Oxygen, and the Potential for Hype

There has been a great deal of research and claims concerning oxygen and wine of late. Much of the research has been from companies with agendas such as bottle closures or oxygen sensors to sell. Most will tell you that oxygen is critical to the aging of wine in a bottle, at least if done just so with their particular brand of closure.

A bit of data pulled from several of these reports adds much more confusion than clarity. A professor at UC Davis was recently cited on a very popular wine site as stating that the amount of oxygen that passes through a healthy cork is about 1mg/L a year (1ppm). The same site ran an article a few years ago from an oxygen testing company that says the sweet spot for ppm of oxygen is 6 and that at 8.5 it is considered a flaw. Another well respected trade magazine cites that depending on the amount of free SO2 in the wine, a fresh white with 6ppm oxygen is ideal.

Have you, as I have, ever enjoyed a twenty year old
Riesling that was fresh and free of oxidation?

So far so good, the 6ppm is consistent, on release, but what about a mere two years later? If the wine has absorbed another 2ppm of oxygen then it is now oxidized, and the flavor will suffer. Again, this is consistent with recent research into shelf life of wines, but what about with your own experience? Do you find that most well stored white wines have completely lost their freshness and have started to become nutty after only two years? Have you, as I have, ever enjoyed a twenty year old Riesling that was fresh and free of oxidation?

Part of what is going on here is the reporting of studies to the general public by people with insufficient knowledge to summarize it well. Very important terms like Redox Potential and the Nernst Equation are rarely mentioned in these articles. Or even simple terms like acidity, temperature and storage conditions. Instead, blanket statements about the need for this lining or that for screw caps, or the viability of this synthetic closure vs. that natural cork are thrown about, usually, again, by those with an agenda.

My favorite (least favorite?) example of how science has been dumbed down for the wine consumer is the term Reductive Aromas. Prior to the popularity of screw caps the term referred specifically to thiols (mercaptans), the nasty smell added to odorless gas so you know when there is a leak. Low levels resemble garlic, higher levels resemble rotten eggs, somewhere in the middle is the more common rubbery aroma. This aroma is all too common in wine, and results when sulfur bonds with hydrogen instead of oxygen bonding with hydrogen. The less techie explanation is that it is due to the lack of oxygen during wine making. Dipping a bit of copper into a wine that has this aroma will change it immediately.

The term Reductive Aromas now seems to refer to the smell that winemakers blame on screw caps being too perfect of a seal. Meaty is a word that is often thrown around. The claim is often that the wines were perfectly made, but suffered in the anaerobic (reductive) environment of the bottle.

In the original sense of the word, the term implies a winemaking fault. In the new sense it implies a fault outside of the control of the winemaker - but which can miraculously be solved by buying the right screw cap liner, or using the right cork or cork alternative.

Some of you may have read my article a few weeks ago about decanting. In that case the young red we tested exhibited no noticeable change from being left out to the air for an hour. The most common response was that we used the wrong wine (even though I invited everyone to duplicate the experiment with whatever wine they think will work better). These replies suggest that the common wisdom is that oxygen greatly changes or improves wine, in an hour (except of course the one wine we tested).

With bottle aging, the common wisdom, and much, but far from all of the research, suggests that oxygen improves wine in the bottle, but slowly. The UC Davis professor tells us that this is because of the 1 ppm ingress of oxygen through natural cork (which he assures us is the best for aging, for this very reason). What he doesn't tell us is that wine has an oxygen saturation point of about 10 ppm at cellar temp and 8 ppm at room temp and most wine is released somewhere near 6 ppm.

So…

Too much oxygen is bad, exactly the right amount is good. Wine needs oxygen to age, but it is saturated with oxygen after only a few years in the bottle. Oxygen is important in the bottle to help reduce the aromas of thiols (presumably by turning them into less stinky disulfides). Yet, the proper use of oxygen during wine making will prevent the formation of thiols in the first place. Gas permeable linings in screw caps, and gas permeable enclosures in general are important because they help prevent unpleasant aromas in the bottle (which may have been prevented in the first place) and allow wine to age, at least for a couple of years. Decanting wine exposes it to oxygen and improves it, except the wine we tested.

To end this, allow me to throw yet another monkey wrench into the mix. The long term studies of scientists like Peynaud and Ribereau-Gayon pretty well shows that wine ages best in a perfectly hermetically sealed bottle, and that oxygen is the bane, not the savior of finished wines. These studies are decades old. Science changes, that is the very nature of science, so it is possible that we have learned more since then, but science can also be done in a virtual vacuum with a goal in mind. Too many of the recent studies seem to completely ignore the importance of electrochemistry in determining the role of oxygen, while stressing the importance of their machine, or perfect wine closure in a bid to convince the science semi-literate, such as consumers, and all too many winemakers.

Confusion and contradictions abound. There is little agreement in scientific circles about the proper role of oxygen in wine, so it is no wonder that so many voices fill the void with cries of aerating tools, decanting instructions, and the perfect way to seal a wine bottle. In the absence of certainty, the loudest voices often win.

Sources:

http://winefolly.com/review/chemist-explains-corks-matter-storing-wine/

http://winefolly.com/tutorial/the-truth-about-oxygen-and-wine-aging/

http://www.winesandvines.com/template.cfm?section=features&content=66003

http://en.wikipedia.org/wiki/Nernst_equation

http://en.wikipedia.org/wiki/Thiol

http://www.dnr.mo.gov/env/esp/wqm/DOSaturationTable.htm

http://www.nanaimowinemakers.org/Steps/H2S_Issues.htm

Traité d'oenologie, tome 1 et 2 - P. Ribereau-Gayon et al.

Knowing and Making Wine - E. Peynaud