Wine Words: Yeasts

Yeast is a very important wine word. Without it grape juice would never become wine.

Read on if you would like to learn more about what I call the 'yeasty beasties' — their role in winemaking, where they come from, and all that good stuff.

Yeasts are a collection of microscopic, single-celled fungi. They are critical in winemaking, (well, in any fermentation process for that matter), as they transform the sugars in the grape juice into alcohol (ethanol). As they transform the sugars into alcohol, they also give off carbon dioxide, which dissipates, unless deliberately retained in the wine, as is the case with the production of sparkling wine.

Yeast Types and Where they Originate
There are two main types of yeasts. These are 'Wild' yeasts and 'Cultured' yeasts. Wild yeasts (often called indigenous or native yeasts) are naturally occurring yeasts. They come from the vineyard and are found on the grapes' skins. Cultured yeasts, on the other hand are a collection of different yeast strains that have been deliberately isolated as 'being strong and desirable', and are then reproduced and sold commercially.

Wild yeasts vary from vineyard to vineyard and comprise a multitude of different strains, whereas cultured yeasts are typically of a consistent type. Wild yeasts are also unpredictable, while cultured yeasts are predictable.

Wild or Cultured, Or Both
When making wine, the winemaker has the option of inoculating the must (i.e the grape juice) with a known, desired, dependable cultured yeast that he/she knows will get the job done successfully. Or, he/she can let nature do its own thing, allowing the native yeasts from the vineyard to ferment the wine.

Many winemakers believe that a native (or non-inoculated) fermentation produces wines with more complexity, as there are many more different strains of yeast at play and, that you get a wine with a true expression of the vineyard's terroir.

However, native yeasts can produce undesirable flavors. They are also weaker than cultured yeasts, and if the population of native yeasts is not strong enough, the fermentation may stop, creating all sorts of other problems for the winemaker.

The prevalence of conventional viticultural practices over the past fifty years, the use of chemical herbicides and pesticides has served to kill off or weaken many vineyard native yeast populations. The return to more sustainable, organic and biodynamic practices is helping rebuild and strengthen these populations.

The most common cultured yeast used in winemaking is Saccharomyces Cerevisiae. Several hundred different strains have been isolated and cultured commercially. Strains differ according to vigor, alcohol tolerance, production / enhancement of fruity esters, temperature tolerance, sulfur dioxide formation etc. For example, many producers of Sauvignon Blanc will deliberately select yeasts that amplifies the zesty, vibrant aromas of the Sauvignon Blanc grape.

As with all things to do with wine, the situation is rarely simple black or white. Because wild yeasts automatically exist, many winemakers allow them to start the fermentation, and then inoculate to ensure a safe, consistent and completed fermentation.

To each his/her own, and to the great diversity in winemaking!

Mary Gorman-McAdams, MW (Master of Wine), is a New York based wine educator, freelance writer and consultant.