Aerating vs Degassing

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MakinMead

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Good afternoon,

So everyone, I am trying to find some clarity on the topic of degassing. It seems to me that everywhere I try to find the answer online I can’t get exactly what I am looking for. My confusion come from the difference in aerating and degassing. From what I understand, you should “aerate” your must prior to pitching your yeast, to oxygenate the must. However then you should “degas” daily for say.. a week. My question is, if you are aerating by vigorously shaking a carboy, or whatever means you use, and when you degas, you are expected to simply stir it or use some fancy drill. While co2 is escaping, is this not also in fact oxygenating your mead? Does the escaping co2 protect the mead from becoming oxygenated by providing a “blanket”? I look forward to some clarification, thanks everyone.
 

Stephmon

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As you mention, you want an aerated must, to give your yeast a stress-free environment. I've seen that quite a few mead makers continue to aerate through the 1/3rd sugar break. So, through the first 1/3rd, aerating and degassing (releasing CO2) can be essentially the same activity (shaking, or whipping). After that point, you don't want to oxygenate, but you still want to prevent a lot of CO2 building up in solution, as it will stress your yeast. I find, that with my 1 gallon carboys, it is possible to put one on my knee and gently swirl out the excess CO2 (and sink any fruit that may be floating) without agitating the surface excessively. Since the CO2 is heavier than air, there should be a 'blanket' of CO2, to prevent a lot of O2, getting in.
Prior to racking, or bottling, I like to vacuum degas, because I find it gets a lot of CO2 out, without the potential of whipping in a lot of O2, if the degassing tool breaks the surface of the wine. It also leaves the lees alone at the bottom, as opposed to stirring them up and requiring a re-settling.
 
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MakinMead

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Well that clears things up, so as I suspected degassing should be more of a gentle stirring process. Helps a lot, I have been treating it that way, but just wanted to make sure I had my facts straight. Thank you!
 
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MakinMead

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Hey grandwizz,

To my knowledge it’s pretty common practice, and some people consider it necessary. I will say that with my first mead, i didn’t know about degassing therefore, did not do it, and it has turned out to be my best batch of 5. My personal opinion is that it helps to an extent, because the co2 is so abundant in strong fermentations, but st the end of the day I feel like sometimes I am working too hard, when I should just let nature run it’s course.
 

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Co2 is poisonous to yeast. Since fermentation creates co2, and often a mead is a higher ABV drink, the build up of c02 can stress the yeast. Degassing or aerating can be beneficial in the early stages to help the yeast along.

Aeration is fine, until the fermentation starts to wind down, say, when the mead gets to 1.020 or below (assuming it will finish dry). Then it should be airlocked and aeration/degassing avoided.

Sometimes I use a bucket for primary fermentation for meads and wines, so that it's easier to stir daily. I cover it with a towel to keep fruitflies and the like out but not airlocked so that the c02 is more easily dissipated. Once fermentation slows, it is moved to a carboy and airlocked for the remainder of the time.
 

biochemedic

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I think a lot of the confusion comes from the term "aeration," which should actually read "oxygenation." I'm sure Dr. Bray, PhD / @loveofrose could comment on this more authoritatively, but in the early stages of fermentation, the yeast do require copious amounts of oxygen to reproduce (hence the term "aerobic phase" of fermentation). You do *not* need to worry about oxidation while you are actively fermenting; the yeast will absorb and use up that oxygen with their metabolic processes, especially in the beginning, when it's actually necessary for their life cycle/metabolism.

"Degassing" does refer to assisting in removal of CO2. Strictly speaking, it's not that CO2 is toxic to yeast, but it lowers the pH -- CO2 is an acid in solution via the equilibrium formula:
CO2 + H20 ↔ H2CO3 ↔ (H+) + (HCO3-)
This is partially why a carbonated version of a beverage tastes different than a still version of the same, and also why sniffing CO2 "burns" your nose! (It's literally an acid burn...)

At any rate, in an active fermentation, there can be enough CO2 being produced to measurably lower the pH to the point it is inhibitory for the yeast. This can be attenuated by avoiding acid additions (including some fruit additions) in early fermentation, as well as by degassing, and also by adding a buffer such as potassium carbonate (K2CO3), as advocated by Bray as well.

Personally, I use an oxygen diffusion stone to add pure oxygen to the mead before pitching, and have occasionally used an additional burst of oxygen in the next 1-2 days, but mostly it's one dose at the beginning. I use the "shake the $h!t out of the fermenter" version of degassing, mostly because it's easier, and I'm all about making it simple if I can. Regardless of how you degas, beware of the mead geyser, or "MEA" (mead eruption/explosion event)! In my case, as I do with all primary fermentations, I use a blow off tube!
 
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MakinMead

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I think a lot of the confusion comes from the term "aeration," which should actually read "oxygenation." I'm sure Dr. Bray, PhD / @loveofrose could comment on this more authoritatively, but in the early stages of fermentation, the yeast do require copious amounts of oxygen to reproduce (hence the term "aerobic phase" of fermentation). You do *not* need to worry about oxidation while you are actively fermenting; the yeast will absorb and use up that oxygen with their metabolic processes, especially in the beginning, when it's actually necessary for their life cycle/metabolism.

"Degassing" does refer to assisting in removal of CO2. Strictly speaking, it's not that CO2 is toxic to yeast, but it lowers the pH -- CO2 is an acid in solution via the equilibrium formula:
CO2 + H20 ↔ H2CO3 ↔ (H+) + (HCO3-)
This is partially why a carbonated version of a beverage tastes different than a still version of the same, and also why sniffing CO2 "burns" your nose! (It's literally an acid burn...)

At any rate, in an active fermentation, there can be enough CO2 being produced to measurably lower the pH to the point it is inhibitory for the yeast. This can be attenuated by avoiding acid additions (including some fruit additions) in early fermentation, as well as by degassing, and also by adding a buffer such as potassium carbonate (K2CO3), as advocated by Bray as well.

Personally, I use an oxygen diffusion stone to add pure oxygen to the mead before pitching, and have occasionally used an additional burst of oxygen in the next 1-2 days, but mostly it's one dose at the beginning. I use the "shake the $h!t out of the fermenter" version of degassing, mostly because it's easier, and I'm all about making it simple if I can. Regardless of how you degas, beware of the mead geyser, or "MEA" (mead eruption/explosion event)! In my case, as I do with all primary fermentations, I use a blow off tube!
Very informative, I appreciate the effort you put in to that. Definitely cleared up everything I was concerned about, and I’m sure helped others. Thanks!
 

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What a great write up indeed, one thing I'll have to mention though is the addition on the potassium carbonate which is a salt based buffer. It will work but the key thing is the salt, may I also suggest calcium as a buffer.
I've not used the potassium as a buffer so I can't say for sure but I have used many times the calcium so that's what I mention, it's a great buffer that will not raise the ph above 5.5 unless used in massive quantities but still once a proper ph is obtained it stops working. It uses what it need and leaves the reast.

In larger ferments we've drilled holes in sterilized oyster shells and suspended them in a ferment, there was obvious loss on them from buffering the ph "working"

I did not weigh them after or before, the ph never dropped below 4 or above 5.

If you have a ph crash, raising the ph will need a strong base. Pickling lime is a great one for the job, make a slurry in a Pyrex measuring cup and add small amounts to get the ph to a proper level, then add the shells to buffer from lowering again.
 

biochemedic

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Thank you for the kind words, and I'm glad to be helpful; it lets me stretch some of the primary science brain cells that I haven't used as much since college!

@Shine0n, "salt" is also a loaded term, as it is a homonym for our perception of a particular flavor component, but technically speaking, all buffers are salts. Salt doesn't necessarily imply sodium, nor a salty flavor. FWIW, we often use other potassium salts, potassium metabisulfite and potassium sorbate, to introduce sulfite and sorbate ions to halt fermentation/stabilize, and in the case of metabisulfite, also for its antioxidant properties.

In both current cases, K2CO3 and eggshell (which is largely made up of Calcium Carbonate , CaCO3), the addition is a source of carbonate ion (CO3--) which helps remove or "buffer" hydrogen ion from solution, thus potentially raising the pH, but also more importantly, helping resist changes in the pH within a certain range. I'm not certain that one is any better than the other, but both probably do the job.

I've sent a PM to @loveofrose to see if he can add anything else to the conversation that we might be missing....
 

loveofrose

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Hello all. In mead making, you are correct that this can be a confusing topic.

Aerating vs Degassing vs Stirring

Aerating or oxygenating would be to blow O2 through a tube in order to encourage yeast reproduction early in the ferment. You would not want to do this later in the ferment as you will oxidize your mead. I’ve heard people swear it improves the mead, but I’ve never needed it because I don’t underpitch my yeast.

Degassing is to put the entire carboy under vacuum to remove all CO2. This is generally done after the ferment is finished to speed clearing. CO2 trapped in solution can prevent yeast from settling to the bottom. This is also why folks who never stir or rack their mead complain it takes forever to clear.

Stirring is really what we mean when we say degassing/aerating. The idea here is to rouse the yeast from the bottom of the carboy so that it doesn’t get buried, Lazy, and give up on making alcohol. At the same time, this removes CO2 from the must and adds some oxygen to encourage yeast health. It’s not as complete as true degassing, but good enough to improve the health of the ferment.

pH vs Buffering and Salt types

Beer making generally adjust pH to around 5 as this is best for beer making. Mead is not beer. In mead, we don’t really want to adjust pH up because lower pH (read more tartness) tends to make a crisper mead. The catch is that if your pH drops too much (<3), most Yeast will give up and stall.

This is where buffering comes in. We want to buffer pH swings (as opposed to adjusting pH) so that acid produced by fermentation doesn’t drop below 3.

In mead, the best buffers are KHCO3 and K2CO3. They have to perfect pKa to buffer the mead in low concentrations as well as good solubility and undetectable taste. In addition, K+ is beneficial for the pumps associated with enabling yeast to pump alcohol out of the cells thus improving yeast health (and allowing you to max out ABV if so inclined). None of these things are true for CaCO3. CaCO3 tends to leave a chalk taste in mead that is not noticeable in beer.

I hope that clears things up a bit!
 
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biochemedic

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Thanks @loveofrose, wasn't sure what the exact rationale for using potassium salts as opposed to calcium was; sounds like multiple good reasons. I had suspected that pK values and buffer ranges may also have been in play, but again, it's been a while since my core science days...

I would point out that further difference between beer and mead is that the pH concerns for beer are more for the mash (which is targeted to a pH of 5.2 to optimize amylase activity) than for the fermentation itself, and the biologic processes of the yeast. I would suppose that there may be additional organic buffer systems at play in beer given the more complicated solution, which may make need for additional buffering agents moot.

Finally, and this is slightly off topic, in terms of the final pH and crispness/mouthfeel/flavor perception, you can (and I almost always do now, following the advice and guidance of a professional meadmaker) adjust things with acid additions *after* fermentation finishes. I find that a little acid blend makes many meads just "pop" a bit more, especially if it's a mead with even a little bit of residual sweetness. Varying the types of acids (citric vs malic vs tartaric) can change the flavor perception quite a bit as well. I have one recipe in particular that benefits from a greater ratio of citric acid, for example. Also, going back to CO2 itself being an acid in solution, if you are fortunate enough to be able to keg, applying even a little bit of carbonation can improve the mouthfeel and flavor perception of a mead.
 

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