http://www.brsquared.org/wine/Articles/surlie/surlie.htm
Sur lie and bâtonnage
(lees contact and stirring)
©Copyright Ben Rotter 2001-2009
www.brsquared.org/wine
"Sur lie" is the French term for leaving the wine in contact with its lees and "bâtonnage" the term for stirring this lees back up into the wine.
Why use lees?
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:
# polysaccharides contribute a roundness and volume to the palate
# mannoproteins can bind with anthocyanins and tannins to increase colour stability and decrease astringency
# the nutrients released from the dead yeast cells assist the growth of malolactic bacteria
# increased palate length, attributed to the late release of particular volatile compounds in the polysaccharide network of the fruit and yeast
# they can assist in protection from oxidation of particular fruit aroma compounds.
# the process of proteolysis, whereby proteins are hydrolysed to amino acids (which can act as flavour precursors, possibly enhancing flavour complexity) and peptides (which travel through the yeast cell walls causing an increase in nitrogen content).
# dead yeast liberate esters, particularly fatty acids with sweet/spicy (fruity) aromas (such as ethyl hexanoate and ethyl octanoate); this coincides with the time when fermentation esters (such as isoamyle acetate and hexyle acetate) experience hydrolysis, resulting in combined sweet/spicy/fruit aromas
# the release of amino acids and nucleic acids can enhance flavours and complex aromas, particularly at the end of the palate
# they yield a sweetness when binding with wood phenols and organic acids
# they modify wine esters and wood aromas
# they provide a natural fining, reducing more yellow colours in whites
# they improve protein stability (current research suggests that lees do this by producing an extra mannoprotein (polysaccharidic molecules which constitute ~35% of the yeast's cell) which prevents polymerisation of tannins, pigments and volatiles; and more of this compound is released when the temperature is increased, and with greater contact time and lees stirring frequency)
# they can assist potassium bitartrate stability, since mannoproteins act as potassium bitartrate crystal inhibitors
# they can reduce colour: the more yeast cell surface area, the higher the absorption and subsequent loss of colour.....................
