Sedimentation & racking
The first clarification is usually carried out by sedimentation where the wine is left for the solids to settle out, after which the wine is racked off the solid lees by pumping or gravity.
Red wines are often clarified in this way, particularly if aged in barrels. The speed of sedimentation is affected by the relative density of the particle, the viscosity of the wine, the wine’s temperature and by convection currents caused by differences in temperature between the top and bottom parts of the tank. Small, shallow containers with poor heat-conducting walls such as barrels are well suited to sedimentation.
Once settling has occurred, the clear wine is racked into another container, leaving the lees behind. Racking can also be used to homogenise the solution, aerate it and remove carbon dioxide.
| Advantages of sedimentation | Disadvantages of sedimentation |
|---|---|
|
It is gentle process, particularly if carried out by gravity It requires no equipment, except possibly a racking valve It allows for a controlled aeration and a homogenisation of the wine |
It is very slow, particularly for wine in large tanks It will never guarantee sterility A considerable volume of lees is produced. |
Some producers of fine wine maintain that any other form of clarification, such as centrifugation and filtration, is detrimental to the quality of the wine, and so will only use sedimentation.
Racking off the gross lees is normally carried out straight after the alcoholic fermentation unless yeast autolysis characters are desired. Further racking can be done three weeks later, and then every 2 months for wines in tanks, and every 3 months for wines in barrels. This can continue until the summer, when the wine is either bottled or stored without racking.
The frequency of racking is often reduced if filtration is used, but it is nearly always carried out before the spring, to remove tartrate deposits which will re-dissolve if the wine temperature is allowed to rise. Many wine-makers rack according to the taste of the wine in order to carry out controlled aerations.
Racking is often preceded by a stability test to see if the wine will stand aeration and always accompanied by a free sulphur dioxide level assessment and adjustment.
After racking, all containers must be kept well topped up with wine, unless blanketed with nitrogen. Carbon dioxide is not used at this stage, as it dissolves in the wine. Care must be taken to anticipate the expansion in volume of the wine in the spring.
