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Head Room - How much is too much?

I am about to do my second BIAB, and it will only yield 3 gallons. My buckets and carboy are both meant for 5 gallon batches.
When switching 2 secondary, I sprayed N2 from a wine saver can, thinking I could drive out a fair amount of the O2. I did not notice an issue, but that could be my unsophisticated pallet (it was a milk chocolate stout. tastes great!) My question is: We need excess headspace for agressive fermenters, but how much is too much? Also, is it more critical for secondary than for primary, since we are supposed to oxygenate the wort before fermentation begins anyway? I’m thinking of doing my primary in a 5 gallon bucket, but then getting a 3 gallon carboy for secondary. Does that approach make sense, or is it critical to eliminate the O2 from primary as well?

When your beer is finished fermenting, any deadspace that can expose your newby beer to oxygen is bad. Dead headspace is much more important if you are going to use a “secondary” because during the primary the yeast are actively producing CO2, but that’s gone when you rack it to another vessel.
Your nitrogen spray may help for a few minutes, but it soon dissipates and leaves the exposure to atmosphere.
So, you need to do something to decrease that headspace to decrease the volume of exposure to oxygen. Increasing your recipe to a 5G batch is the best option. A 3 gallon carboy would be another best option ( other than skipping the “secondary” completely). Adding enough marbles to fill the space would be another option, but I’m sure we’re talking about hundreds of marbles, so probably not practical.
I’m sure you will get lots of “Why are you bothering with a secondary anyways?” questions, so start thinking about your why now…

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I’m a proponent of secondary. When I first switched to 10gal batches I used 2 carboys. I experimented and would transfer one to secondary and keep the other in primary. Secondary always cleared quicker and better.

Secondaries are safe if you rack carefully. There is enough dissolved CO2 in the beer that unless you haphazardly rack you won’t need to worry about O2 uptake.

There are 2 beers that I won’t secondary; wits and wheats.

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Absolutely correct. I stopped using secondary vessels for a while, but went back to using them because I’m throughly convinced that they result in better beers (especially when the brews are stronger styles). Of course, this is a conclusion one has to reach (or refute) for one’s self since everyone’s situations, palates, threshold of acceptance, and personal tastes differ.

For whatever it’s worth, I’ve never had an oxidation or infection issue that was the result of transferring to a secondary (even with brews that have bulk aged in secondary for upwards of 8 months). The key is cleanliness , and gentle racking into a (preferably) co2 purged carboy. After transfer to a 3 gal carboy, you will have very little headspace to worry about and since the transferred beer will still have yeast in suspension, the small headspace will still be blanketed with co2.

To “secondary or not” is a topic that modern homebrewers can get pretty passionate about. And there is no right, wrong, or universal best way…individual experimentation is the only way to arrive at the method that meets one’s own standards. I’ve tried it both ways. Your mileage may vary, but for me, the secondary wins.

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Thanks for all of your input. My understanding of yeast is that it will consume both sugar and O2, so the oxidation we are trying to avoid will be limited by the head space we leave in the sealed fermenting vessel (unless you have a poor seal). Is that correct, or is the oxidation independent of the yeast, and just affecting the beer directly, (and slowly) without significant consumption of the O2 during oxidation?

I really did not want to get into the debate of secondary, or not to secondary. In my case, I am doing an oak barrel aging for longer duration, and want my large open bucket available for another brew. I have used both methods, and believe my beer has better clarity when I add a secondary fermentation step, but my observance is subjective and not scientific. I’m still experimenting between the two approaches, and see practical value, as others noted, when wanting to add flavoring to the agings.

Most of all, though, reading the history of where secondary fermentation came from and why has been really interesting - and not alway shared in the books. Thanks again, all for the advice.

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How about before you rack your beer in a secondary why not fill the Carboy with co2 that what there is no air in there to worry about

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