Sunday, July 28, 2013

Lees aging of sparkling wines: The story behind the headline

In my series comparing Champagne, Franciacorta, Prosecco, and Cava, I touched on the topic of yeast autolysis and its positive impact on wines that gain their bubbles through a second, in-bottle fermentation. In this post I provide additional details on the autolysis process, its by-products, and the benefit they confer on the hosting wine.

Murli Dharmadhikan (Yeast Autolysis, defines yeast autolysis as "... self-destruction of the cellular constituents of a cell by its own enzymes" following its death. Figure 1 below shows the component parts of a healthy yeast cell while Figure 2 shows an overview of the process  -- autolysis -- that occurs once that yeast cell has consumed all of the available nutrients and dies. At a high level, autolysis encompasses (i) the degradation of intracellular materials and (ii) degradation of the cell wall.

The detailed autolysis process is shown in Figure 3 below. The yeast extract, product of the degradation of intra-cellular material, is confined to the cell until such time as the cell wall becomes porous enough to allow the material to seep out. It should be noted that degradation and compound creation continues outside the degraded cell walls.

Figure 3. Details of yeast autolysis
The lees-aged wine is enriched by the compounds released during the constituent-degradation process. Compounds released during autolysis include (Thierry Binder, Cremant d'Alsace, TONG #13; Dharmadhikan):
  • Nitrogenous compounds
    • Amino acids -- known to enrich mouthfeel; aroma precursors of acacia honey notes
    • Polypeptides -- sweet and bitter taste; precursors of the autolytic aromas of brioche and toast
    • Peptides
    • Nucleic acid components
  • Polysaccharides -- originates from breakdown of cell wall components
    • Degradation products are glucose and mannose
    • Mannoproteins increase mouthfeel and foam stability as well as contributing to fineness and persistence of bubbles
  • Fatty acids -- important for foam stability, mouthfeel, and flavor. Can be involved in the formation of esters, aldehydes, and other volatile compounds
  • Volatile components
    • Heavy esters
    • Terpene components
    • Higher alcohols
    • Other volatile components.
The factors that influence the quality of the autolysate include temperature, wine pH, ethanol content of the wine, and the duration of yeast contact. Autolysis is generally conducted at temperatures between 15 and 18 degrees centigrade and with wine pH held between 3 and 4. This ensures a slower rate of autolysis, allowing the winemaker to benefit from longer lees contact.

The in-bottle fermentation process, with its associated autolysis, strips out aspects of the varietal flavors and replaces them with signature yeasty, sourdough flavors.  The longer the wine remains on the lees, the more pronounced these flavors become. Also, the longer the residence on the lees, the richer the wine.

Champagne is legally required to remain on the lees for at least 16 months, if a non-vintage, and at least 3 years if designated as vintage.  Quality houses normally age their non-vintage wines for 3 to 4 years and their vintage wines for 7 to 8 years.

Non-vintage Franciacorta wines are aged for a minimum of 25 months with 18 of those months being on the lees in the bottle.  Vintage wines are aged for a minimum of 37 months with 30 of those months being in the bottle on the lees.  Riserva wines are aged for 5 years on the lees.

In order to be called Cava, the sparkling wine made in the region has to be aged a minimum of 9 months before being taken to market.  Many producers age their wines for 2 to 4 years in order to provide wines with more character.  To be classified as Gran Reserva, a Cava has to be aged for at least 30 months.

©Wine -- Mise en abyme

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