Why is that some wines can be enjoyed only months after bottling yet deteriorate with aging, whereas others are undrinkable when young and improve with time? The answer is that each wine contains its own special blend of acids and sugars, minerals and pigments, phenols and tannins that determine how a wine tastes when it is young and how that taste evolves over time. Great wines tend to have more of these flavorings and aromas than other wines, yet too many acids and tannins create a harsh, bitter taste. Over time, if the detrimental effects of temperature and oxygen are controlled, the elements that give wine its character can come to terms with one another and work together in harmony to produce a valued aged wine.
But what are the flavorings and aromas, those essences that give wine its character? Acids and sugars are the basic ingredients for fermentation. They are the nutrients from which alcohol and glycerine are created. The vine draws minerals and pigments from the air, soil, sunshine and moisture-in short, from what the French call terroir-and these elements are what give wine its color. Phenols give wine its aroma. Tannins are natural preservatives that, when young and upstart, impart a taste to wine which is described as astringent. Yet, tannins mellow with time and ultimately convert to sediment that collects on the bottom of the bottle.
The French drink a lot of wine, and spoilage of newly bottled wine was a problem that greatly concerned them in the mid-1800s. Thus, the great French scientist Louis Pasteur was asked in 1863 to find out why some wines tended to go bad soon after bottling, and he found that too much contact of the wine with air promotes the growth of vinegar-producing bacteria. Yet small amounts of oxygen also allow wine to mature. Apparently, there is enough oxygen dissolved in wine to last it for many years, even when wine is stored in an airtight bottle. This dissolved oxygen reacts slowly with compounds in the wine to mature them, and may continue to react after maturity to cause the wine to deteriorate.
Wine begins to age as soon as it reaches the fermentation barrel where the wine begins to absorb oxygen through the barrel staves. Thus, stave thickness, the wood the staves are made from, whether the staves are encrusted with tartrate crystals, the barrel capacity, whether the barrel stands in a draft, and whether the wine is racked from one barrel to another are all important factors.
The wood, usually oak, is especially important because the wine picks up from it extra tannins that affect the taste, and extra phenols that affect the aroma. The strongest flavors come from the most aromatic oaks, and the greater wood porosity, the greater the aroma. The most porous woods tend to come from the fastest growing trees. Because oak from the Limousin forest in France is fast growing, it is quite aromatic and imparts a strong "oaky" taste to the wine. American oaks from the wet forests of the California Coast impart sweet, spicy flavors, and German oaks from the cold forests of the Baltic have low-porosity wood that imparts delicate aromas. The slowest growing oak comes from the Tronça;is forest in France, and many consider it to be the best.
If left too long in the barrel, wine may lose color and fruitiness, and start to taste flat. Yet the same wine if bottle aged will retain its fruit, body, and acidity for several years, and perhaps improve. Many wines are barrel aged longer than necessary, yet barrel aging is much cheaper than bottle aging.
The aging process that takes place in a corked bottle is quite different from that which takes place in a barrel. Rather than absorbing oxygen in the bottle, the wine gradually loses it. Although the wine does pick up a little oxygen through the cork, the aging process depends mainly on the small amount of oxygen originally in solution in the wine. Because the oxygen supply is steadily reduced as the wine matures, the process is called "reductive aging".
Tannins are the main preservatives in red wines, and during bottle aging the tannins react with pigments and acids to form new compounds, some of which absorb back into the wine to alter its taste, whereas others settle to the bottom as sediment. This results in slow destruction of the pigments, and red wines ultimately lose their color. The tannins also loose their bitterness, the aromatic qualities of the phenols are enhanced, and the acids and alcohol react with oxygen to lose their harshness by forming new organic compounds called esters and aldehydes.
Because white wines have far fewer tannins and phenols, their main preservative is acidity. White wines with sufficient acidity, as with some Chardonnays, may mature as long as reds, and in a few cases, as with some Rieslings and Chenin Blancs, even longer. As white wines mature they gradually darken to gold, and the acids and phenols mellow to create tastes and aromas described as honeyed, nutty or oily.
Although aging may mellow, and in some opinions enhance, the flavor of a wine, the best time to drink most wines is now. If freshness and fruitiness are what we desire, then there is no point to keeping the wine for years and years, except to find out what happens when the freshness and fruitiness are gone. Some of course disagree, and for a few the older the vintage the better. Most of the dark red wines can be drunk after five years or so, but a few need more time; and a few tannin-rich wines may lose their sweetness long before the tannins mellow. To learn more about storing wines, we suggest you see what Robin Garr has to say about it on his web page.
To learn more detail about how grapes are crushed, how the wine itself is made, and the art of the winemaker you might want to see what the All About Wine website by Hugh Johnson has to say about it.
Alcohol - The presence of alcohol is one of the things that distinguishes wine from other beverages, and grape juice is particularly well suited for the production of alcohol by fermentation. In fact, grape vines are unusual in that they store carbohydrates in the berry as glucose and fructose (6-carbon sugars). Most fruits store carbohydrates as starches and sucrose (common table sugar). The distinction is that glucose and fructose are easily fermented, whereas starches and sucrose require conversion first to glucose and fructose.
Astringent - The description for the lip- and gum-puckering, bitter taste of acids, in particular the tannins (tannic acids) that give character to many wines.
Esters and Aldehydes - Are straight-chain hydrocarbons, containing some double bonds, that may be present in wines. Esters, in particular, are important since they are odorous compounds that can add strong flavors. Esters may be produced by some types of yeasts during fermentation, but in most cases they are produced during bottle aging by reactions of acids in the wine with the alcohol.
Fermentation - The process by which microscopic organisms convert carbohydrates to energy. The organisms responsible for this process in grapes are yeast, and the yeast convert glucose and fructuose (6-carbon sugars) to energy with the by-products of alcohol (ethanol), glycerine (another type of alcohol) and carbon-dioxide. Although any number of different types of yeast may be involved, those of the genus Saccharomyces are most important as far as wine making. The fermentation reaction in most cases is anaerobic, that is it takes place in the absence of oxygen.
Anaerobic conditions are important, because oxygen, in other words exposure to air, encourages the growth of vinegar-producing bacteria, in particular bacteria of the genus Acetobacter. These bacteria create energy by converting alcohol (ethanol) and oxygen to acetic acid (common vinegar). Thus, in air-tight containers, fermentation by yeast results in the conversion of 6-carbon sugars to alcohol, whereas exposure to air results in the conversion of alcohol to vinegar.
Sulfur dioxide is a pungent gas that most yeast can tolerate in small amounts, but which tends to inhibit the growth of most other micro-organisms, particularly vinegar-producing bacteria. Sulfur dioxide is also an antioxidant that prohibits oxidation, or "browning", of grape skins. Thus, most winemakers add small amounts of sulfur dioxide to their wine while it is in the fermenting barrel.
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