Wine aging, also known as maturation, is a very common practice, especially in the production of premium wines. It can be defined as the period between alcoholic and/or malolactic fermentation and the moment the wine is drunk by the consumer.
The physical and chemical conditions the wine may be kept in during this aging period vary widely and depend on the philosophy or production approach of each winery.
Broadly speaking, types of aging are classified according to the environmental conditions the wine is kept in and how those conditions affect its oxidation-reduction (redox) potential, or, in simpler terms, the degree of oxidation of the wine’s components.
On this basis, there are three types of aging:
- Oxidative aging
- Reductive aging
- Mixed aging
Oxidative aging (Sherry, Port and others)
This is the type of aging typically given to the famous Sherries and Ports. It is generally carried out in wooden containers of varying size, usually barrels, in which part of the container is deliberately left filled with air, and therefore with oxygen. This oxygen dissolves into the wine in amounts that vary with temperature.
During oxidative aging, the higher concentration of oxygen in the wine raises its redox potential, or, in simpler terms, increases oxidation. Oxidative aging has been used as a traditional winemaking technique in many countries, producing excellent wines that display all the sensory characteristics of this type of aging. Among the best-known wines aged this way are Sherry and Port, to name just two. With these wines, it is often said that “oxygen makes the wine”.
There are two main types of oxidative aging used in winemaking:
- Biological oxidative aging, or aging under flor
- Chemical oxidative aging
Biological oxidative aging, or aging under flor
In this type of oxidative aging, colonies of flor yeast are allowed to form a film on the surface of the aging wine. Through oxidative enzymatic reactions, they produce new chemical compounds in the wine that give it its unique sensory characteristics (aroma and taste).
Chemical oxidative aging
Unlike biological oxidative aging, here the wines are fortified with grape spirit before aging begins, or in its early stages, to an alcohol content of between 17 and 19% v/v. This prevents microorganisms from developing in the wine, including flor yeasts as well as acetic and lactic acid bacteria.
These fortified wines are then aged oxidatively. Since there are no microorganisms, the aging is purely chemical, so the compounds formed are different from those formed during biological oxidative aging, and the resulting aromas and flavors are different as well.
Reductive aging
Reductive aging is characterized by the complete absence of oxygen, or a drastic reduction in the wine’s oxygen content. With wines made by reductive aging, the guiding idea is usually that “oxygen is the enemy of wine”. This type of aging is found in some white wines that have been made with no exposure to air and then bottled under an inert gas atmosphere. One of the aims of this approach is to extend the life and persistence of the wine’s varietal aromas.
Mixed aging
This type of aging is probably the most widely used for “traditional” red wines today. As its name suggests, it consists of first subjecting the wines to a very tightly controlled oxidative aging phase. This part of the process is usually carried out in wooden containers, either traditional barrels or larger vessels, where the wine undergoes controlled oxygenation and partial oxidation of some of its essential compounds. This allows new substances to form that improve the wine’s sensory qualities. The oxidative phase usually lasts between 6 and 24 months, although some wine regions and producers extend it to as long as 36 months.
The oxidative phase is followed by a final reductive phase, which usually takes place in the bottle once the wine has been bottled. When the wine is bottled with the help of inert gases, the conditions inside the bottle allow its redox potential to fall gradually, while a series of chemical transformations takes place that improves the wine’s taste.
Some winemakers believe that mixed aging is what allows a wine to develop its “bouquet”. It is also said that in this type of aging, oxygen is needed only in small amounts and only in the first phase.