When yeast cells die their cell walls breakdown, gradually releasing such compounds into the wine as polysaccharides (e.g. glucose), amino acids (and peptides), fatty acids, and mannoproteins. The compounds released can influence the structural integration of the wine in terms of phenols (including tannins), body, aroma, oxidative buffering and wine stability. |
At the end of alcoholic fermentation, yeast cells autolyse. Yeast autolysis is a slow process involving hydrolytic enzymes which act to release cytoplasmic (peptides, fatty acids, nucleotides, amino acids) and cell wall (mannoproteins) compounds into the wine.
The primary reasons for sur lie ageing are usually based on stylistic goals: to enhance the structure and mouthfeel of a wine, give it extra body (an impact of polysaccharides on astringency), and increase the aromatic complexity, flavour/aroma depth and length. Lees also absorb oxygen, assisting in maintaining a slow and controlled oxidation during maturation. Lees stirring can increase the release of yeast compounds into the wine bulk. Stirring can result in a creamy, viscous mouth feel, and can enhance flavour complexity.
Some of the compounds from broken down yeast cells also contribute to wine in the following ways:
Lees stirring (Bâtonnage)Bâtonnage is the French term for stirring the settled lees back into the wine.
Method of stirringAny method which redistributes the lees thoroughly provides an adequate method of stirring, but different methods contribute differently to wine.
On a moderate scale (such as a single barrel), this can be achieved by the use of a steel rod with a paddle at the end which can be placed in the tank or barrel and spun with an electric drill attached.
On a small scale, a food turntable can be used which allows the process to be controlled externally.
It is important to note the oxygen exposure that each method exerts on the wine, since this will significantly impact wine development.
Intensity and frequencyLees should be stirred thoroughly, bringing all lees into suspension. The more lees there is, the more frequently stirring should be conducted.
Oxidation and vesselOxygen uptake during lees stirring plays an important part in wine development. For example, oxidative stirring increases acetaldehyde concentrations and may increase the acetic acid concentration. The oxygen uptake of a wine under stirring should be factored into the decisions made on maturing schedules.
The vessel type and size in which stirring is conducted should also be noted.
Benefits of stirringStirring redistributes the previously mentioned desirable compounds (such as polysaccharides and mannoproteins) of dead cells into the wine mass and, in the case of barrel ageing, re-exposes the wine to the wood at the bottom of the barrel.
Lees absorb oxygen, in the absence of which the wine will become reductive. Bâtonnage, or lees stirring, helps prevent this by redistributing the lees (especially at reductive-point zones) into the wine mass and potentially exposing the wine to some oxygen.
Living yeast cells can enzymatically consume ruptured dead yeast cells. Excessive pressure (such as experienced in large tanks) on dead cells is a primary cause for yeast cell rupture. This is called yeast autolysis and is the process that the traditional Champenoise method employs to gain yeasty/bready notes in Champagne style wines. Yeast autolysis is not usually a feature of sur lie ageing since this process tends to occur at time scales greater than those usually used for sur lie (i.e. around a year as opposed to the several months of sur lie ageing).
Stirring tends to enhance the characteristics of sur lie ageing, diminishing fruitiness and slightly reducing wood/oak influence.
In general, stirring is used to:
Stirring in-barrel with a Boswell Company stirrer.
This image ©Tom Shudic 2004. Used with permission.
The OXOLINE stirring system
Stirring with a food turntable
Extreme schedulesSome of the winemakers of top white Burgundy provide perfect examples of extensive sur lie schedules:
Dom. Valette (Vinzelles, Burgundy) wines experience 12-24 months on lees with regular stirring prior to MLF, then only occasional stirring post-MLF.
Under Guillaume de Castelnau, the wines at Dom. Génot-Boulanger (Meursault, Burgundy) are fermented with fine lees (and possibly gross lees if it is "healthy") before being stirred every 7-10 days and "led toward an oxidative state, and then brought back into a reductive state prior to MLF." Following MLF they are placed in stainless steel "to gain body".
Of course, not all white Burgundy producers use extreme schedules. Alain Coche of Dom. Alain Coche-Bizouard (Meursault), for example, lees stirs once every 8 days for about 4 months and once every month thereafter.
Generally speaking, those who bulk age sur lie for longer periods of time tend to stir less, and vice versa.