Alcoholic fermentation


Effected by the yeast Saccharomyces cerevisiae

Oenococus oeniAlcoholic fermentation, which lasts 4-8 days, is effected by the yeast species Saccharomyces cerevisiae.

To multiply, Saccharomyces cerevisiae consumes the nutrients in the must and its metabolic activity is expressed in two essential phenomena: the conversion of sugars in the must into alcohol (ethanol) and the production of volatile compounds responsible for the aromas of the new eau-de-vie.

Conversion of sugars in the must into alcohol

In practice, the winemaker monitors the progression of alcoholFermentation alcoolique par mesure quotidienne de la température et de la densitéic fermentation, measuring must weight daily with a “mustimetre”: the density decreases as the sugars are converted into alcohol.

This reaction generates energy (an exergonic reaction).

Some of the energy produced is used by the yeast to continue to multiply (the yeast population reaches 100 million cells per ml [3 billion cells per ounce] during alcoholic fermentation), but most of this energy is released in the form of heat, which explains the rise in temperature during alcoholic fermentation.

Above 30°C (86°F), the temperature may inhibit yeast growth, which is why winemakers must monitor the fermenting wine’s temperature daily.

Production of volatile compounds generating the aromas of the new eau-de-vie

Parallel to the conversion of sugars into alcohol, the yeast produces a wide variety of volatile compounds that will be concentrated during distillation. These will generate the new eau-de-vie’s aromas.

The primary volatile compounds synthesised by the yeast are aldehydes, higher alcohols, fatty acid esters, acetate esters, or esters of higher alcohols.

composes_volatilsThe formation of most esters is sought after, for these compounds give the eaux-de-vie fruity (banana, pear…) and floral (linden flower, rose, violet…) aromas.

Other compounds produced by the yeast (higher alcohols, ethyl acetate, acetaldehyde…) contribute aromas to the eau-de-vie but can alter quality when they exceed known concentration thresholds.

Necessity of controlling the progress of fermentation

cinetique_fermentaireTo control the synthesis of all these volatile compounds, an optimum fermentation cycle must be established – in other words, a cycle that will lead to the optimum development of Saccharomyces cerevisiae. This means:

● rapid start of alcoholic fermentation to avoid the appearance of undesirable micro-organisms during the prefermentation phase (there will be no SO2 to remove them!) ;

● steady progress ;

● complete consumption of sugars by the yeast.

The winemaker’s goal is to avoid :

● significant temperature increases related to a fermentation that is
proceeding too quickly (e.g., 4 days) (see 4-day graph) ;

● fermentation that proceeds too slowly, which could lead to premature multiplication of bacteria and overlapping of alcoholic fermentation with malolactic fermentation, which must absolutely be avoided. (lactic acescence, which will be revisited in the chapter on MLF) (see 10-day graph).

Yeast addition: A winemaking practice that improves control over alcoholic fermentation

The practice of yeast addition has become increasingly widespread. This practice consists in seeding the must with a select population of Saccharomyces cerevisiae of about 1 million cells per ml (about 30 million cells per fluid ounce).

The goal is twofold :

● ensure rapid start to fermentation

● populate must with a strain with known qualities.

Indeed, it is important to know that there is great variability within the species S. cerevisiae: strains belonging to this species present physiological characteristics that can differ widely. The analytical and sensory profile of a wine is therefore dependent on the strain of Saccharomyces cerevisiae that dominated in the must during alcoholic fermentation.

The necessity of controlling the formation of volatile compounds in the wines has led researchers and oenologists to select the best-suited strains. Today, most winemakers seed the must with selected yeasts.

Strain FC9, for example, was selected in the Cognac region vineyards as a result of an extensive ecological study performed in the 1990s.

Addition of nitrogen in case of deficiency

Un ajout judicieux d'azote perment d'augmenter la vitesse de fermentationHowever, the practice of yeast addition and choice of a particular strain of yeast are not always sufficient to ensure good development of fermentation.

The nitrogen content of the must plays a prime role here, for it is a limiting factor in yeast growth.

If the must is nitrogen-deficient, fermentation will be too slow and the risk of lactic spoilage will be very high.

If the must is nitrogen-deficient, fermentation will be too slow and the risk of lactic spoilage will be very high.

These additions encourage yeast growth and therefore the speed of fermentation. Ecologically speaking, they are preferable to excessive vine fertilisation.

Current regulations allow the addition of ammonium salt to the maximum dose of 100 g/hl (1.4 oz/gal). However, few situations require such supplements. Additions in the region are often around 20 g/hl (0.3 oz/gal). Exceeding the 50 g/hl (0.7 oz/gal) total dose of supplements is discouraged, at the risk of seeing significant changes in the analytical profile of the resulting eaux-de-vie.

Other factors influence yeast metabolism: presence of must deposit, temperature…

La fermentationOther factors influence the production of volatile compounds and, consequently, the aroma profile of the new eaux-de-vie. Some of these factors include :

● greater or lesser presence of must deposit ;

● temperature (controlled temperature of about 22°C (72°F) yields more aromatic esters).

The aroma profile of the new eaux-de-vie is also closely tied to the composition of the fermenting must. This relationship is complex and explains differences in vintages. The Station Viticole is currently studying this subject.

Next : malolactic fermentation