Wine and chemistry

I’ve been reading The Billionaire’s Vinegar recently, the story about wine unearthed allegedly owned by our third President, Thomas Jefferson. While I’m only half way through, this “mystery” revolves around the discovery and authenticity of wine bottles with the inscription “Th J” and the intense bidding wars for these prizes unleashed at auction. You can read my review on the book by clicking here. What caught my eye was a discussion about the roll that oxygen plays in the maturation or spoiling of wine, in this case, a wine allegedly two hundred year old. It caused me to pause, reminding me that basic chemistry is so vital to the quality of the end product that we enjoy.

Oxygen was discovered by Joseph Priestly in 1774 when upon burning Mercury Oxide, noted that an odorless gas allowed a candle flame to burn far longer than anticipated. In 1775, Priestly placed a mouse in a closed jar with oxygen and to his astonishment, it survived 30 minutes and was revived without incident. Oxygen is the third most abundant element in the universe after hydrogen and helium and constitutes just short of 21% of our atmosphere. There are two forms of the element that make life possible for all of us. One is diatomic oxygen (two atoms combined to form O2) and the other O3 or Ozone, a layer high up in the atmosphere that protects us down here from the hazards of ultra violet radiation. Ironically, Ozone is a pollutant at the surface and can be a component of smog.

Ok, enough basic chemistry. Let’s get down to the Dr. Jekyll/Mr. Hyde personality of oxygen during and after wine production. As a primer, let’s establish the stages of wine production in very simple terms and then parse out how oxygen does its work: there’s harvest, crushing, pressing (the process of forcing juices from the grapes) and fermentation, storage, bottling and at some delightful point, popping the cork or unscrewing the cap.

In the big picture, oxygen can be a friend, an enemy or both to any given wine. Big, bold tannic red wines with lots of astringency benefit from some oxidation, oxygen facilitating the break down of these tannins, allowing them to reform into polymers. These polymers have a soft and smoother mouthfeel, making the wine more balanced. Alternatively, give oxygen the freedom to react with alcohol for a prolonged period of time and you risk transforming the alcohol into acetic acid (the acid in vinegar).

Oxygen begins to react with grapes post harvest as soon as they’re crushed, making it advantageous to transport harvested grapes in shallow containers where “gravity” pressing is less likely to occur. Many vintners prefer to have the crushing and destemming apparatus close to the pressing vats in order to reduce oxygen contact. Unless strict precautions are taken during pressing, it’s a given that oxygen will react with the juices causing some browning or oxidation of the liquids. Think about how cut apples, avocados or newspapers turn yellowish-brown upon exposure to oxygen. To counter this natural process, many vintners add small quantities of sulfur dioxide to the mixture, inactivating the enzymes responsible for oxidation.

Depending upon the category and style of wine desired, the vintner has a choice during the pressing and fermentation phase of either exposing or limiting the must (skin, stems, seeds and juices) to oxygen. If the wine is destined to be a bold and tannic red, the must is likely to be an astringent “soup” that could benefit from oxygen exposure. This can be accomplished by performing this phase in an open wood or cement vessel, allowing some oxygen seepage and leaving the top open. Stirring and punching down the cap will serve to expose all of the must to air, as well. This results in the slow breakdown of some of the more astringent elements such as tannins which then recombine into compounds (polymers) that yield a softer feel. (If this process occurs in the environment of a closed bottle, you’ll find a carpet of these polymeric sediments layered in the most dependent part of the bottle after a period of time).

Alternatively, if the red is to be light weight in style and targeted for current consumption (vs. aging), minimal oxygen contact is desired, the focus being on retaining the light and lively freshness of the wine. This can be accomplished by timely crushing, fermentation and transferring the must to a stainless steel fermentation tank (or a wood vessel but for just a short time in order to limit oxygen exposure).

The delicate flavors and aromas of white wines are far more susceptible to oxidation than reds requiring vintners to be extra diligent in the process. Over oxidation in whites can spoil the fresh, delicate flavors and produce very perceptible and unpleasant acetaldehyde and vinegary aromas. Thus it’s very important that the crushing process be accomplished in the most efficient manner. The issue is not quite as sensitive with reds as their complexity often “hides” potential defects.

Fermentation of the red must takes place prior to pressing in a vessel that can range from wood to cement to stainless steel. The vessel can be closed or open allowing the must to have less or added contact with oxygen. Again, this decision often lies with the anticipated weight of the wine.

Alternatively, fermentation in whites occurs after the juices are pressed off. Many whites are fermented in closed, stainless steel vats in order to limit oxygen contact. CO2 produced from the fermentation process in concert with adding an inert gas such as Argon can be very efficient in excluding oxygen. Fresh, acid driven wines such as Sauvignon Blanc and German Rieslings would be good candidates for this style of fermentation. Some whites, such as Chardonnay, do well with barrel fermentation in order to give the wine that full and buttery personality so often associated with the California style. Here, the wood allows for some very slow diffusion of oxygen into the wine, which some vintners feel adds additional flavor nuances. Unoaked or “naked” Chardonnays have become more common in recent years, these wines stored in stainless steel containers to eliminated oxygen and retain the pure freshness of the Chardonnay grape.

Once the fermentation phase is complete, the juices are them removed and stored. Depending upon the wine, oxygen contact will be either allowed or restricted. As you might gather, the meaty and tannic red wine will likely welcome a little oxygen contact in order to slowly soften the elements. These wines are usually stored in wood vessels (barrels) allowing for very slow oxygen diffusion during the storage process. Lighter reds and the lighter, acid driven whites such as Sauvignon Blanc are likely to be stored in stainless steel tanks in which the ullage space will be filled with inert gas to limit oxidation.

Once the storage process is complete and bottling is begins, the producer has the option of spraying carbon dioxide and an inert gas such as Argon into the bottle to reduce exposure to oxygen. This can be very crucial, especially if the wine is a lively white or red to be consumed in the near term. On the other hand, a deep, bold and tannic driven wine meant for long term aging, not only would not suffer from a little remaining oxygen, but may well benefit in the months and years ahead, the magic of chemistry softening the tannins and bringing the wine’s elements into ideal balance. This is how the truly collectible reds from regions such as Bordeaux and Burgundy find their way to the top of the quality charts.

Meanwhile, I’ll be fascinated to continue on through The Billionaire’s Vinegar to read the tasting notes of those who have been offered such a unique opportunity to taste history. If the bottles of wine containing the inscription “Th J” really do represent an era of wine bottling 200 years ago, how do you think the wine will taste?