Fermentation in winemaking

Lore & Background

The natural occurrence of fermentation means it was probably first observed long ago by humans. The earliest uses of the word 'fermentation' in relation to winemaking referred to the apparent 'boiling' within the must from the anaerobic reaction of yeast to sugars and the release of carbon dioxide; the Latin 'fervere' means, literally, to boil. In the mid-19th century, Louis Pasteur noted the connection between yeast and fermentation, in which yeast act as catalyst and mediator through a series of reactions converting sugar into alcohol. The discovery of the Embden–Meyerhof–Parnas pathway in the early 20th century contributed further to understanding the complex chemical processes involved.

In winemaking, distinctions are made between ambient yeasts naturally present in wine cellars, vineyards, and on grapes (sometimes called a grape's 'bloom' or 'blush') and cultured yeasts isolated and inoculated for use. Common genera of wild yeasts include Candida, Klöckera/Hanseniaspora, Metschnikowiaceae, Pichia, and Zygosaccharomyces. Wild yeasts can produce high-quality, unique-flavored wines but are often unpredictable and may introduce less desirable traits or spoilage. Traditional winemakers, particularly in Europe, advocate ambient yeast as a characteristic of terroir, while many winemakers prefer controlled fermentation with cultured yeast, most commonly Saccharomyces cerevisiae ('sugar yeast'), which includes several hundred strains that affect heat, vigor, and flavor. Alternative non-Saccharomyces cerevisiae yeasts are increasingly used for greater complexity.

Upon introduction of active yeasts to grape must, phosphates attach to sugar, and six-carbon sugar molecules split into three-carbon pieces, undergoing rearrangement reactions. Carboxylic carbon is released as carbon dioxide, and the remaining components become acetaldehyde. In the absence of oxygen, acetaldehyde is converted by reduction to ethanol. A small amount of acetaldehyde is converted by oxidation to acetic acid, which in excess can contribute to the wine fault known as volatile acidity. After yeast exhausts its life cycle, it falls to the bottom as sediment called lees. Yeast ceases activity when all sugar is converted or when alcohol content reaches 15% per unit volume.

Reader's Guide

Fermentation is central to winemaking, determining alcohol content, flavor, and aroma. The process involves careful management of sugar content, yeast strain, and temperature. White wines are typically fermented at 18–20 °C, though higher temperatures may be used for complexity; red wines are fermented at 20–30 °C, though cooler temperatures can bring out fruit flavors. Fermentation generates residual heat, which winemakers control through vessel size or cooling devices, ranging from placing vats on ice to tanks with built-in cooling rings. Risks include stuck fermentation and wine faults such as volatile acidity, which can be addressed with sulfur dioxide, though excess SO₂ can itself cause faults. Winemakers may stop fermentation early to retain residual sugar for dessert wines by cooling the must or adding alcohol to create fortified wines. The ethanol produced acts as an important co-solvent for non-polar compounds. The use of different yeast strains is a major contributor to wine diversity, even among the same grape variety. After a winery has operated for years, few yeast strains remain active; active dry yeasts reduce strain variety by outcompeting naturally present strains. The metabolism of amino acids and sugars creates volatile compounds (aldehydes, esters, fatty acids, fusel oils, hydrogen sulfide, ketones, mercaptans) and non-volatile compounds (glycerol, acetic acid, succinic acid) that contribute to wine's sensory profile. Some yeasts generate volatile thiols for fruity aromas, while Brettanomyces yeasts produce the 'barnyard aroma' characteristic in some red wines.

Did You Know?

• The Latin word 'fervere' means 'to boil', referring to the apparent boiling of must during fermentation.
• The most common cultured yeast used in winemaking is Saccharomyces cerevisiae, also known as 'sugar yeast'.
• Methanol in wine typically ranges from 0.1 to 0.2 g/liter and has no adverse effect on people.