Which yeasts are used to make wine?

What is an indigenous yeast in oenology?

It's a unicellular fungus that can be found on grape skins, but also in its environment : vineyards, viticultural and winemaking equipment, cellars, etc. The majority of indigenous yeasts belong to the family of apiculate yeasts, known as non-Saccharomyces. These include Kloeckera, Candida, Brettanomyces and Pichia.

Some of these indigenous yeasts are, however, Saccharomyces-type strains. These are the strains present in the growing, harvesting and winemaking equipment. Originally inoculated in previous vintages, they are basically exogenous yeasts that have become indigenous by colonising the various surfaces in contact with the grapes or must. These are know as "ambient yeasts.

What is the function of indigenous yeasts in winemaking?

These micro-organisms initiate the spontaneous fermentation of the grapes , following harvest. Non-Saccharomyces yeasts are the first to act, feeding on the sugars in the must to produce ethanol the alcohol in wine. However, the resistance of these yeasts to ethyl alcohol remains low: once the must reaches 4 to 6° alcohol, most of them die. They are then replaced by more resistant environmental and exogenous yeasts, which take over and continue the fermentation process.

What are the benefits and risks of using indigenous yeasts in wine?

Various studies have shown the beneficial effect of these yeasts on the organoleptic properties of a wine, particularly its fermentative and varietal aromas. By allowing the natural expression of a terroir and its biodiversity, they benefit the aromatic complexity and authenticity of a vintage.

Spontaneous fermentation plays a central role in biodynamic wine-making , but the process is difficult to control. The major risk is the appearance of visual, olfactory or gustatory defects in the wine, or in its ageing potential.

What is exogenous yeast?

This is a strain resulting from selection and cultivation in the laboratory, often sold in France in dehydrated form. This is called active dry yeast. These yeasts are intended to be rehydrated and then deliberately introduced into the vat during the wine-making process: this is known as yeasting. This step takes place during the vatting process for red wines and following the settling process for rosé and white wines.

The most common exogenous yeasts are Saccharomyces cerevisiae , which are also used for brewing beer and leavening bread. They are particularly popular in viticulture, even organic viticulture, as they are resistant to high levels of alcohol and sulphur and to the low pH of wine. However, other strains of exogenous Saccharomyces yeast are also selected for winemaking, depending on the wines produced:

• Bayanus,
• Pastorianus,
• Beticus,
• Uvarum,
• Fermentati,
• Paradoxus.

What is the role of these in wine-making?

The primary use of exogenous yeasts is to guide the fermentation process in such a way as to make it easier and safer. This is known as 'guided fermentation', as opposed to the spontaneous fermentation of fruit. Among the selection of nearly 300 exogenous yeasts on the market, each has proved its worth in the laboratory, making vinification more predictable and reducing the risk of organoleptic deviations in the cuvée.

But each yeast also has a specific impact on a wine : its alcohol content, colour, aromas and ageing potential. Some yeasts also have a specific fermenting power and vigour for rapid fermentation (starter yeasts) or at a certain temperature, or for setting the foam. Exogenous yeasts serve to enhance the aromatic and qualitative potential of a wine, whether white or red: winemakers therefore need to choose the yeasts that will bring out the excellence of each grape variety they vinify.