Oxygen Suckback

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I know there has been a lot of discussion on this subject but can anyone tell me approximately how much CO2 I would need to capture for oxygen suck back prevention? That is how much CO2 approximately would be sucked back during a cold crash of a 23 litre brew. I believe if the container is not large enough then once the CO2 is depleted it will start sucking oxygen anyway.
Thanks Brian
What type of fermenter do you have ?? That may help people answer the question you have asked
A quick google finds a few threads elsewhere, some of which have calculations listed to work out the difference in volume based on temperature etc, but short answer, there are a lot of variables including fermenter size, shape, volume, headspace... there are 'co2 harvesters' out there (essentially two mason jars connected with tubing, purged with co2 during ferment, then sucking back the co2 after the fact. Of course there's potential the co2 mixes with the o2 in the jars, so may not be fully purged via this method (and as you suggest, after a certain point may bring in o2).

Some people purge a keg and hook this up, I've been using a couple of PET bottles with carbonation caps / tee piece - fill with sani, push out with co2 (to try and remove as much o2 as possible from the get go) then add a few psi of co2. Insert tubing into the airlock / bung hole to plug, attach disconnect to PET bottle and this allows a slight reservoir (and some co2 in the headspace). Been doing this with a brewbucket which can hold a couple of psi. It definitely pulls in co2 and some absorbs into solution, so I do re pressurise the PET bottles a couple of times over the few days cold crashing.

Mylar balloons also work great, and can hold more co2, and a simple visual indicator (I have limited fridge space hence going for a more compact setup).
Hi MCG, I have added a second air bubbler hole in the lid of my fermenter and plug one until fermentation has been active for a couple of days, this allows the co2 created to purge the oxygen from the fermenter, then after the two days I have a mylar balloon setup to plug into the second hole this then fill the balloon (it's probably 2 litres in size) and when I cold crash the co2 from the balloon empties into the fermenter, ( it has never completely emptied after cold crashing). I do this mainly for piece of mind rather than having real concern about O2 affecting the brew. I say this because from everything I have read Co2 (being heavier than O2) forms a barrier protecting the O2 from contacting the beer while it's in the fermenter. So in short I don't think, from what I have read, there is an issue with O2 suck back during cold crash. I'm sure if I'm wrong someone will set us straight.
God I wish this was an easy one to answer.
The biggest single factor will be the solubility of CO2 in the beer at the two temperatures being looked at.
Let’s just assume that you fermenter at 20oC and cold crashed at 0oC so a change of 20 degrees.
If you look at a good carbonation table (I like the metric one in Braukaiser) and don’t even think of trying to do these calculations in PSI, oF or any other stupid units.
At 20oC you would have 1.7g/L of dissolved CO2, cool to 0oC and that goes up to 3.2g/L so a difference of 1.5g/L in say 20L of beer that is 30g. As there is ~22.71L of gas in a Mole of any gas (STP) and CO2 has a mass of 44 you would need 30/44*22.71=15.5L just to account for what goes into solution. You would also need to account for the contraction of the gas due to temperature and a couple of other bits and bobs so I would take a punt on around 1L of CO2/L of beer.

Paddy Melon
I not really arguing with you about your process, Just take note that the protective layer of CO2 is largely an illusion. Yes while the beer is actively fermenting you will keep O2 out but once you get close to an equilibrium it all goes to shite, Oxygen will try to treat the contents of the fermenter (also bottles and kegs) as a vacuum.
And here I was sitting in a state of total calm and bliss thinking that I had conquered the O2 suck back issue....... Now I'm shattered. The mylar balloon at least gives me some peace of mind (even if it is only a non sense). I do, however, purge with co2 when bottling so hopefully am reducing some potential O2 contamination.
Thanks Mark.
Late to the party on this one… recently upgraded from plastic buckets to brew tech SS fermenter. I also added a blowoff instead of using airlock/ bubbler.

Not noticed on previous ferments, but on the current batch when I cold crashed - it has the dreaded suck back. Hardly any sanitiser left in the blowoff container.

I ve taken a sample of the ale & there is no off smells or taste, so will keg today. Just thinking of process for next time. Do I move back to airlock instead of blowoff… something I should also do when cold crashing?
I gave up on the collection of CO2 from the ferment, why, well the fermenting gas coming off isn't just a pure CO2 but the gas carries a lot of undesirables, DMS hydrogen sulfide generally things that you don't want in the beer, so what you are sucking back is really contaminated CO2. Better to fill the balloon with CO2 from your bottle, I realise that oxygen is the enemy but more of an enemy to commercial breweries who's bottes and kegs go in some cases, long journeys where the welfare of the beer is is at the mercy of the elements. As for me, I prefer bottle conditioned the bottles are conditioned at the same temperature as the ferment, and stored at around 14 to 15 C and drunk within a month, or two at the most. What do I do to prevent suck back, nothing, I have gone back to the gladwrap over the top of the fermenter while cold crashing.
In Germany where they aren’t allowed to buy CO2 they only use the CO2 from the second half of the ferment.
The first half is regarded as too contaminated to be worth trying to recover.
The gas from the second half is washed dried, filtered through sawdust and activated carbon often diatomaceous earth, then liquefied for storage and reuse.
It always bemuses me that home brewers think just storing some ferment farts in a plastic bag will be just fine.
On this occasion I'm agreeing with WEAL, it’s at best a waste of time and effort and, at worst, bad for your beer.
Doing some very basic research (or even just a quick Google) could save a lot of time.

I like to make wort adhering to the Reinheistsgebot.
When fermenting, I allow the ferment to gas off. The final .010 points, I then close and regulate the pressure until it is complete. I do find the flavour is cleaner to my taste.
Pressure transfer to keg. This ensures that no oxidization occurs throughout my process.

I'm definitely a rookie but sorry guys some of you are waaaay overthinking this! haha!

MHB: I'm normally a man of science but homebrewing can be far from lab conditions and rarely follows the rules of science. I did a really high pressure fement recently at 25degC and 25psi. My initial thoughts were that the cold crash would just follow the same column and just lower in temp seeing as the CO2 is locked in the fermenter but after cold crashing to 2degC the pressure was sitting at ~10psi and not 4psi like the table suggests. So there's a chance if you do this, you will end up with a beer with a higher CO2 volume than you intended. This of course is trusting the pressures on a cheap arse gauge :confused:

Paddy Melon: I'm with you on this one. I don't use a balloon but if the balloon fills up and then shrinks back down during cold crash but still has some sort of pressure remaining, it's as close to pretected from O2 as you could ever want.

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