Ferment 3 gal. Mead in 6 gal carboy
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TipsyDragon
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no. the CO2 put out by the fermentation will sit on the surface of the mead protecting it and displacing the air in the carboy. not to mention the fact i am fairly sure there will be more than 3.5 gallons of CO2 produced during fermentation.
That would be ok for primary, but you will want to rack it to a 3 gallon carboy after primary. It should age in the carboy for quite a while, and you want to keep it topped up to prevent oxidation. You may need to rack it a couple of times before aging it, depending on how much lees drop out.
Yoop's right...for secondary, there's way too much headspace left over. Some will say they've never seen a need to top up, but as someone who's moved mead up and down the steps (upstairs to rack it over in the kitchen and then back down stairs to the ferment room) -- the headspace killed me. I've had 2 batches go "bad" due to likely oxidation from this factor alone - which is why I will no longer move mead after primary fermentation is done.
TipsyDragon
Well-Known Member
thats because there is no out gassing after a few rackings and the mead is coming in direct contact with air. you could fill the head space with dry ice fog. its pure CO2 and will push out any air thats in there preventing oxidation.
True. As long as you put co2 in the carboy to displace the oxygen, you're (in theory) fine using a larger vessel for a small batch.
gratus fermentatio
Well-Known Member
+1. Do yourself a favor & get a 3 gallon carbouy, it's well worth the $15 - $20 you'll pay for it & it'll pay for itself the 1st or 2nd time you use it. Regards, GF.
GNBrews
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Here's the trouble. During primary fermentation, headspace is not an issue because the must is continuously producing CO2, which displaces the oxygen. Any oxygen that isn't displaced is absorbed into the must, and is consumed by the yeast. The airlock keeps any further air exchange at bay.
You might think that since CO2 is heavier than air, it will sit like an impenetrable blanket on top of the fermented mead. However, air is 21% oxygen and atmospheric pressure is 14.7PSI. Since there is no oxygen inside the carboy, atmospheric oxygen is now attempting to diffuse, with .21 X 14.7 = ~3PSI, on all of the outer surfaces of the sealed container. The airlock is keeping that oxygen out. While it's true that CO2 is heavier than air, as soon as you remove the airlock for any reason, that ~3PSI of oxygen will rush into the open hole of the carboy and become part of the gas mixture inside. The limiting factors of how much of that oxygen is absorbed into the mead is how much of the mead's surface area is exposed and how much oxygen is available in that empty space.
If the carboy is close to full, the liquid level is pretty close to the curvature of the neck, and so there is just a 3-4" circle of exposed liquid and a small volume of empty space containing oxygen. That little bit of oxygen will be absorbed into the mead, and you'll be back to a mostly inert (CO2/Nitrogen) headspace. The small amount won't likely give much off-flavor unless you repeat the opening process unnecessarily often after fermentation has completed.
If you have only a partially full carboy, you now have a very large surface area of liquid to absorb oxygen from a large amount of headspace. This fairly certainly guarantees serious oxidation and off-flavors.
Moral of the story is that if you want to use container with a large amount of headspace, it's ok for primary fermentation, but don't open the carboy until it's completely done. Then immediately transfer the contents to smaller containers where you can minimize headspace for aging. Those gallon wine jugs work well for that purpose.
You might think that since CO2 is heavier than air, it will sit like an impenetrable blanket on top of the fermented mead. However, air is 21% oxygen and atmospheric pressure is 14.7PSI. Since there is no oxygen inside the carboy, atmospheric oxygen is now attempting to diffuse, with .21 X 14.7 = ~3PSI, on all of the outer surfaces of the sealed container. The airlock is keeping that oxygen out. While it's true that CO2 is heavier than air, as soon as you remove the airlock for any reason, that ~3PSI of oxygen will rush into the open hole of the carboy and become part of the gas mixture inside. The limiting factors of how much of that oxygen is absorbed into the mead is how much of the mead's surface area is exposed and how much oxygen is available in that empty space.
If the carboy is close to full, the liquid level is pretty close to the curvature of the neck, and so there is just a 3-4" circle of exposed liquid and a small volume of empty space containing oxygen. That little bit of oxygen will be absorbed into the mead, and you'll be back to a mostly inert (CO2/Nitrogen) headspace. The small amount won't likely give much off-flavor unless you repeat the opening process unnecessarily often after fermentation has completed.
If you have only a partially full carboy, you now have a very large surface area of liquid to absorb oxygen from a large amount of headspace. This fairly certainly guarantees serious oxidation and off-flavors.
Moral of the story is that if you want to use container with a large amount of headspace, it's ok for primary fermentation, but don't open the carboy until it's completely done. Then immediately transfer the contents to smaller containers where you can minimize headspace for aging. Those gallon wine jugs work well for that purpose.
