Alan has the right of it, in that CO2 is not an anti-oxidant. But because CO2 is heavier than oxygen and will displace it at the surface of wine, aiding in preventing oxidation. Argon does the same thing, it’s just a barrier. And not a particularly effective one. But typically the heavier gases will also absorb into the solution, so even in a closed tank the argon or CO2 layer needs to monitored. And if you have any turbulence at all, it’s likely some oxygen will be absorbed.
But if you are bubbling O2 through the liquid itself it should be taken up by the solution, even if inefficiently. My guess is that the lees ability to absorb dissolved oxygen is more likely why the micro-ox would appear inneffective (though a number of other things could also be the reason and without a significant amount of analysis this is all just wild speculation).
I get the idea of CO2 in solution slowing down the rate at which oxygen can be absorbed (a cup that’a half full of water would slow down the rate at which milk could be absorbed by the glass-crude analogy for this group I’m sure) but for micro-oxygenation where the oxygen is added very slowly, the absorbtion of that oxygen should still be handled by the available space, no?
Sorry to do this again, but some clarification is needed here. There is a common perception that “heavier” gases settle lower, and create a protective barrier. But that’s not really true.
If you were to spray some argon gas on top of a tank, it might very well “settle”, but not because it’s heavier, but because it’s probably colder, having cooled during the process of escaping through a tiny nozzle. Then it would start to diffuse, and mix with the surrounding air, a process that takes on the order of minutes.
In a real situation, you would cover and enclose the tank, then pump in nitrogen or argon, and seal the tank as much as possible. That would keep for much longer, with outside air not able to mix easily.
But if you just put a heavy gas and a light gas together, they will not separate to a heavy and light layer, they will eventually mix uniformly.
I suspect the notion of CO2 providing some kind of protection against oxidation comes from what I theorize is the effect of CO2 bubbling up during fermentation. That more physical process probably does scavange other gases, like O2, and expel them at the surface. But that’s a different scenario than, say, increasing the CO2 concentration at bottling, and expecting that to provide chemical protection against O2, instead of using SO2.
No reason it shouldn’t. The idea is to replace the volume of wine you remove with argon, instead of the air that would get in if you just opened the bottle and poured. The goal is not to introduce more oxygen into the head space than was already there in the unopened bottle.
I think the big ?-mark I have is the speed of these processes. Coravin claims the wine will stay fresh for years. You’d think that with the massively increased surface area (and volume) would make the oxygen ingress/reactions happen a lot faster?
I think the idea behind coravin is sound. The one problem area seems to be that the hole punched in the cork, though it closes up to a large degree, still leaves a much clearer path for air ingress than the original cork. Maybe if you waxed the top of the cork after using the coravin, it would restore the integrity?
IIRC, this is true. As CO2 bubbles up and out from the wine, it also takes some O2 along and pushes it out of the solution as well. There needs to be quite a bit of O2 in the must before the fermentation, but the combination of yeasts expending the O2, and the CO2 that they produce expelling the remaining O2 usually depletes most of the O2 from the wine. Of course processes like racking, punch-downs, pump-overs and lees stirring can be used to introduce some new O2 into the wine.
However, I remember reading multiple research papers or summaries in which the experiments clearly show that dissolved CO2 slows down O2 dissolution into the wine considerably. This has nothing to do with the free CO2 in the headspace, but what is in the wine. So increasing the CO2 concentration at bottling might not provide chemical protection against O2, but it still does provide protection. I guess that’s more like physical protection than chemical?
I try to see if I can find any papers or anything on the subject that I could link here.
From the conclusion part: In the case where there is no production of CO2 but where a high initial dissolved carbon dioxide concentration is present, the “protective” effect acts only by reducing the rate of oxygen transfer. For instance, in the case of an initially carbon dioxide saturated wine, our previous work has shown a 10-fold decrease of the observed kLa values, corresponding to correlatively strong reduction of the oxygen transfer fluxes (Devatine et al., 2007).
For a contact with a gas at a given kLa value, 95% saturation is obtained at a time equal to 5/kLa. So, these lower kLa values result in an important time delay in reaching oxygen saturation in initially carbon dioxide saturated solutions. Indeed, for short accidental contacts with air, the protective effect is efficient.
Are you sure about this? I’ve known of people dying in breweries and wineries from co2. When lower doors of fermentation tanks are opened post fermentation there is a high concentration of co2 that settles to the ground level. If someone breathes in enough at standing level to pass out, then they fall to the floor where the concentration is higher and they die if no one rescues them soon enough. Not common but it is a real danger. My guess is @Rick_Allen is familiar with this.
People often incorrectly say hot air rises. Well, yes, it does but it is because cold air is more dense (ie a higher concentration of mass) and forces the warmer air upward as the colder air sinks due to gravitational effect of of the mass. Similarly, co2 has a higher density than o2 and will settle lower unless there is some energy, such as wind, to mix the two.
Fascinating. My own knowledge is limited to the military realm, where one element of CBRN / NBC defense is having a positive overpressure system. And they did teach us that it was not completely effective.
It’s very different releasing a giant pulse of CO2 (will cause unconsciousness at like 1% concentration in air for a few minutes, way before it displaces significant volumes of the O2), which is usually what happens when a fermenter/cylinder fails, onto a factory without ventilation,…than having a closed bottle with a very-slowly-semipermeable cork with a mix of O2 and CO2.
O2 and CO2 are intrinsically miscible (see: our atmosphere) and the timescale of complete mixing in a small volume at room temperature is on the order of seconds, not weeks-months!
The 2016 Juvé y Camps Cava Gran Juvé & Camps Gran Reserva Brut I had recently was a good example of the C in CBRN, and some natural wines (and I say this as a fan of them) most definitely fall under the B. But none have to do with overpressure…
Alan is correct. The gases will eventually mix equally throughout the vessel. That mixing doesn’t happen immediately though. When we were purging air out of tanks, we could add CO2 to the bottom, and push the air out an opening at the top. The problem is that it took a lot of CO2 to scour out the last of the air, so we ended up wasting a lot of CO2. CO2 is expensive enough that this isn’t a good approach. We now fill a vessel we want to purge with water, and then push the water out with CO2. Usually we try to do this to multiple tank at the same time, so you just push the water to the next vessel that needs to be purged.
A couple points about your comment: The amount of CO2 thrown off by an active fermentation is tremendous. The place where I’ve heard about people having issue is when they were doing punch-downs. In those cases, the CO2 is rising off the must, and dilutes the oxygen in the air to the point where the person passes out and falls into the vessel.
CO2 and air (with all its components) mix to be very nearly uniformly distributed. But, it takes some time and if there is a source of CO2, it will initially fall before it’s mixed. You can find various Youtube videos of people using baking soda and vinegar to create CO2, then extinguishing a candle flame by pouring the CO2 onto the flame.
In the atmosphere, convective processes keep the gases mixed down at the elevations where we live (so-called troposphere). Under the influence of gravity alone (no convective processes), a gas in the atmosphere would evolve to a distribution that exponentially decreases with height/elevation, with the characteristic scale length depending on the molecular weight of the gas (and the temperature). So, solely from gravity, the concentration of different gases would vary as you move to higher elevations, but they would all be present. The scale length of a typical gas is around 8 km, so you would have to move up quite a distance for mixture of oxygen and CO2 and other gases to change very much. This is just from gravity, winds, etc., mix more thoroughly.
Deaths happen with fairly regular occurrence in breweries and wineries from co2 falling from tanks after the lower door is opened. I thought you might be more aware of this. I’ve known directly of two instances.
You can definitely see behavior that seems to be consistent with the idea that “heavier gases sink, light gases float”, but in every case, you can find a reason why: like a bunch of cold CO2 was released from a tank, it quickly displaces much of the surrounding air, and settles to the bottom of a tank, something like that. But over time, as the temperaturue equilibrates, and mixing is allowed to occur, everything in the room, or tank, will mix uniformly.
There is a gravitational element to separating molecules of different weights, that’s how centrifuge isotope enrichment works. But you need a much greater difference in effective gravity potential to achieve that than a few inches or feet separation (edit: just saw Al’s 8km comment, which is what I was referring to).
Sudden release of large amounts of a gas is a real risk. In my field of MRI, where both liquid nitrogen and helium are used, there is a lot of emphasis on safety in handling those gases and working around magnets. There have been a number of deaths in the industry caused by an unexpected magnet quench, or mistaken release of a cylinder into a small enclosed space. It’s a serious risk. And I’m aware of some fatalities in the wine industry as well, where too much CO 2 built up in an enclosed space.
Yes, CO2 produced by a tank, or pumped in through the bottom valve of a gassing bell, is extremely effective at displacing air because of its density. It will eventually mix given enough time, but if you can displace the air and then seal the tank, you’re golden, as Alan was saying. Nitrogen is abysmal and mixes so effectively with air as to be barely worthwhile. Not too surprising given that air is 78% nitrogen. I’ve done plenty of wine movements and treatments where nitrogen is used and you see big oxygen pickups in the wine. Change the gas to CO2 and the DO barely budges. But CO2 is expensive and can be dangerous in some settings, so is more rarely used. And you can use nitrogen somewhat effectively, you just have to be thoughtful and use quite a lot of it.
High dissolved CO2 in a wine does indeed lower oxygen solubility to a sufficiently high degree that it seems to slow the overall oxidation of a wine. In the winery, keeping some dissolved CO2 in a wine should therefore confer some protection during wine movements. During bottle aging specifically I am less convinced that this is a major factor, since oxygen ingress is so much slower than oxygen consumption that all bottled wines have essentially 0 dissolved oxygen at any given time, so solubility is not the limiting factor you might expect it to be. Still, it should, and does seem to, have some impact. Much more going on in Champagne though, and the low phenolic load, metal content, low pH, and several other components may have impacts that probably aren’t well understood.