The effects described in the following section concern only the fine lees. The coarse lees have no beneficial effect on the wine and are potentially a source of bitterness, vegetal notes and reduction. There is not really any value in keeping them in the wine.
This technique is particularly beneficial for dry white wines but can also be applied to certain red wines. Stirring rebalances the wine's redox potential and thus has a buffering effect. It reduces oxidation by consuming oxygen in the wine through the release of glutathione and reduces the quantity of reductive molecules such as H2S, by adsorption.
From an organoleptic perspective, polysaccharides from the yeast walls are released, which stabilizes the tartaric acid by reducing crystallization and therefore the deposit of tartar crystals. This is very beneficial for white wine stabilization. In addition, polysaccharides can increase the perception of fullness and richness. However, this effect can become too pronounced in certain situations, and frequent stirring of the lees is not always the best solution.
Regarding the colorimetric profile, stirring the lees eliminates the oxidation compounds that cause wines to turn yellow and therefore gives wines a cleaner, purer colour.
If the lees are not stirred for a long time, there is a risk of a reductive effect. They settle in the bottom of the container in very reductive conditions, which can cause the wine to have a reductive odour. These risks are more significant when the wine has undergone a reduction period during alcoholic fermentation.
Stirring can favour the development of spoilage microorganisms such as B. bruxellensis. Nutrients are released during stirring and the resuspension of these organisms encourages their development in the wine. Vigilance is therefore required if this practice is applied to red wines, in which case microbiological analysis is recommended.
It can be appropriate to stir the lees at various stages in the winemaking process. It should be noted that the lees' antioxidant properties are most pronounced during the first six months after fermentation and decrease thereafter.
The first period during which stirring the lees is beneficial is just before or during malolactic fermentation in order to detoxify the environment, notably by eliminating some of the short-chain fatty acids and releasing nutrients.
This also protects the wine against oxygen and thus from ethanal production in the event of languishing MLF.
For white wines, stirring is beneficial at the beginning of the ageing process to reduce crystallization.
During the first six months, it helps to maintain a balanced redox potential and improve the perception of the wine's volume and richness. Stirring press wines is also useful to eliminate oxidized compounds and those responsible for causing yellow colouring.
The frequency with which the lees is stirred is decided depending on the wine's properties and the taste objectives.
Traditionally, stirring frequency ranges from twice a week during early ageing and MLF to once a month thereafter.
The best strategy is tasting with regular observation of the wine's colour and organoleptic profile.
For wines presenting microbiological risks, stirring should be avoided or only implemented at the beginning of MLF when there is still a high level of lactic bacteria activity.