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Sunday, April 22, 2012

"OXIDIZED WINES". Sometimes things are not what they seem...

After tasting an old wine from Argentina for "#MalbecDay" (picture), an interesting discussion developed this week on twitter: Oxidized wines.

Apparently, some people seem to be troubled by the fact that a wine is named "oxidized", while others seem to have problems to appreciate a wine that, as a sensorial perception, seems to be oxidized.

According to Jamie Goode ( "It's not easy to define when a wine is oxidized to the point of faultiness and when it is showing signs of oxidation consistent with normal aging. Except in extreme circumstances, this decision is not always clear-cut".

I agree with Jamie. There are more shades of grey when it comes to wine oxidation than many people (who think they know all about wine) care to admit.

Let's try to understand why it is not "clear-cut" by stating the obvious: Wine contains ethanol => The oxidation of ethanol can produce volatile compounds (acetaldehyde being the most important volatile wine carbonyl). Well, acetaldehyde is a molecule that is highly reactive. It displays aromas and flavors of straw, roasted nuts, and apple-like.

It would be simple to describe a wine that shows these characteristics (straw, roasted nuts, and apple-like aromas and flavors) as "oxidized", but things are a little more complicated than this:

  • Acetaldehyde can be formed both chemically (by wine oxidation - chemical formation of acetaldehyde relies on exposure to oxygen), but it can also be formed biologically as Saccharomyces cerevisae (most used commercial yeast) excrete acetaldehyde during the initial phases of alcoholic fermentation.
  • The odor threshold for acetaldehyde varies considerably (0.5 mg/L - 10 mg/L).
  • The addition of SO2 to stop alcoholic fermentation (as in the production of wines with residual sugar) leads to the formation of acetaldehyde.
  • Unless wines are handled carelessly (or, on purpose oxidized, such as in the production of Sherry or Madeira wines) after alcoholic fermentation, most of the acetaldehyde found in wine actually comes from yeast activity.

Some important conclusions can be drawn from these facts:

  • A wine can be perceived as oxidized by a sensitive taster to acetaldehyde, while another taster doesn't have that same impression. The difference in threshold might be as high as 20-fold!
  • A wine cannot be branded "oxidized" by the simple fact that acetaldehyde is perceived in the wine. The production of this volatile compound may have had a biological formation (and nothing to do with excessive exposure to oxygen).
  • If you are ever asked this question "You don't know oxidized wine when you taste it?", your answer should be: "No, I don't. I can't know if a wine is oxidized when I taste it (one hundred percent of the time), because the tell-tale aromas and flavors of acetaldehyde (green-grass, apple-like, nutty) may not have been caused by oxidation at all. Simple logic.

This is what Patrick Farrell MW (my mentor) had to say about it:

"One can form many different oxidation compounds, acetaldehyde being just one, either through exposure to oxygen or via microbial action, often the two in combination. The exposure can be accidental or purposeful. I pick up a walnut note on the finish of microx wines. Flavor compounds oxidize as well, some for the better and others for the worse.

Now, there’s oxidation in the bottle. Here’s where things get tricky. Sometimes, it is from dissolved oxygen in the bottle, other times insufficient SO2, and sometimes the combo. It is possible for a wine to oxidize in the bottle, without exposure to oxygen. Keep this in mind in that oxidation as a chemical term is different from oxidation as a wine term. Oxidation reactions involve electron transfer and do not require oxygen. So, it is possible to have a well stored bottle of wine, often white, that tastes oxidized, yet the closure has been fine."

I hope this helps you to understand a little better "oxidized wines". Sometimes things are not what they seem...

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Richard Gibson said...

Just to clarify a point here - yes, yeast produce acetaldehyde during fermentation. The produced acetaldehyde is usually strongly bound by the sulphur dioxide added after fermentation is complete and cannot be detected on the nose. If oxygen exposure of the wine occurs and SO2 is depleted to a low level, the bound acetaldehyde is liberated and the wine appears aldehydic. So, aldehyde produced by yeast can become apparent due to oxidatve processes that have an impact on SO2 levels. More oxygen exposure can drive further aldehyde production due to conversion of ethanol...a much slower process than SO2 depletion.

Luiz Alberto said...

Hi Richard, thanks for taking the time. Much appreciated.

Yes, you are correct. Acetaldehyde is the most important SO2 binder (about 80%), along with pyruvic and ��-ketoglutaric acids.

It's also worth mentioning the fact that acetaldehyde is re-utilized by yeast in the second half of the alcoholic fermentation and it is degraded by lactic acid bacteria simultaneously with malic acid during MLF.

And that, to a certain degree, acetaldehyde will be used by chemical reactions, such as the one that occurs during red wine aging - polymerization of phenolics.

I started this discussion because I believe that wine can taste "oxidated" without undergoing a truly oxidative process (in the sense that the wine was overexposed to oxygen).

Are you in agreement with that?

Please keep in mind that enology is not my background. I'm just a student trying to put together pieces that will help me and other fellow students of wine to pass their exams.

Thank you,

Anonymous said...

Tasting "Vin Jaune" from the Jura region in France the initial impression of the unfamiliar taster is that the wine is oxidised and faulty. However it is made in a similar way to sherry (but not fortified) and it has a unique and not unattracrtive character once one has accustomed the palate to it. The oxidation is deliberate and gives it it's character. See Jancis Robinson's Oxford Companion to Wine.

Duane Bowman said...

Luiz - Saw your comment on the Blog. You said - "I started this discussion because I believe that wine can taste "oxidated" without undergoing a truly oxidative process (in the sense that the wine was overexposed to oxygen)."

In that you are on solid ground - well, somewhat.

First we have to agree on what we mean by "oxidized." Usually the textbook perceptual characteristics of "oxidized" are 1) browning, 2) reduced fruit flavors and 3) distinctive aromas/tastes (aldehydes, acetic acid, ethyl acetate).

Does having one of those absent the others mean a wine is oxidized, or must all 3 be present to qualify? You've focused only on the presence of aldehydes to be synonymous with oxidized. Maybe that's a bit too restrictive a definition?

The reason I say that is because there are various paths to getting one or more of these characteristics at a time. Phenolic/enzymatic as in laccase, chemical (requires a catalyst) and microbiological as in yeast and lactic acid bacteria actions will yield them individually or in combination. All require some level of O2 at some point in the process, but the levels need not be "excessive" nor long lived exposures.

Now for the big question - do we care to be so precise in our perceptual definition of oxidized? Well, for judging wines and just detection of a "flaw" probably not. Which makes your observation above right on. But for winemakers the differences in what's seen (the presence or absence of "oxidative" attributes) will mean very different corrective actions.

And sometimes we even "oxidize" wines on purpose!

Rob Hansult said...

I think technically you make an interesting point I don't necessarily disagree with.

From a tasting/drinking perspective the issue is simpler to me, especially having a nose with a fairly low threshold for oxidative aromas.

For me the question is the age appropriateness of the level of oxidation. Is it in a "normal" range? Advanced? Retarded?

This is an important question for my palate regarding many aged wines since I value fruit quite highly, & the vibrant fruit of a high quality wine is usually the first victim of advanced oxidation.

This is also obviously not to say that oxidative aromas/flavors are in themselves "bad" - as you stated.
I Love aged Sauternes (don't reach their peak until some oxidation has occurred), Sherry, barrel-aged Muscats, Madiera, etc.

Just my 2 cents.

Cheers, Luiz!

Chris Williams said...

The presence of Acetaldehyde is not the only indicator of oxidation. More importantly is the whole aroma and flavour profile of the wine. If the fruit aromas are dull and lifeless, that is a good indicator of oxidation in most wines. The timing of oxidation is also important- a wine can be oxidized in barrel, bottled and then move into a reductive state in bottle, which will have a dramatic effect on the flavour profile. Some wines are made "oxidatively" -old style white Rioja for example, but are certainly not oxidized. Oxygen has just been influential in building the flavour profile at certain stages of production.