Maturation of Wine
Barrel Management during Wine Maturation
Barrel aging is the cornerstone of the process called elevage, which is the French term meaning raising or upbringing used to describe what happens to the wine between fermentation and bottling. The wine's elevage can last for a few months to many years, during which time the wine's flavors integrate and mature. The winemakers' choices during the aging process, including how long to age the wine and how much to manipulate it, will have a huge impact on the taste of the finished product.
Choosing the Right Barrels
Depending on wine type, style, and economic considerations, winemakers need to choose the right barrels. Barrel aging must enable wines to develop their full character, yet remain in balance. If the oak/wine match is not perfect, there is a risk of acquiring a dominant woody character that overpowers the wine and dries it out rapidly. Ribereau-Gayon (2006b) has enunciated the criteria for selecting barrels based on the type of wine:
Racking the Barrels
As the wine matures, primarily red wines, the polymerization and precipitation of phenolic compounds continues, resulting in the development of lees at the bottom of the barrels. This polymerization of phenolic compounds brings about clarity of the wine and color intensity and stabilization.
Frequency of Racking
Vintners have different philosophies in the frequency of racking the barrels. Generally, a wine is racked at least two or three times during its life in a barrel. In Bordeaux, during the first year, red wines with good aging potential are racked at the following times: when the alcoholic and malolactic fermentations are completed (to clarify and degas the wine), at the end of winter (to eliminate sediment) and before summer (to adjust the free SO2 level) (Ribereau-Gayon et al., 2006b).
Racking Regimes
Great care is taken to prevent exposure of the wine to oxygen by using inert gases while transferring, filling barrels, filling headspace, and any other movement that could introduce oxygen. The use of a dissolved oxygen meter is frequently used in the cellar to alert the staff to possible oxygen problems. Racking can either be done by pumps or pressurization systems.
Wine Evaporation Rates
When a wine is stored in a barrel, some of it is lost due to evaporation. Generally, the loss is about 2 to 5 percent in volume per year in moderate climates or humidified barrel rooms. The typical 225-liter (59gal) barrel can lose anywhere from 21 to 25 liters (5-6gal) (mostly alcohol and water) in a year through evaporation. Water and ethanol are smaller molecules that diffuse into wood as liquid and escape as vapor. The evaporation rate depends on the temperature, air movements, and the relative humidity in the cellar. Humidity affects the kind of constituents that escape, with alcohol becoming more concentrated in wine stored under conditions of low humidity and weakening with high humidity.
Topping Off the Barrels
The wine level in oak barrels drops rapidly after filling due to evaporation or a leaky barrel/bung. Topping off is a procedure in which wine from the same lot is used to fill the headspace (ullage) in a barrel so that oxygen is not available to support the proliferation of spoilage microorganisms such as Acetobacter and film yeasts (Figure 8.2).
Bungs
AEffective seals coupled with a regular topping routine are essential to the barrel-aging program. In tightly bunged barrels, the diffusion of water and ethanol out of the barrel creates a partial vacuum in the developing headspace.
Barrel Aging Time
The length of wood maturation depends on grape variety, concentrations of phenolic compounds, wine style, kind of barrel, winemaker's preference, and consumer choice. For example, a wine with higher concentrations of phenolic compounds will need longer aging time to soften the tannins. Maturation in wood can vary, time wise, from 3 months to 3 years or even longer. The white wine that is most often seen aged in oak is Chardonnay and depending on the time the wine remains in the barrel 3 to 12 months.
Aging in Barrels of Different Ages
Winemakers will often use a variety of barrels made from different woods, different ages, and even tanks to mature their wine. This creates varied taste profiles, which work well when blending, to give the wine more complexity. Achieving a fine balance of oak aromas and obtaining the right level of oak influence are part of the art of a skilled winemaker.
Adding Tannins or Oak Replacements
Tannins protect wine from oxidation during barrel aging. The wood tannins extracted from a new barrel protect the wine from over-oxidation during the slow process needed for tannin polymerization and wine development.
Organoleptic Evaluation
Routine organoleptic evaluation of the wine during maturation can alert the winemaking team to the early development of potentially large problems. Winemakers will taste the wine frequently to ensure the flavors are just right. They do this with a wine thief, a special tool that extracts a small amount of wine from the barrel.
Wine Barrel Racking Systems
Barrels are typically stacked on racks that can be moved with a forklift, instead of moving them manually. All barrel stacking systems, including those that don't use a rack, can be defined as either rack-on-barrel or rack-on-rack systems. The weight of the stacking structure is therefore either borne by the barrels or alternatively by the barrel racks.
Rack-on-Barrel System
The rack-on-barrel system has a simple and economic design and it is easiest to stack, but old barrels that may have weak staves cannot always bear the weight which could result in the collapsing of the entire stack of barrels (Figure 8.4).
Rack-on-Rack System
Several racks on the market use various corner-post riser designs that shift the load from the barrels to the rack itself. In addition, there are portable barrel racks that are designed to stack so that the barrels are offset in a manner mimicking a conventional pyramid stack.
Cellar Temperature and Humidity
Controlled temperature and humidity are most recommended for barrel storage. Barrel aging cellars must be kept at relatively low temperatures, with no sudden variations and without excessive aeration or ventilation.
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