Wine aging is, at first sight, decently easy with big rewards. Buy a bottle, forget about it for a year or five, and get a completely new wine to drink. Some cheapish bottles can turn into memorable sensory experiences with age and the great wines require it to reach their full potential. Some wines only make it a couple of years before they are better suited for cleaning the toilet bowl.
Basically, what happens during wine aging (from a sensory perspective) is that the characteristics the wine has gained from the grape varietal and the fermentation are diminished and replaced with flavors that give the wine complexity. These original attributes are traded for more subtlety in fragrance, mouth feel and flavors, enhancing the wine in the process. When it comes to white and rosé wines, and many red wines as well, aging dissipates the yeasty, spritzy characters in newly fermented wines.
The pigments in red wine stabilize and over time, the wine gets a smoother mouth feel and a new, more complex, scent- and taste profile which some people (including most wine professionals and many wine connoisseurs) prefer while most general consumers tend to enjoy the fruit-forward tones in young wines. However, if the wine has a short shelf life, which is the case for many lower priced wines made for immediate consumption, the primary characteristics (from the grape) or secondary characteristics (from the fermentation) are not replaced by interesting tertiary characteristics (from aging).
Putting wine down to age is easy, using wine chemistry to plot the path your wine will take over time is challenging. Many different chemical reactions take place in that bottle at the same time, where some are known and some are still not understood, even by wine chemists.
Cracking open the (bottle) science
If you are a real wine geek perhaps you are wondering what actual chemical reactions occur in the bottle to cause those changes we love (or hate, in case that anniversary wine you saved for years was opened past its prime). Not all chemical reactions result in changes above the detection threshold but many do and those are the ones we will look at. These are the most important and interesting chemical reactions in an aging wine bottle. The kind of stuff you can throw around at your next wine tasting and make the rest of the group swoon at your deep understanding of wine. Ready?
Many wines improve in quality during barrel and bottle storage. Basically, wine aging can turn cheap bottles of wine into memorable sensory experiences. Not all chemical reactions result in changes above the detection threshold but many do and those are the ones we will look at. (Photo by Marco Massimo / Pixabay)
What primarily happens in the bottle over time is that the organic compounds that create flavors and scents as well as the structural elements interact with each other and change character. This includes aromatic esters, phenolic compounds including phenols, anthocyanins, flavonoids, astringent tannins and polyphenols, as well as acids and alcohol. Some, like the tannins and pigments, will polymerize which means that they start clumping together over time. This makes them heavier and rounder, changing the way they feel (softer) or break light (redder/more brownish) or sometimes making them so big and heavy that they fall out of the wine entirely and create that sediment you have probably seen at the bottom of an older bottle. The result is reductions in bitterness and astringency for tannic red wines. Other compounds break down over time or change their formation, like a harsher acid changing into to a softer one.
During fermentation, a group of aromatic compounds called fruity esters are produced, especially in white wine. During aging, these hydrolyze (break apart) back to their original acids and alcohols. The brain associates fruity scents with sweetness, which is why some beginner wine drinkers will describe a completely dry, fruity wine as sweet. This loss of fruity flavors make it seem like the wine is less sweet once aged. Like for whites, rosé wines lose most of their fruity aromas in 1-2 years.
During the aging period, acidity decreases, additional clarification and stabilization occur as undesirable substances are precipitated, and the various components of the wine form complex compounds affecting flavor and aroma. (Photo by Thomas Verbruggen on Unsplash)
Oxidation - going easy on the air
Of all the chemical reactions in wine, oxidation is perhaps the most important one to get a grip on. It’s the same reaction that will brown an apple after it is cut. Oxygen that exists in the small amount of air at the neck of the bottle or enters the bottle at low rates through the cork produces is key in the oxidative reactions in the wine. In chem-speak, it means an electron (or hydrogen atom) leaves a compound present in the wine and attaches to the oxygen molecule. The resulting changes in the wine compound itself can result in a new aroma forming. Take tartaric acid for example, an acid present in many wines. When it oxidizes it creates some of the pleasant tertiary aromas lauded by collectors of aged fine wine. When a wine oxidizes too far, that acid will turn the wine into vinegar.
The speed, type and frequency of those oxidative reactions change based on the bottle closure (cork, screw cap, plastic…), on the pH of the wine and on storage temperature as well as on which phenolics are present in the wine from the beginning. The phenolics, hundreds of compounds that affect taste, structure and color of the wine, include tannins and the color pigments called anthocyanins (which also give red wine it’s antioxidant health benefits). Oxidation of acids can give both darker colors and softer acids. The color of a red wine as it ages will shift from purplish red to a clear, bright red and then to a more brownish, terra cotta tint. For white wines, which hold less pigment to start and can be almost clear when young, the color change will go from lemon/greenish to yellow, gold and finally brownish shades. Rosé wine doesn’t have much pigment so when the few pigments in the wine oxidize, the wine can turn orangish within a year or two.
Oxidation is one of the key processes in aging wine but, logically, also the reaction that might put your wine “over the hill” if allowed to go break down the wine too much. You may have heard the term “premox”, or prematurely oxidized, often in the case of premium white burgundies from the 90’s. In this case, the wine has gone bad much quicker than expected. Red wines can also age prematurely, getting early distinct prune/fig odors that mask the other components of the wine.
Oxidation of red wine over time. The color of a red wine as it ages will shift from purplish red to a clear, bright red and then to a more brownish, terra cotta tint.
Reduction - hoping for more from less
The winemaker can decide to let more or less oxygen be part of the winemaking and bottling process, which will reduce or increase the oxidative effects in the wine over time as well. A winemaker that strictly limits the amount of oxygen (in the form of air) that the wine comes in contact with both during the winemaking and bottling and chooses closure that allows for less or no air to enter the bottle is said to work reductively. This will keep the fruit flavors around for longer as less oxygen is around to break them down. If done too strictly, there can be some sulfur notes in the wine that hopefully dissipate with aeration or age.
Some wines, such as sherries and vin jaune, are made with so much oxygen present during winemaking that they are already stable and will not be much affected by more oxygen once opened. This is called “oxidative winemaking”. That’s why the sherry bottle keeps well for many days, even weeks or months after the first pour. These wines change with cellar age thanks to other reactions.
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What changes the aging process?
If I knew the answer to this, I’d be rich. Oh, you are still asking about wine? Ok.
How the wine fares over time depends on the wine’s aging potential, which is related to the presence and type of those structural components such as tannin, pigment, acidity and other compounds that can also provide new flavors when the fruit flavors get broken down.
If the wine is exposed to too much ultraviolet light, some of the compounds that would otherwise be stable might also deteriorate, stripping the wine of flavor. Tannins in the wine will do the opposite. If they don’t bind to each other, they can bind with aromatic chemicals in the wine and keep them in the wine over time. That is why a tannic red wine can hold on to its fruit characteristics longer, while simultaneously developing the tertiary - aged - flavors. Tannins act as a preservative and can keep the wine fresh by preventing oxidation. High acidity in a wine also acts as a preservative and will generally make for better aging potential which is one of the reasons champagne does well over time.
The vintec guide to perfect cellaring and serving temperatures.
Storing temperature is absolutely key. A wine that can be stored for 20 years at 10 C might make it 10 years at 20 C. Aging speed approximately doubles with every 10 degrees C storage condition. The slower the aging, the more complex the wine becomes. Above 30 degrees C the changes are rapid and the results are not very good. If the temperature is high, 30 degrees C, the wine can lose approximately 80% of those fruity esters we mentioned before in as little as two years! A good reason to have a quality wine cellar solution in place.
The aging potential of a wine depends on your personal preference as well. Those preferences are determined partially by genetics (!), partially by exposure. Some people are not very sensitive to tannins and astringency but like the fruit notes. If this is you, drink the wines younger rather than older. If you prefer smooth wines and enjoy the aromas of tobacco, leather, underwood and complex, more subtle flavors? Give the wine time and let good ol’ chemistry work some magic.
Other important chemical reactions
Hydrolysis - the chemical breakdown of a compound when it comes in contact with water. This can change the way it tastes and feels.
Polymerization - the joining of several similar molecules to form a polymer. for example, tannins can bind together and form larger, softer, tannin complexes or become so heavy that they fall out of the wine.
Structural rearrangement - changes in the structure of the molecules in the wine
Volatilization - making certain molecules ready to evaporate when the bottle opens, making them dissipate quicker or making them readily available for the enjoyment of your nose
Autolysis - a break down of the yeast cells left in the wine into smaller components.
Really important in champagne where a lot of the aging pre-bottling is done on the lees (which is a fancier name for dead yeast deposits)
Oak absorption - integrating the oak flavors into the wine and making them smoother and less prominent
Reference: Ronald S. Jackson, Wine Science: Principles and Applications, fourth edition, Academic Press.