We all know that, once you open a bottle of wine, it starts turning into vinegar. And oxygen is bad for your beer, mead or wine – you should do everything you can to protect your batch from exposure to outside air! So what happens if you slip up a bit? If your beer, mead, or wine has been opened at all, is it going to turn into vinegar?
If your beer, mead, or wine has a powerful pungent or sour smell or taste, it has likely been contaminated with Acetobacter and turned into vinegar. This is typically a result of both sanitization issues and exposure to oxygen post-fermentation and can be prevented or mitigated with better practices.
Your homebrewed alcohol turning into vinegar actually is one of the bigger concerns, compared to the other possible contaminants I’ve written about before. While other infections are unlikely or basically impossible – and even if they do happen, your batch is still probably drinkable – ending up with vinegar is very possible (if you don’t protect your brew enough) and can be disappointing.
It’s even a concern with commercial alcohol! I actually don’t drink wine very often – not because I don’t like it, but because I feel like I have to commit to a whole 750ml bottle in one or two nights or else it’ll become too vinegary after it’s been opened.
Basically, though, you don’t have to worry about it if your processes are good enough. Even if your batch gets contaminated with the bacteria that produce acetic acid, it cannot be converted into vinegar unless it’s exposed to enough oxygen.
So take precautions to minimize exposure to oxygen – don’t open the vessel more than you need to, and keep headspace to a minimum once fermentation is complete – and you should be just fine.
And if something does happen (like the liquid in your airlock evaporates – which is something I’ve seen happen to people on Reddit too many times) then it’s not the end of the world. The silver lining is that vinegar is still useful and can be enjoyed, just not for drinking. Use it for cooking, and chalk it up to a lesson learned.
How to Know If Your Batch is Turning Into Vinegar
This might be a little blunt, but honestly, the best way to determine if your beer, mead, or wine has been contaminated with acetobacter and is being (or has already been) turned into vinegar is if it smells or tastes like vinegar.
I can provide some flavor and aroma descriptors to compare to, but the easiest way to figure it out is to open a bottle of white, apple cider, or any other vinegar that you have in your pantry, sniff it, and compare that smell to your batch.
What does vinegar smell like? It’s a very pungent and powerful smell. It can also be described as sour or puckering. If your batch smells like this, then it’s likely been contaminated and has been turned into vinegar.
This powerful or pungent smell and flavor are very different from the smell and flavor of ethanol (ethanol is alcohol, which is good – that’s what you’re going for).
Very young meads and wines (and some beers, especially the high-abv ones) have a powerful alcohol smell and taste. This can often come across like jet fuel or rubbing alcohol – you can easily compare the scent to a bottle of vodka or medicinal alcohol. Additionally, I’ll often get solvent-like or rubber flavors from young batches.
If you’re getting these sorts of smells and tastes, your brew simply needs to age a bit. These will fade with time.
Do note that vinegar has a very distinct sort of sour smell and taste compared to other contaminants. Vinegar is a specific kind of acid (called acetic acid) with its own characteristics.
If you’re getting sourness that’s more tart, you might be looking at something like a Lactobacillus infection instead. Click here for more information on that.
Why Does Your Alcohol Turn Into Vinegar?
Alcohol is turned into vinegar in the presence of acetic acid-producing bacteria, Acetobacter.
Acetobacter consumes alcohol in a metabolic process that produces acetic acid as a by-product, much in the same way that your yeast is consuming sugar and producing alcohol.
Acetic acid is what vinegar is, by the way – the term “vinegar” literally means “sour wine.” Most vinegars are a liquid solution with a low percentage of acetic acid.
What Is Acetobacter (acetic acid bacteria)?
I won’t go too deep into the biology and chemistry of Acetobacter and the creation of vinegar. However, I will try to explain it at a high level – it’s useful to understand why things are the way they are so that you can understand the context of what that means and what you can do about it.
Acetobacter are a group of bacteria characterized by the ability to convert ethanol into acetic acid in the presence of oxygen. Like yeast (and other microbes we may encounter in brewing) there are many strains of Acetobacter, each with their own characteristics, tolerances to abv and pH, and flavors that they produce.
I’ve stated before about how fermentation is a fantastic preservative, and that the yeast, in an attempt to outcompete other microbes, create an environment that is hostile to most bad bacteria. Acetobacter is the exception – the only way to prevent them is to keep them out in the first place and/or don’t expose the batch to oxygen (more on that further down).
Since it needs ethanol to survive, Acetobacter is fairly tolerant to high alcohol. Most strains can survive in a solution up to about 8% or 10%, but there have been documented cases of strains living in even 15% or 15% batches! This is actually one of the reasons it’s suggested that your meads and wines be at least 12% (and most commercial wines are), as that should protect it from Acetobacter – although it’s not a perfect solution.
Beer doesn’t have to be such a high abv, since hops are part of the recipe. Hops have a preservative effect as well, and while they are not perfect either, they do tend to keep Acetobacter from working. That’s why most beers simply become oxidized and don’t become vinegary.
Acetobacter can also function all the way down to a pH of 3.0, although they would much prefer a higher pH around 5.3 to 6.0. Most brews finish in the 3.0 to 4.0 range, so the bacteria is perfectly happy.
How Does Acetobacter Produce Vinegar (acetic acid)?
The conversion of ethanol to acetic acid is a metabolic process, which the bacteria use for energy.
It’s very similar to the metabolic process that yeast use to convert sugar into alcohol, or that we use to convert food into energy.
The process first converts the alcohol into acetaldehyde (which, itself, is often an off-flavor in brews) which is then converted into acetic acid.
One important thing to note is that oxygen is a necessary part of the metabolic process – without it, Acetobacter cannot perform this conversion.
What this means is that, even if your batch has been contaminated with Acetobacter, vinegar cannot be created without both ethanol and oxygen. Nothing can happen to your brew prior to fermentation (when there’s no alcohol), nor can anything happen if the batch is never exposed to oxygen post-fermentation.
How Does Acetobacter Contaminate Your Brew?
The conversion of ethanol into vinegar also requires the Acetobacter bacteria to be present in the batch.
If your sanitization practices are good, then Acetobacter should not be present in your batch when it is put together and you pitch the yeast. However, no one is perfect, and as the process goes on, chances arise for contamination: while you should be doing what you can to mitigate it, you’ll still likely open the container to check gravity, rack to secondary, or bottle/keg the batch.
When someone is intentionally making vinegar, they often “innoculate” the batch with Acetobacter much like you would pitch yeast. However, instead of making a starter or adding a package of dry yeast, Acetobacter is typically added either by using a “vinegar mother” or by adding a bit of unpasteurized vinegar (which will have the bacteria in it, much like unpasteurized alcohol would have yeast).
However, while these methods are the most reliable way to ensure the bacteria makes its way into the back, intentional inoculation is not the only way that it happens.
Acetobacter is an airborne microbe, and if your batch is exposed to the air for too long, there’s a good chance that it can become contaminated.
Every time you open the fermenter and expose the batch to air, there is a chance for Acetobacter to be introduced. Additionally, if you aerate the batch prior to pitching the yeast by vigorously shaking the vessel, you’re incorporating outside air – which may contain Acetobacter.
And even a small number of these bacteria can reproduce quickly and grow a large colony in the presence of both ethanol and oxygen!
Fortunately, as has already been stated, it does require both of those things. Simply having Acetobacter in your brew will not result in vinegar – or any other problems, for that matter.
Will Open Fermentation Contaminate My Brew? Will It Become Vinegar?
Open fermentation is a method where you don’t use an airlock during active fermentation. After you pitch the yeast, you actually leave the fermenting vessel completely open, leaving the lid off of it. Most brewers will cover the opening with a cheesecloth, muslin bag, or similar just to keep insects like ants and flies out, but that’s it.
I’ll dive more deeply into what open fermentation is, how to do it, and its pros and cons in the future. But for the purpose of this article, it does beg the question: if exposure to air can result in your brew becoming vinegar, won’t open fermentation cause that?
Open fermentation does not result in the batch becoming contaminated with Acetobacter or turning into vinegar. The yeast will consume any oxygen that enters the liquid, and gasses produced from fermentation will blow airborne contaminants away. You just need to seal it up before fermentation is complete.
In summary, while the batch is actively fermenting, there will be enough external pressure to push away all airborne contaminants. According to John Palmer, it should actually be enough pressure to keep flies out, so it should definitely keep Acetobacter (which cannot move itself and is subject to the movement of air) out as well.
So, if the batch is not already contaminated, it will not get contaminated, as long as you do open fermentation correctly (and close it at the right time).
If Acetobacter is already present in your brew somehow, it should not be able to produce any noticeable amount of acetic acid. During active fermentation, it would have to compete with yeast for access to any oxygen that enters solution – and, as already stated, the bacteria work much more slowly, so the yeast would outcompete them at this stage.
There is one study on microbe levels in kombucha that tangentially touches on this subject. I would have liked to see them measure the change in levels of acetic acid during the first few days (when active fermentation is happening and krausen levels are high) but they do not. It is impossible for me to conclusively say that no acetic acid can be formed during that stage without being able to directly measure it, although I do have an idea for a future experiment to test if any difference can be detected by taste.
However, based on everything we know about how these bacteria work and how fermentation works, I think we can safely say that your batch is safe from becoming vinegar at this stage – the key is sealing it up at the right time! You must do it before fermentation is complete, while krausen is dying down and it’s still off-gassing.
Additionally, brewers perform open fermentations all of the time and they don’t have issues with Acetobacter.
If Your Batch Has Turned to Vinegar, Is It Safe?
If your batch has been contaminated with Acetobacter, and has been turned into vinegar, it is safe for consumption. However, it will no longer be what you expect: the alcohol will have been converted to vinegar, and it will not taste like beer, mead, or wine.
You likely will not want to sit and sip on a tall glass of beer, mead, or wine that has been turned into vinegar!
However, vinegar is wonderful for cooking, pickling food, and other things!
You’ll probably want to just chalk this one up to a learning experience, and be more careful with the next batch. But this one is not a dumper!
Rather than tossing the batch, you may want to bottle it and enjoy it – just use it a little bit differently than you had planned.
Can A Batch That’s Turned Into Vinegar Be Salvaged or Fixed?
Unfortunately, once ethanol has been converted into acetic acid, there is no turning it back.
If your batch has been completely turned into vinegar, that’s the end of it. Now, you have vinegar, and will need to use it as such.
However, the process to turn alcohol into vinegar under normal circumstances is a fairly long and slow one. Acetobacter requires plenty of oxygen to undergo its metabolic process, and even if your batch is contaminated and has been exposed to the air, the surface area of the liquid is likely fairly small relative to the volume of the batch, which creates a bit of a limiter in terms of how much oxygen can get infused into it.
People who are intentionally creating vinegar will often use O2 injectors or aquarium airstones to speed this process up, forcing extra oxygen into the brew so that the bacteria has enough to work with.
But if the batch is simply sitting out, exposed to the air, it can take months to be completely converted to vinegar.
That’s one benefit of using carboys for bulk aging – by dramatically reducing the surface area of the liquid, you can reduce the exposure of the liquid to oxygen, protecting it from oxidation and from turning into vinegar.
And, even if your batch has been contaminated and exposed to the air for a moment, the Acetobacter can only convert ethanol so long as there are oxygen molecules available. That is to say, if there is only a little bit of oxygen, then only a little bit of ethanol will be converted before the bacteria goes back to sleep.
What does this mean? Well, even if you have to open up your vessel and expose it to air, if you minimize the exposure, you can minimize how much of your alcohol gets converted.
I’m guessing that, for most batches at the homebrew level, there’s probably a little bit of this conversion – and a little bit of acetic acid in the final product. However, it’s usually so minimal that it can’t really be detected by taste alone.
How do you minimize exposure to oxygen?
This is a continuous discussion on this website, because there are a ton of methods and best practices you can follow to protect your batch and improve your results, but a handful of good options are:
- Don’t open your fermenter unless absolutely necessary.
- When you do need to open it, only leave it open for a very short time – as short as possible!
- Additionally, open it the least amount you can. Vessels with lids that have a separate hole for airlocks and bungs, like the Little Big Mouth Bubbler and BrewDemon fermenters, are perfect for this.
- Purge any vessels or bottles with CO2 prior to siphoning liquid into them.
- Ideally, used pressurized vessels and closed transfers so that you never have to open the vessel at all.
By limiting the batch’s exposure to oxygen, you reduce the conversion to acetic acid.
A Note on Aging, Or: A Little Bit of Acetic Acid Might Be Desirable!
For many recipes and styles, I wouldn’t worry too much about a tiny bit of acetic acid conversion.
In fact, it can be a fundamental, and even desirable, part of long-term aging.
For many production red wines, it’s considered “best practice” to use a decanter or to let the wine “breathe” before tasting it. This exposes it to air, and allows the bacteria to begin producing acetic acid.
You wouldn’t let it sit out for days or even hours, but after a few minutes, the wine “opens up.”
For many beers, meads, and wines aged long-term in oak barrels, this same thing happens to them very methodically and slowly. Wooden barrels are porous, which allows a tiny bit of air to penetrate the vessel and enter the liquid over time, resulting in a very slow and very small amount of oxidation over time.
This is actually desirable, in some cases. Obviously not all recipes and styles benefit from this (I imagine a hazy IPA would not be good after long-term aging and even a minor amount of oxidation) but for many, it can build character.
My point is: don’t worry so much. If a tiny bit of acetic acid has built up in your batch, it might actually be a good thing. Try it out, and if it tastes good, enjoy it!
If it is a little too sharp, it might benefit from long-term aging, either in secondary or in the bottle.
Obviously, though, if it’s fully converted to vinegar, it’s probably not going to be good for drinking.
Conclusion
If it smells like vinegar, and it tastes like vinegar, it’s probably vinegar.
But if your beer, mead, or wine has been contaminated and turned into vinegar, it’s not the end of the world! Keep the batch, use it for cooking, and try again.
As long as you follow best practices, and meticulously manage your brew to limit exposure to oxygen, however, you should be fine.
And just a little bit of oxygen and acetic acid conversion may not be a terrible thing, depending on what you’re trying to make.
