does the headspace/surface area become a concern after fermentation slows?
Are you guys that are doing long primary's using buckets or 5 gal carboys with blowoff tubes? Will I be ok leaving a beer in primary ( in a 6.5 gal bucket ) for 2-3 weeks while the yeast finishes cleaning up? My assumption, because I haven't seen this addressed anywhere is that there's nothin to worry about. However it does beg the question then; why is headspace/ surface area an issue in secondary but not in primary?
Hope this question makes sense, I'm typing On my phone while waiting for a haircut:)

1 Answer 1


The reason it's not an issue in primary is because the headspace is full of CO2 - all the oxygen has been purged and replaced with CO2 many many times over during fermentation.

When you rack to secondary, the headspace is full of air - 21% oxygen - so you want to minimize the headspace to reduce both the surface area of the wort and the amount of oxygen available. Also, since secondary is started before FG is reached, there is still some small CO2 production going on that will push out the small amount of air. But there wouldn't be enough CO2 produced to push out all the air from the headspace you get in a bucket, unless you are truly doing a secondary ferment, such as with additional fruit.

  • That makes sense. Would opening the bucket to peek or take samples for checking gravity matter? In other words, will that allow CO2 to escape and be replaced by oxygen?
    – Forty-Two
    Sep 8, 2012 at 16:52
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    Yes, there will be some diffusion of air if you open the lid, but that too will be expelled if fermentation is still active. But even if no co2 is being produced, the amount of oxygen and the rate of uptake is very slow when the wort is stationary.
    – mdma
    Sep 8, 2012 at 17:05
  • If you ferment without going to secondary, you should rig your equipment so you can do everything without popping the top of the bucket. I have two ports in my bucket lid. I pull the blow off and replace that with a racking cane that has a stopper on it. The second port, I remove a solid stopper and replace it with a stopper that has a tire stem. Then I add CO2 from a bike tire filler to get the racking siphon going.
    – Dale
    Sep 19, 2012 at 23:32
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    @Dale, I do the same, using a stainless tire stem that's permanently fixed. The end of the cane has a corny liquid QD - by inserting the racking cane into the headspace, I can then fill the keg and the hose with co2, and then just pushing the cane further down starts the transfer to the keg.
    – mdma
    Sep 19, 2012 at 23:41
  • @mdma, That's an awesome/elegant technique!
    – Dale
    Sep 20, 2012 at 15:19

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