On the footsies of vinegar flies

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Aug 9, 2023
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I've read countless posts in countless threads on this forum warning of the dangers of fruit flies landing in your wine causing it to spoil and turn to vinegar. To me this has always seemed a bit ridiculous, and contradicts a lot of how I think about about fermentation and microbes.

What is Life?
You, blue whales, petunias, bacteria and our beloved yeasties are all essentially wet sacks of enzymes - overall simply facilitating the fundamental chemical reaction of life: sugar + oxygen --> CO2 + water. As long as the first two ingredients are in our environment we skim off some energy along the way to do all the things we consider interesting, but the fundamental driving force is a very basic chemical reaction going from a high energy state to a lower one. When deprived of oxygen, some of us are clever enough to hitchhike on a different chemical reaction. We humans can take advantage of (sugar --> lactic acid), yeast happily surf down (sugar --> CO2 + boozeahol), and the bacterium acetobacter uses a fairly unique reaction to skim it's energy off of: (alcohol + oxygen --> acetic acid (vinegar)).

A simple problem, a simple solution.
Notice that last equation has oxygen on the left hand side? Confirmed by my armchair biochemistry and wikipedia searching, there is no way for acetic acid to be made in a fermentation situation without the presence of oxygen. Yes, the enzymes inside acetobacter can facilitate a reaction that produces vinegar, but only in the presence of oxygen. So long as we properly exclude oxygen, adding acetobacter, from the feetsies of flies or elsewhere, will never produce acetic acid. And we can be pretty sure we have excluded oxygen when we're fermenting something alcoholic because yeast would much rather suck oxygen out of its environment than produce booze.
My point is that acetic acid bacteria, as just a bag of enzymes, can only catalyze the oxidation of alcohol, but they can't do anything without a fresh supply of O2.

Need some more proof?
Sure, acetobacter need oxygen, but despite our efforts maybe there is enough oxygen left lying around in headspaces, or still dissolved, to let that vinegar fly make our wine into vinegar? Well, let's do the math:
  • A 5-6ish gallon batch of wine is roughly 20 liters.
  • At, say, 10% abv that means we have 2 liters of pure alcohol
  • Density of alcohol is 789 g/L so we have 1578 grams - call it 1500g of alcohol
  • Molar mass of ethanol is 46g/mol, so we have 1500/46 = about 33mol of ethanol (which means the number of molecules of alcohol in our batch is about 2... with 25 zeros after it!)
  • Each molecule of alcohol can be turned into one molecule of acetic acid with one molecule of O2, so to turn it all to vinegar we need 33mol of oxygen as well.
  • One mole of any gas at room temperature and pressure occupies 24 liters of volume, so the oxygen required is 24 * 33 = 794, or about 800L of pure oxygen
  • Air is about 1/5th oxygen, so we need about 800*5 = 4,000L = 1,060 US gal of air to turn your fairly conservative 10% batch to vinegar. And, that amount would have to be bubbled and re-bubbled through so that all the oxygen was used up. I would guess that really at best a tenth of the oxygen in contact could actually be pulled into the wine, putting the amount of air required closer to 10,000 gal!
Of course, all of the alcohol doesn't need to be turned to vinegar to ruin a batch, but its in no way a trivial amount of air that's needed either. To turn half a percent of the alcohol to vinegar would still require about 50 gallons of fresh air to somehow contact your wine.

Another way of thinking.
Less important are the actual organisms (mold spores, bacteria, fruit flies, etc. ) in your fermenter, but much more important is the environment you maintain in there. Microbes like all living beings are just riding and facilitating some pretty basic chemical reactions, but those reactions can only take place if all the ingredients are already there in the environment. I'm sure some of you more refined geezers out there could taste the parts per million of acetic acid introduced my smaller amounts of air, but I most often seen this warning given to beginners, and still the amount of vinegar is completely dictated by how well you exclude oxygen. Not by the presence or absence of a single germ, or fly. Acetobacter is in the air as well, and probably already in the must - files or not. On the other hand, adding a tablespoon of "vinegar flies" to your brew (not recommended) won't make a difference vinegar-wise if everything's good and airtight.

Closing thoughts.
I love to wild ferment whatever I can get my mitts on, and that means I accept all microbial (and sometimes insect) gate-crashers to my fermentation party. So far, I've never had a noticeable issue with acetic acid, because I focus on keeping my fermentation environment one in which yeast and only yeast thrive. Yes, oxygen is present earlier in the fermentation, but alcohol is produced under anaerobic conditions, and acetobacter needs alcohol. Yes, I still sterilize my equipment, mainly to discourage other nasties that can survive in the early ferment before the alcohol makes things safer.

To me, the myth of "vinegar flies" makes a lot of sense - fruit flies are airborne creatures, so if they are touching your wine, SO IS AIR!

--Yeasty Boy
Welcome to WMT..

I have asked the question in the past myself. I read somewhere that 1 fruit fly can ruin an entire commercial batch of wine. I suppose in theory it could. If it introduced acetobacter that was not present before... AND all of the other variables were in place to allow the wine to spoil, then yes I suppose a fruit fly can ruin a wine.

It doesn't make it a guarantee though, and the only proof I need are the many fruit flies I have found floating in my musts that over a year later still have not turned to vinegar.
there's also two other components to consider:

1. once the wine reaches 10%+, it becomes exponentially harder for it to take hold
2. (for those who do so) it'll likely be stabilized long before it can take hold

as a bit of a germaphobe, i get far more worked up thinking about what that fly may have been crawling around in prior getting into my wine...(hint...poop)

i've seen videos and read articles about people who intentionally TRIED to turn their wine into vinegar by literally adding vinegar to the batch, and it wouldn't take hold or took months and months (I think this further corroborates point 1, since they added it after fermentation had occurred)
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Welcome to WMT!

I agree with your assessment and your perspective, with one* exception. I have NOT seen countless posts on this forum warning about fruit flies and vinegar. I have seen plenty of posts that explicitly mention that acetobacter need oxygen, and we are advised to control oxygen exposure for that reason as well as other oxygen-induced flaws.

*Okay, another small exception is that wine yeast will produce alcohol anaerobically even in the presence of oxygen, i.e., even if they could respire aerobically, provided the sugar content is high. It is called the Crabtree effect, and it probably evolved to give S. cerevisiae a leg up over other yeasts in high-sugar environments.
@yeasty_boy Welcome to WMT!

Thanks for sharing your thoughts. I am less concerned about fruit flies than I was when I first started. As you pointed out, acetobacter use an aerobic process to convert alcohol to acetic acid. Once primary fermentation is finished, we limit oxygen exposure and use Kmeta to remove oxygen from the wine. So we already have some good safeguards against making vinegar.
Thank you all for your warm welcome to the forum! :)

@sour_grapes You are right I'm probably just extra sensitive to seeing this "myth" - I first heard it from my father many years ago when we used to brew together. I did the calculations back then which felt like 'closing the case', so seeing this it go unchallenged a few times in more recent threads may have, ahem, triggered me a bit. Also very interesting about the Crabtree effect, thank you!

-- Yeasty Boy
@yeasty_boy Thanks for posting this information. And, thanks @sour_grapes for pointing it out...

I recently asked to have the definitive conversation about fruit flies after doing a search of the forum for "fruit fly/fruit flies" and not finding much except fear inducing warnings to keep those little critters out of the wine or all that Cabernet Sauvignon is destined to transform into something akin to Balsamic! My crush pad brings them in and they meander to the winery. To be clear, the "winery" is my garage...:h
Yeasty Boy - Maybe you just have way too much time on your hands. But thanks for the great review anyway. I had a tough time in qualitative chemistery, (had it was 60 years ago) and would have to do a lot of study to get your results. Good work!
I hate those darn flies anyway.