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The advantages of resting on the yeasts.
Among the changes that distinguish the modern cellar approach compared to the practice adopted in the past, the refinement sur lie occupies a place of importance.
Initially adopted in some areas of France (Burgundy, Loire) for the conservation and evolution of white wines as well as red wines, today the rest sur lie It is widely used throughout the so-called Old and New World.
The market demand for full-bodied, rounded and typical, but at the same time stable, white wines has pushed more and more producers to adopt this technique, also thanks to its indisputable benefits and despite the fact that it presents some risks in the event of incorrect management of the process.
The stop sur lie consists in keep the wine in contact with the sediment of dead yeasts at the end of alcoholic fermentation (fine lees) for a period that can vary from a few months to a year.
If the main effect sought at the taste level is the direct sensorial contribution of the mannoproteins released by the yeasts, this technique also determines important modifications of the chemical-physical characteristics of the wines by intervening on their evolution during conservation.
The yeast cells that die at the end of fermentation actually begin a process of autolysis (self-digestion) that leads to the partial breakdown of the cellular part of the yeast that is rich in mannoproteins, mannans and other polysaccharides that play an important role both at the organoleptic level and at the level of wine stabilization. Overall, most of the autolysis process occurs in the first trimester and then slows down.
As autolysis proceeds, other cellular components such as polypeptides (small portions of proteins) and nucleic acids are also released into the wine, which play a very important role in the taste component of wine. Oenological and biotechnological research has made the advantages of refinement available. sur lie for all wineries, minimizing the risks associated with the technique.
In fact, there are many biotechnological products derived from yeast that can achieve different effects on the treated wine, contributing significantly to the qualitative increase of the wine.
The pros
Increased body
The rest of the wine on the yeasts allows an increase in body thanks to the release of polysaccharides. In order of importance, yeasts are the second source of polysaccharides for wine, after those contained in grapes (acid polysaccharides) and in addition to those, not positive, possibly provided by the Botrytis cinerea (glucans).
The polysaccharides contained in yeast are mainly mannoproteins and glucomannoproteins.
Mannoproteins constitute about 80 percent of the exocellular polysaccharides and contain 90% mannose and 10% protein – hence their name. Glucomannoproteins represent about 20% of the remaining exocellular polysaccharides and contain glucose (25%), mannose (25%), and protein (50%).
The release of these two types of polysaccharides from the yeast cell wall occurs through the action of enzymes, such as endo-β-(1,3)- and endo-β-(1,6)-glucanase.
The quantity of mannoproteins extracted by the yeasts during alcoholic fermentation and subsequently during refinement on the lees, thanks to the autolysis process, depends on the type of yeast used.
It should also be remembered that the initial composition of the must in pectic colloids significantly affects the release of mannoproteins: the more abundant the pectic colloids, the lower the release of mannoproteins.
Softening of the tannic sensation
The refinement sur lie It involves an increase in the perception of soft components on the palate, particularly in red wine, in some cases, a less aggressive tannic texture can be perceived.
Ability to absorb oxygen
Since the lees compete with the oxidizable substances in the wines for the consumption of oxygen, this means that they act as "lightning rods" in favor of otherwise vulnerable compounds such as, for example, anthocyanins and leucoanthocyanins.
Oxygen consumption by the lees is greatest in the early stages of autolysis and decreases as the process progresses.
This determines an initial risk of reduction, with in some cases the need to supply minimal quantities of oxygen with microoxygenation or decanting, but in the long run it generates a protective power against oxidation.
Anti-pinking factors
Pinking is a phenomenon typical of some white wines, especially Sauvignon Blanc, which causes a temporary grey-pink colour in the presence of oxygen.
The phenomenon is not yet fully understood, but there is a clear correlation with the oxidation-reduction potential of wine.
It has been seen how free mannoproteins help to reduce the vulnerability of many wines to this phenomenon and in general wines that have been kept on the lees for a long time tend to be less affected by pinking.
Color stability
The stabilization of the red color of wines is a consequence of maintaining anthocyanins in solution in a non-oxidized form.
Free anthocyanins are very susceptible to oxidation, while those bound to tannins are much more stable over time. Tannin-anthocyanin complexes can in turn combine with yeast-derived parietal polysaccharides (mannoproteins and glucomannoproteins) resulting in further long-term stabilization.
In wines with very low tannin content such as Pinot Noir, the technique of elevage sur lie It is essential for stabilizing the little color typical of this variety.
For more colorful varieties the stop sur lie It can have little impact, because it happens at a time when the color stabilization games are already done.
Today, thanks to biotechnology, it is possible to exploit this mechanism already during the maceration phase thanks to specific products derived from yeast in combination with tannins.
Greater aromatic persistence
If it is true that mannoproteins, released during the refinement phase, have various effects not only at an organoleptic level but also at a physical and chemical level, it is equally true that among these, the broadening of the aromatic profile of the wine as well as the strengthening of its persistence are among the most appreciated benefits.
The interaction they have with the aromatic substances in fact determines not only an increase in the aromatic spectrum, which is enriched with the so-called baking notes where not also pastry notes, but also an increase in persistence: the bond is not stable and the mannoprotein makes the aromatic compound more resistant.
Lower volatility of aromas
Fine lees generate nitrogenous compounds with a strong reducing power and therefore capable of acting as antioxidant protectors of fragile compounds, such as fruity ones.
Furthermore, mannoproteins tend to “absorb” some odorous substances, due to the interaction of the latter with the protein part of the mannoproteins.
Among the compounds that benefit most from this situation, β-ionone should be mentioned in particular.
Tartaric stability
The type of stability induced in wine thanks to the presence of mannoproteins, concerns in particular tartaric stability.
The action of mannoproteins is long-lasting.
In fact, the mannoproteins released during autolysis interact with the first phase of crystallization of tartaric acid salts, preventing the aggregation of crystallization nuclei, avoiding the formation of larger crystals which usually then precipitate creating the familiar cloudy bottom.
It must be said that the action of mannoproteins alone is often not sufficient to guarantee complete tartaric stability, especially in young wines.
Protein stability
In this case, mannoproteins act as a protective colloid for the proteins present, preventing the protein micelles from reacting with other substances, first forming a flocculus and then precipitating, giving rise to the so-called protein casse.
However, this action is not strong enough to lead to the complete replacement of bentonite, which is used for this very purpose.
Decrease in the concentration of polyphenols (collage)
The rush – if you don't want to call it a mania – to go allergen-free has led to a rethink not only of the fining of white wines but also of the clarification of red wines.
Resting on the yeasts also helps here.
Mannoproteins released into wine by yeasts during autolysis can interact with wine colloids, preventing their aggregation.
In fact, wine tannins constitute colloidal particles and tend to associate with each other or to interact with wine proteins (the so-called collation mechanism), creating larger aggregates that tend to precipitate and thus lead to phenomena of colloidal instability.
Mannoproteins, thanks to their carbohydrate and protein nature, ensure a bivalent interaction with the phenolic colloidal particle and with the must-wine, preventing the colloids from forming insoluble aggregates.
Ability to fix sulfur compounds
Among the many advantages of lees, one also concerns the ability to fix sulfur compounds, specifically thanks to the mannoproteins present in the cell walls of yeasts.
However, care must be taken, as the bonds between the yeast cell walls and the sulphur derivatives require the mobilisation of a divalent cation such as copper and only thiols can be eliminated.
Consequently, it is not possible to eliminate disulfides by copper-based processes.
Facilitation of malolactic fermentation
The rest on the yeasts also has effects on the subsequent treatments carried out on the wine.
Mannoproteins and especially the cellular contents of autolysing yeast cells act as activators of the lactic flora and therefore play a positive role in the initiation of malolactic fermentation.
It has been demonstrated that the waiting times for the start and the duration of MLF are reduced in proportion to the levels of mannoproteins released by the yeasts.
In cases where FML is not desired, especially in white wines, it is convenient to use highly purified yeast-derived biotechnological products that provide mannoproteins without providing other growth factors for lactic acid bacteria.
The cons
Process duration
The major drawback of the rest on the yeasts concerns the extensive duration with which the lees release the polysaccharides.
The process is in fact very slow, also due to the low winter temperatures, and months of pause are often necessary to find evident advantages.
This translates into a greater economic burden and also entails the risk that prolonged contact leads to abnormal fermentations or generates reduced odors.
The use of enzymes with β-glucanase activity allows to significantly speed up this process, obtaining good performances after just a few weeks of contact.
Absorption of anthocyanins
During the initial stage of fermentation, the yeast cell walls can absorb pigments and anthocyanins, with an intensity that varies depending on the yeast strain used.
Typically, yeast that absorbs fewer phenolic compounds onto its wall will then have less color loss.
A 2005 study on Corvina grapes (Experimental Microvinification Trials for the Analysis of Corvina Grape Anthocyanins signed by Tosi, Malacrinò, Guzo, Zapparoli) has also demonstrated that, while acylated anthocyanins tend to be retained by the faeces, non-acylated ones are less subject to this phenomenon.
In any case, from a practical point of view, to avoid color losses from the lees, it is sufficient to choose a selected yeast with a low capacity for adsorbing anthocyanins.[/vc_column_text][/vc_column][/vc_row]