What happens as wine ages? And how do you know when it will be best to drink?

When wine sits in the barrel or the bottle, the flavors, aromas, and colors change.

Not every wine will benefit from aging. A bottle made to be opened immediately, as most wines today are, will turn brown and lose all of its fruit character if you cellar it for years. The wines that age best, chief among them Bordeaux, have relatively high levels of tannins and acidity.

Tannins are a category of chemicals that come from grape skins and seeds. They have an astringent, somewhat bitter taste and make your mouth feel dry. But over time, tannins “soften” because they polymerize, or form long chains with each other. The tannin polymer molecules feel and taste less harsh, says Ken Fugelsang, a professor of enology at California State University–Fresno.

As a wine ages, it also develops a stronger aroma. Before tannins chain up, they “hold on” to volatile aroma chemicals, keeping them from evaporating, says John Clews, vice president of vineyard and winery operations for Clos Du Val in Napa, California.

Like cut apples, vino turns brown when it comes into contact with the air. As it sits in the barrel or in a bottle with a cork (which is porous and lets a little air in), a new red wine, initially deep purple, turns reddish, then slightly orange, then dull brown. In whites, the wine first loses the greenish hue that marks a very young bottle, then develops a golden-brown tinge. Just as lemon juice keeps cut fruit from browning, acid in wines slows the oxidation process, which means that acidic wines are better candidates for long aging.

Oxygen also affects the aroma and flavor of wine. In the best-case scenario, fruit flavors and smells in young wines fade and combine with wood and alcohol notes to produce something that’s more winy than fruity. Age a wine further and you’ll get leathery, earthy notes in a red and nutty, bready flavors in a white alongside the fruit, Clews says. It’s this complexity that inspires connoisseurs to pay hundreds of dollars for a long-aged bottle.

These reactions aren’t totally understood by chemists. In fact, the jury’s still out on how screw caps, which create a virtually airtight seal and are relatively new, will affect aging.

Both acid and tannins act as preservatives, slowing oxidation and decelerating the flavor-changing reactions. Whites, which are very low in tannins, are less worthy candidates for aging. But highly acidic grapes like Chenin Blanc and Riesling are exceptions. Some German Rieslings, which are extra-acidic thanks to a cool climate and long growing season, can age even longer than many red wines, says Mulan Chan, French regional and Rhône Valley wine buyer for K & L Wine Merchants. Another exception is white Burgundies. The Chardonnay grapes that go into these wines are grown at much cooler temperatures in Burgundy than they are in California, raising their acid content.

So how do wine critics decide that one year’s crop of Bordeaux will be best in 12 years and another’s in 30? By considering how previous vintages in a particular style and from a particular region fared and how the weather from the year of the vintage in question affected tannins and acidity in the grapes, it’s possible to make an educated guess about when a wine will be best. But it’s mostly a matter of personal taste, Clews says. However, there are a few wine-analysis companies out there that can measure tannin and acid profiles and by comparing them to other wines can scientifically determine how a wine will age. Though the practice isn’t common, it’s catching on, Fugelsang says.