A wine’s character starts with the microscopic organisms that fuel fermentation. In Part I of our new series, wine expert Maurizio Broggi explores wine yeasts and reveals how their behavior influences aroma, structure, style, and quality.

Yeasts are single‑celled fungi responsible for alcoholic fermentation. Their metabolism and the production of alcohol depend on the availability of oxygen. In its presence, they respire aerobically and reproduce rapidly, but produce only water, CO₂, and heat. Under anaerobic conditions (i.e., without oxygen), they instead convert sugars into ethanol – the key product of alcoholic fermentation – along with CO₂ and heat. In addition to ethanol, yeast metabolism generates glycerol, acids, esters, and a broad range of volatile compounds, all of which contribute to the structure and sensory profile of wine. Of the more than one thousand volatile aroma compounds identified in wine, at least four hundred are attributed to yeast activity.

Broadly speaking, yeasts in winemaking can be grouped into two main categories: inoculated and native yeasts. Inoculated yeasts are cultured strains deliberately added by the winemaker to carry out fermentation, a process commonly referred to as inoculated fermentation. Native yeasts (also known as wild or indigenous yeasts) include those present on grape skins and in the vineyard environment, as well as ambient yeasts found in the winery and on winemaking equipment. When native yeasts drive fermentation, it is typically referred to as spontaneous fermentation (also called native-, wild-, or indigenous-yeast fermentation).

When discussing wine yeasts, the term genus describes a broader family of related yeasts, while species refers to a more specific grouping within that family. In winemaking, alcoholic fermentation is most commonly driven by the Saccharomyces genus, with Saccharomyces cerevisiae typically acting as the dominant species. Several hundred distinct strains exist within the Saccharomyces cerevisiae species, differing in their fermentative behavior and metabolic outputs. Saccharomyces cerevisiae is particularly well suited to winemaking, as it tolerates high acidity, relatively elevated alcohol levels (commonly 13–15% abv, with some strains reaching 16–17% abv), low-oxygen conditions, sulfur dioxide (SO₂) levels typically used in winemaking, and a wide range of fermentation temperatures.

Among the most relevant non-cerevisiae Saccharomyces yeasts in winemaking are Saccharomyces bayanus-type strains and Saccharomyces uvarum. For example, Saccharomyces bayanus-type strains are widely used in sparkling wine production because they ferment reliably at cool temperatures, tolerate elevated CO₂ pressure, and provide a clean aromatic profile. Saccharomyces uvarum is particularly cold-tolerant and can dominate the early stages of fermentation under very cool conditions, especially below 15 °C (59 °F).

Although Saccharomyces species are usually present on grapes only in very small numbers, they frequently take over fermentations once conditions become favorable. Indeed, many wineries harbor a persistent “house” yeast strain on winery surfaces. Even a small initial population of Saccharomyces can multiply rapidly under suitable conditions. Typically, in most fermentations, Saccharomyces cerevisiae eventually becomes the dominant species once conditions shift in its favor. Sulfur dioxide additions further suppress wild yeasts by limiting microbial competition for nutrients and oxygen.

Native yeasts include a range of non-Saccharomyces species that live on grape skins and in winery environments. They may also be carried by air currents and insects such as fruit flies. The most common include Hanseniaspora (also known as Kloeckera) and Candida, with Pichia and Metschnikowia, among others, typically present in smaller numbers. In spontaneous fermentations, these species often initiate fermentation when sulfur dioxide additions are low or absent.

Hanseniaspora tends to dominate early in fermentation, producing esters, glycerol, and ethyl acetate. At low concentrations, ethyl acetate can add a pleasant aromatic lift; at higher levels, however, it can compromise the sensory profile of the wine. Candida can tolerate higher alcohol levels (up to ~10% abv) than most non-Saccharomyces species, which are generally inhibited at relatively low alcohol levels (often below ~4–8% abv).

Owing to their limited alcohol tolerance, these wild yeast species usually decline as fermentation progresses, allowing Saccharomyces species to take over. Temperature also plays a significant role in wild yeast activity: pre-fermentation cold maceration (cold soak) conditions favor Hanseniaspora and Candida, while Saccharomyces species are more competitive at warmer temperatures. Pichia is typically found on damaged fruit and is often associated with high levels of volatile acidity and spoilage.

1. Why are yeasts essential in winemaking?

Yeasts convert grape sugars into alcohol, CO₂, and heat during fermentation. Their metabolic by-products – acids, esters, glycerol, and volatile compounds – shape a wine’s texture, aroma, and overall style.

2. What is the difference between inoculated and native yeast fermentation?

Inoculated fermentation uses cultured Saccharomyces strains added by the winemaker for consistency, while native (spontaneous) fermentation relies on naturally occurring yeasts from the vineyard and winery environment, often leading to greater complexity but less predictability.

3. Why is Saccharomyces cerevisiae the dominant yeast in winemaking?

S. cerevisiae tolerates high acidity, alcohol levels up to ~15% (or higher in some strains), low oxygen, and typical SO₂ additions. These traits allow it to outcompete other yeasts and complete fermentation reliably.

4. What role do non-Saccharomyces yeasts play?

Species such as Hanseniaspora, Candida, and Pichia often start fermentation in low-SO₂ or native fermentations. They contribute unique esters and textures early on, but usually die off once alcohol rises, allowing Saccharomyces to finish the fermentation.

For more insights into wine yeasts, keep an eye open for Introduction to wine yeasts: Part II, where cultured yeasts, fermentation styles, and the role of the winemaker are in focus.

Want to read more? Take a look at some of our other blogs:

• • Sulfites in wine: Part I
  • Introduction to wine faults: Part I
  • The Méthode Champenoise