From Fermentation to Bottle: Malolactic, Filtering and Fining, Barrel Aging
The winemaker may choose to allow a wine to undergo a second fermentation which occurs due to malic acid in the grape juice. When malic acid is allowed to break down into carbon dioxide and lactic acid (thanks to bacteria in the wine), it is known as "malolactic fermentation," which can impart additional flavor to the wine. A "buttery" flavor in some whites is due to this process. Since malic acid is perceived as more sour than lactic acid, the process also reduces the perceived acidity of the wine.
Malolactic fermentation is much more prevalent in red wines than in whites, with the smell of apples in white wine denoting the presaging the presence of malic acid.
After fermentation, there still may be a lot of stuff floating around in the wine which some winemakers want to remove. There are various ways for the wine to undergo this "clarification" (for example, strain the wine through something like cheese-cloth, called "filtering"), but the most common way is called "fining."
When you make jellies, the recipes may sometimes call for adding egg whites. The materials that cloud the jelly are captured by the egg and you get a nice, bright result that looks really good in glass jars. It's the same with wine, even down to using egg whites. Except that the most common materials used for fining are gelatinor bentonite (a type of clay).
When and where to use heavy filtering and fining is highly controversial, since removing these substances prevents the wine from obtaining flavors from them, affecting the character of the wine. You are certain to hear complaints about "over fined and filtered wine." The implication is that such wines will have less flavor. For this reason some wines will say on the bottle that they are "unfiltered."
The winery may then keep the wine so that there can be additional clarification and, in some wines, to give it a more complex flavor. Flavor can come from wood (or more correctly from the chemicals that make up the wood and are taken up into the wine). When wood aging is used, wines are stored most commonly in oak barrels. It it is considered by many that French oak barrels give the best flavor and that they must be replaced after several years of use. American oak is used by some producers and you can usually tell the difference. Other producers will buy the older, used French oak barrels and create wines that some feel are inferior (but they probably are less expensive). Some wines may never see anything but stainless steel and the glass that they are bottled in. In any event, using oak barrels puts an "oakiness" characteristic in wine. The wine may be barrel aged for several months to several years.
Ignoring any additional processing that might be used, you could empty the barrels into bottles and sell your wine. However, during the barrel aging, the smaller containers may develop differences. So the winemaker will probably "blend" wine from different barrels, to achieve a uniform result. Also, the winemaker may blend together different grape varieties to achieve desire characteristics. For example, blending a little Merlot into a Cabernet Sauvignon can give is a more "mellow" taste. This process also temporarily creates very purple stained teeth in the red wine maker. Other blends may seem unusual. Recently I had a blend of 50% each Chardonnay and Viognier. (I liked it.)