Evaporation not Oxidation

Interesting summary of how the chemistry of decanting and other quick impact techniques to ‘open up’ wine work.

I didn’t read the more detailed research referred to in the article, but it makes sense to me that evaporation is the culprit/agent that affects the wine, not oxidation. At least over a few hours. Seems like temperature, therefore, is incredibly important (as I assume that a few degreees warmer fluid will evaporate faster).

Any thoughts from the chemists here? Or philosopher-chemists?

Another nail in the coffin of the slow-ox technique.

“Slow ox” is a new term for an old technique, which has already been exposed as a misnomer. In other words, another nail in the coffin of a stupid term that should have already stopped being used.

Rich - Thanks! This is a terrific, concise piece packed with info.

I’ve been looking for a good discussion of the role of evaporation as well as oxidation, and this is it. It was nice to find support for the view I’ve pressed (harped on?) in discussions here of decanting and oxygen absorption: “Since most oxidative reactions take several hours or days to show up at detectable levels, evaporation remains the prime source of change in decanted wines.” Most people here have assumed that breathing is all about oxygen reactions.

Pop and pour: The article says that a lot of evaporation occurs immediately on opening, which explains the experience most of us have had where bottles show a huge burst of aromas when first opened.

Decanting: The sections covering what happens later were even more illuminating:

In most situations—say, dinner at a restaurant—decanting takes place in a matter of minutes, so oxidative changes are limited, according to Ugliano. “Oxygen reacts with wine quite slowly—over hours or days—so it’s not obvious that aroma-impacting chemical reactions will take place,” he says. “Of course, if you decant and then wait for a couple of hours, some changes due to oxidation will probably take place. However, most people don’t do that.”

That confirmed my anecdotal experience over many years of wine drinking that some wines really do require long decant to show the most.

Differences between grape types: This part was even more fascinating, and I’d never come across this info:

Individual wines react to oxygen differently, so the oxidative changes that occur can manifest in various ways. Some grape varieties have aromas that derive from thiols—the blackcurrant note in Cabernet Sauvignon, for example, or the passion fruit in Sauvignon Blanc—and > some of those thiols can be broken down relatively quickly by oxygen, correlating with the loss of some fruity aromas. > > Other varieties contain high levels of antioxidant substances, such as glutathione or phenolic compounds like tannins and anthocyanins, which will intercept oxygen and prevent it from changing the aromas> , in some cases even over several days.

That could explain why some grapes such as nebbiolo and syrah can require particularly long decants.

Yes. In fairness to Francois Audouze, I don’t think he ever used that term. As I recall, he always referred to the technique (popping a cork on an old wine and removing a bit to taste before letting the bottle rest) as a way to let bad smells blow off.

This article doesn’t discuss it, but Jamie Goode has written elsewhere on factors affecting evaporation.

For example, sugar in a wine tends to makes it more aromatic, because the sugar competes in the solution (i.e., the wine) with the volatile aroma compounds, and the latter come out faster. That explains why sweet wines are often more aromatic.

Conversely, he once described experiments altering the alcohol level of the same wine. At some point, as the alcohol rises, aromas drop off because the alcohol makes it easier for the aroma compounds to remain dissolved in the wine.

Sorry John, this is incorrect. Look up Raoult’s law. Adding a non-volatile solute will decrease the vapor pressure of both the solvent, and any other volatile solutes.

I’m dubious that evaporation of alcohol has much if anything to do with how we perceive a wine to change with aeration over an hour or two. We can all do the experiment pretty easily, but I’m also dubious that you can lose 1% ABV from a glass over an hour. Maybe if you’re swirling it often, coating the side of the glass, where it could more easily evaporate.

Alan,

Wouldn’t this also depend upon the ambient temperature as well as the temperature of the wine? My elementary guess would be that at colder temperatures, evaporation is slower and accelerated are warmer temperatures?

Cheers

Larry, sure, of course. I’m just assuming they did this at some reasonable drinking temperature (which, presumably, would be cooler for a white wine, but still not particularly high for red).

All I know is what I’ve read from Jamie Goode, PhD.

Jamie has posted here, maybe he’ll see this and clarify. But from the way you describe his sugar claim, it doesn’t make much sense.

Not a chemist (just a simple little ole country computational physicist…spoken in my finest SamErvin drawl), but I play one on the “BillNye, The Science Guy”
TV show.

It’s pretty well accepted that oxidation plays little/no role in the decanting of wine and the resulting changes. What are the oxidation products in red wine?
Browning and precipitation of sediment as the tannin molecules polymerize. You don’t see that at all in a decanted red wine over a number of hours. So
clearly oxidation is playing no role, the reaction rate for oxidation in wine is just too slow.

That being said; if you decant (don’t even have to decant it) a very old red wine (like 20-30 yrs old) that is in a highly reduced state, where the oxidation
reaction rate will be much/much higher; it is not at all uncommon to see a noticible browning of the wine over an hour or two time. Can’t speak as to
the formation of sediment, though. Never have looked very closely.
Tom

I read the article a few days ago, then I read the portion of the abstract of the underlying study which was available at the link. Without a subscription, I couldn’t look at the whole study.

The stunner for me was the finding, shown in the graph, that a 15% abv wine, after two hours of breathing (at least under their lab conditions, which included a fair amount of airflow) becomes a 14% abv wine and after six hours, an 11.8% wine. I’d never seen anything in the various discussions of the effects of aeration claiming such a dramatic difference. The article notes that other compounds one may want in the wine also evaporate in that time, but none so quickly as the alcohol.

I’m not sure I buy everything they say. As Alan said, losing a whole percentage of alcohol in an hour is a little hard to believe. And does that mean from the bottle, the decanter, or the glass?

Also, oxidation can take a long time, but if I recall from my chemistry classes a long time ago, pure oxygen is pretty reactive, and I’d imagine that a lot of reactions take place fairly quickly. As far as changes in aromatics, the most volatile chemicals will evaporate first won’t they? So that leaves the less volatile chemicals. But we don’t get a lot of information from the wine by sniffing it anyway. We get a lot more information by putting the wine in our mouths. That’s when we get most flavors and aromatics, based on reactions with air flow, saliva, etc.

As far as some grapes behaving differently, I think it’s well-known that some are more reductive, some less, they have different chemical make-ups, etc.

Anyway, interesting article.

The article suggests that there’s much more than reductive or not:

Some grape varieties have aromas that derive from thiols—the blackcurrant note in Cabernet Sauvignon, for example, or the passion fruit in Sauvignon Blanc—and some of those thiols can be broken down relatively quickly by oxygen, correlating with the loss of some fruity aromas. Other varieties contain high levels of antioxidant substances, such as glutathione or phenolic compounds like tannins and anthocyanins, which will intercept oxygen and prevent it from changing the aromas, in some cases even over several days.

Jamie G says in I Taste Red about how some chemicals enhance the aromas of other chemicals, and some suppress other aromas. (I have to assume TCA is an example of the latter.) So wine from a particular kind of grape can respond uniquely to evaporation and oxygen.

This article also says that reductive aromas can simply mask other scents, but the former tend to be highly volatile, so they dissipate more quickly than the other aromas. So it’s not just a matter of a grape having a reductive profile on the nose; that may have consequences for how it responds to extended breathing.

some chemicals enhance the aromas of other chemicals, and some suppress other aromas. (I have to assume TCA is an example of the latter.) So wine from a particular kind of grape can respond uniquely to evaporation and oxygen.

Understood. Didn’t mean to limit it to redox - I was just using it as an example.

We also know that various compounds don’t “blow off”, they react with oxygen to form larger, less volatile compounds.

As far as evaporation in an hour or two, I knew I had this somewhere! Some time ago, America’s Test Kitchen referred to some stats the USDA published on how much alcohol remains when you’re cooking with it:

stirred into hot liquid: 85% (this would be like mulled wine)
flamed: 75%
no heat, stored overnight: 70% (they didn’t note the number of hours but left this in a pot)
baked/simmered, alcohol stirred into mixture:
15 minutes - 40%
30 minutes - 35%
1 hour - 25%
1.5 hours - 20%
2 hours - 10%
2.5 hours - 5%

The vapor pressure of alcohol mixed with water, as in wine, is a lot lower than it would be from pure alcohol, and no heat is applied to the wine aside from the few degrees it might warm up if taken from a cellar, all of which would argue against losing much to evaporation in the context of an open wine bottle over dinner.

Interesting article.

So next time I pop a high-alcohol wine, I just decant and let it sit for a while?

Joking aside, if the wine is “hot,” will decanting bring it more into balance?

This is, more or less, Waterhouse’s claim for what happens when you aerate a wine. Nothing more.

Waterhouse?

I’m certainly aware of reductive scents blowing off. But I hadn’t ever thought about that process delaying the perception of other elements.

John, I’ve quoted this many times over the years in these aeration discussion:

To help me evaluate the Wand, the Clef and the whole idea of enhancing freshly opened wine, I called on two friends, Andrew Waterhouse and Darrell Corti. Mr. Waterhouse is a professor of wine chemistry at the University of California, Davis, and a specialist in oxidation reactions and phenolic substances, including tannins. Mr. Corti is the proprietor of Corti Brothers grocery in Sacramento, one of the most influential wine retailers in California, and a recent inductee into the Vintners Hall of Fame.

We met at Mr. Corti’s house for an afternoon of taste tests, lunch and discussion. Some tests were blind, others open-eyed. By the end, we had indeed detected some differences between carafes and glasses of wine that were treated with the Wand or the Clef, and the wines that were left alone. The differences were not great, and not always in favor of the treated wine, which usually seemed to be missing something.

Mr. Corti said: “There do seem to be differences. The question is, are they important differences? You could buy a lot of good wine for the price of that wand.”

He also pointed out that the Clef is a very expensive version of the copper pennies that home vintners have long dipped into wine to remove the cooked-egg smell of excess hydrogen sulfide.

Mr. Waterhouse thought the elimination of sulfur aromas is all that these accessories — or, for that matter, aeration — had to offer.

“A number of sulfur compounds are present in wine in traces and have an impact on flavor because they’re very potent,” he said. “Some are unpleasant and some contribute to a wine’s complexity. You can certainly dispose of these in five minutes with a little oxygen and a small area of metal catalyst to speed the reactions up, and change your impression of the wine.”

But Mr. Waterhouse maintained that no brief treatment could convert the tannins to less astringent, softer forms, not even an hour in a decanter.

“You can saturate a wine with oxygen by sloshing it into a decanter, but then the oxygen just sits there,” he said. “It reacts very slowly. To change the tannins perceptibly in an hour, you would have to hit the wine with pure oxygen, high pressure and temperature, and powdered iron with a huge catalytic surface area.”

So why do people think decanting softens a wine’s astringency?

“I think that this impression of softening comes from the loss of the unpleasant sulfur compounds, which reduces our overall perception of harshness,” Mr. Waterhouse said.

With devices debunked and aeration unmasked as simple subtraction, the conversation turned to genuinely useful tips for handling wine.

Sorry, I haven’t committed all your posts to memory.

In any event, the text you quoted seems to be focused on oxygen’s interaction, not differing rates of evaporation or masking (either by covering up or by chemical interaction with our sensory apparatus).