Maturation takes place between the end of fermentation and bottling. Once the wine is bottled, ageing takes over.
Winemakers have two objectives when maturing their wine:
The length of maturation depends on the type of wine being produced. For simple wines maturation will last a few weeks or even days. For most wines, the maturation period will be a few months but for high-quality wines, maturation can take two years or more.
Depending on the type of wine being produced maturation may take place in large scale tanks or in small volume oak barrels.
After fermentation wine still contains small particles of grape material, yeast, and bacteria suspended in the liquid. Known as lees, these tiny particles could produce a second fermentation if they come into contact with any residual sugars so winemakers clarify the wine to remove these particles.
The processes below are the most commonly used methods of clarification but for mass-produced, commercial wine the winemaker could employ a process of centrifugation, spinning the wine so that the lees are pushed towards the edge and removed, or pasteurisation where the wine is heated briefly to very high temperatures.
The most common method and typically used for quality wines. This method is often used in conjunction with filtering before bottling.
The process of racking involves pouring the wine from one container to the other many times leaving the lees behind at the bottom of the container on each pour. A bit like separating an egg yolk from the whites.
The racking process also helps to oxygenate and soften the wine and also releases residual carbon dioxide that was produced during the fermentation process.
Racking may be carried out two to four times per year if the wine is spending a long period of time in oak barrels.
Often used before bottling whether or not racking has taken place.
Fining takes advantage of flocculation, a process whereby particles suspended in a liquid are removed by the addition of a clarifying agent.
For red wines, the best clarifying agent is egg white (about six per barrel) while for white wines casein (phosphoproteins commonly found in milk) is best. Bentonite clay (natural clay) is another common clarifying agent.
When the clarifying agent is added to the wine it binds with the lees, which then sink to the bottom of the wine over a period of about 10 days. The winemaker then employs the racking process to remove the lees.
After fining red wines lose some of the roughness caused by tannins and gain softness and finesse.
This process can be used in conjunction with the racking process during maturation but normally takes place before bottling.
Wine is passed through a plate or membrane that contain either small or large holes. Where the holes are small (less than one-thousandth of a millimetre) not only are the lees removed but the wine is sterilised as well as the holes are too small for bacteria to pass through.
Wine can also be matured sur lie (on the lees). Allowing the wine to mature sur lie can help retain a small amount of residual carbon dioxide, creating a slight tingling sensation on the tongue. This process can also help develop the wine so that it becomes more complex, soft and rich.
Fining and filtration are often carried out before bottling.
Blending is an optional step in the maturation process.
Blending is a difficult technique and involves the harmonious combining of wines sourced from different vineyards and different grape varieties, from vines of varying ages or from various barrels.
Blending can take place throughout the maturation process right up to the point of bottling. In the case of Champagne, it is done before maturation.
Red wines and some white wines require aeration when maturing. Aeration is provided by the racking process, by the slow diffusion of oxygen through the seams of the barrel, or by a process whereby oxygen is slowly released into the wine by mechanical means (microbullage).
For the production of most wine, aeration should be minimal. When wine comes into contact with air it can promote the growth of bacteria and alter the colour and taste of a wine. To avoid these problems the winemaker will add sulphur and/or carry out a process known as topping up.
To avoid oxidation the vessels containing the maturing wine must be kept completely full.
As the wine matures it also evaporates so the winemaker compensates for this loss by topping up the vessel.
Sulphur in the form of sulphur dioxide is used when making wine.
The addition of sulphur helps prevent oxidation and also acts as an antibacterial agent. However, it can also cause some defects in wine.
Its use is restricted by European regulations. Whilst chemists are looking for a replacement nothing yet has been found however, its use has been reduced by improving other processes such as hygiene and filtration.
Some producers do attempt to make wine without sulphur but very few manage to produce a bottle without faults. Sometimes these faults can add character to the wine but in other cases, they render it completely undrinkable.
Minor faults may consist of an aroma of staleness, fermentation gas, or sulphur. If these faults are found it may be possible to rectify them by decanting the wine before drinking to allow it to oxygenate.
The stale smell appears when the wine has been deprived of oxygen for a prolonged time. Fermentation gas should have disappeared if the wine was aerated before bottling. The smell of sulphur is caused by a dosing error.
Major faults are irreparable.
These faults consist of mercaptan (smell of rotten eggs caused by the reaction of fermentation yeast with sulphur), oxidation (a deterioration of the wine’s aroma caused by prolonged contact with air and insufficient sulphur), and acetification (caused by a bacterium that makes the wine taste of vinegar).