Have you ever had the pleasure of sipping on a 50 year old wine? If so, do you do it regularly? If you do, can I come to dinner sometime? The reality for most of us wine drinkers is that popping open aged wine is an infrequent occurance. However, there is a perception that aged wines are by necessity better than younger wines. What is happening in a bottle that is aging, though? And what allows those really remarkable bottles to sit for generations?
There are a few major components of a wine that allow it to age-- protection from oxygen, the way it is stored, and the concentration of certain compounds in the wine. White wines and red wines, because of their differences in desired characteristics, concentrations of tannins and other anti-oxidative compounds, age differently.
The aging process is generally broken down into two phases. Maturation is the time post-fermentation during which the wine sits in barrel or tank. Time in barrel or bulk tank typically lasts anywhere from 6-36 months. During this time, the wine may be treated with clafiying agents, racked a number of times, and generally has some degree of contact with the oxygen in the atmosphere through the bung in the barrel and through general permeation through the wood. During this time, the yeast left over from fermentation drops out of solution, and the initial concentration of fruity esters drops as they transform into other compounds. Since most white wines are prized for their fresh, delicate fruit aromas, it is generally best to consume them young, before these esters have broken down too much. This is why you tend to see more recent-vintage whites on the shelves than reds.
Reductive aging begins when the wine is bottled. There are a few major factors that influence the aging process of the wine in bottle: oxygen, temperature, light, and pH.
Once in bottle, the wine's contact with oxygen is greatly reduced, with the only access being through the closure (cork, synthetic, or screwcap). The presence of oxygen in bottle can lead to a myriad of issues, and certain types of wine are more prone to oxidation than others. This is why prior to bottling, it's important to protect the wine from oxygen pick-up. Excessive oxygen in bottle can lead to browning of both white and red wines, as well as oxidative aromas and flavors-- those of bruised or rotting fruit. Acetaldehyde concentration increases, and certain phenols can become oxidized giving the wine flavors such as cooked cabbage.
Temperature can mean the difference between a fresh-tasting wine and a tired wine. Most chemical reactions speed up at higher temperatures, and those involved in wine aging are no exception. One example of this would be the appearance of a gasoline/kerosene aroma in Riesling-- a compound known as TDN accumulates quickly once storage temperatures creep into the 70s and 80s.
Light exposure plays a role in wine aging and quality, as ultraviolet light leads to the production of sulfur off-aromas that can make white and sparkling wines, especially, taste skunky.
The pH of the wine has a huge effect on aging, because the higher the pH, the more preservatives needed to protect against oxidation. The pH affects the equilibrium of phenolics, and esters in the wine. At higher pH you can shift the phenolics in a wine into a form that makes them more susceptible to oxidation. This is partially why higher acid wines with low pH tend to have more longevity. Additionally, the presence of a high concentration of tannins also has an antioxidative and slightly antimicrobial action, thus conferring to red wines an additional measure of protection that slows the effects of aging. And it's the slowing of the aging process that allows us to taste over a longer period of time, and choose the moment when we deem a wine "just right" to drink.
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