How Wine is Made:
Alcoholic Fermentation

By Cole Swanson
November 11, 2024

Alcoholic Fermentation

Yeast eat sugars of harvested grapes and metabolize the sugars into alcohol (ethanol) while also releasing carbon dioxide and an important compound called acetaldehyde. Additionally, fermentation provides color and tannins to a red wine and flavors to all wines.

Sugars

Sugar, or glucose, is one of the most essential parts of fermentation, because without it there is nothing for the yeast to consume. This starts in the vineyard. Winemakers will examine the grapes on the vine for “brix,” or the amount of sugar within the grapes. When the brix level reaches its desired amount, the grapes are ready to be harvested and transferred to the winery.

At the winery, they are sorted for any faults, such as rot, before being placed into a fermentation vessel. It is in there that yeast is added and fermentation begins. 

Yeast: Saccharomyces Cerevisiae

The most common commercial yeast strain is called saccharomyces cerevisiae because it is able to withstand high levels of ethanol and sugars; it is also common when making bread and other foods. As the yeast eat the grape’s glucose, they metabolize it into carbon dioxide and an important compound called acetaldehyde, which helps the yeast to continually eat glucose and produce ethanol, the type of alcohol found in wine. It is also critical for flavor profiles, and so it is monitored closely to ensure there is not too much or too little.

Inert Vessel

The most common type of fermentation vessel is a steel tank, also known as an inert vessel. Inert vessels allow for temperature control and anaerobic winemaking, or disallowing oxygen to come into contact with the wine. 

Fermentation can only be successful within certain conditions. Because yeast are living organisms, they cannot survive if the fermentation temperature is too low (<53 Fahrenheit) or too high (>89 Fahrenheit). Therefore, winemakers prefer to use steel tanks (inert vessels) in order to properly control temperatures.

Inert vessels also play a key role in anaerobic winemaking. Oxygen can decrease the amount of fruity flavors, and so if a winemaker wants to retain a fruity profile, a steel tank is ideal. Winemakers may even go as far as to add carbon dioxide or sulfur dioxide to the tanks before adding the grapes so the tank itself has as little oxygen as possible from the start.

Fermentation is complete once there is no more sugar for the yeast to eat. This is a natural fermentation completion, but sometimes winemakers may stop fermentation early, such is the case when making sweet wines.