Yeast in winemaking

Lore & Background

For most of wine history, winemakers did not understand the mechanism that turned grape juice into wine. They observed fermentation as a frothy, bubbling process, described as 'boiling' or 'seething', and the word 'yeast' itself etymologically means 'to boil'. In the mid-19th century, French scientist Louis Pasteur, tasked with studying wine spoilage, discovered the connection between microscopic yeast cells and fermentation, showing that yeast converts sugars into alcohol and carbon dioxide. The exact metabolic pathway, the Embden–Meyerhof–Parnas pathway, was not elucidated until the 20th century.

The yeast species now known as Saccharomyces cerevisiae was first identified in late 19th-century enology texts as Saccharomyces ellipsoideus due to its elliptical cell shape. Over the 20th century, more than 700 strains of this species were identified, with differences mostly minor but including vigor, temperature tolerance, and production of volatile sulfur compounds. In modern winemaking, winemakers can select from a diverse range of commercially available yeast strains, each offering distinct characteristics that influence aroma, mouthfeel, and fermentation kinetics, allowing precise control over the wine's character.

Reader's Guide

Yeast's primary role is to convert glucose in grape must into ethanol via fermentation. In the absence of oxygen, yeast cells reduce acetaldehyde into ethanol, recharging the NAD+ co-enzymes needed for metabolism. Ethanol is released as a waste product, and if fermentation runs to completion, all fermentable sugars are consumed. Winemakers may stop fermentation early to retain residual sugar for dessert wines by dropping temperatures, sterile filtering, or fortifying with brandy to kill yeast. Unintentional stoppage, when yeasts exhaust nutrients before reaching dryness, is called stuck fermentation.

Beyond alcohol, yeast produces other compounds that influence wine. Glycerol, produced early in fermentation, contributes body and slight sweetness; some winemakers favor conditions that promote it, such as selecting certain yeast strains or increasing oxygen exposure. Other by-products include methanol from pectin demethylation, fusel oils from amino acid decomposition, and succinic acid formed early in fermentation. The yeast Brettanomyces may be viewed as either a wine fault or, in limited quantities, as an added note of complexity. Saccharomyces bayanus, tolerant of 17–20% alcohol, is often used in fortified wines and high-sugar harvests.

Did You Know?

• The word 'yeast' etymologically means 'to boil', reflecting the frothy appearance of fermentation.
• Louis Pasteur discovered the connection between yeast and fermentation while studying wine spoilage for the French government.
• More than 700 different strains of Saccharomyces cerevisiae have been identified.