yeasty_boy
Junior
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:
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
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!
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
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