How Wine is Made: Alcoholic Fermentation

ALCOHOLIC FERMENTATION

By Cole Swanson
November 11, 2024

Yeast eat sugars (glucose) of harvested grapes and metabolize them into alcohol (ethanol). 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.

In addition to alcohol, carbon dioxide is released as well as an important compound called acetaldehyde.

Additionally, fermentation is responsible for a wine’s flavor, and, in the case of red wine, color and tannins.

A wine’s flavors and overall profile can be manipulated more depending on the type of fermentation vessel the winemaker chooses, such as a steel tank (AKA inert vessel), barrel, or concrete egg. Each type provides different characteristics, though a steel tank is the most common choice.

SUGARS (GLUCOSE)

Sugar, or glucose, is essential for fermentation, because without it there is nothing to be coverted into alcohol.

Before grapes are harvested, their internal sugar levels, also known as “brix,” are examined. Grapes are only plucked when thier brix level reaches a desired amount. At this point, the grapes have ripened enough for the winemaker to transfer them to the winery and begin the fermenation process. 

YEAST: SACCHAROMYCES CEREVISIAE

Yeast is a fungus that is used to transform sugars and startches into carbon dioxide, and alcohol during fermentation.

There are about 1,500 species of yeast, though the most common commercial yeast strain used for fermentation 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 ethonal (alcohol), wich will create an important compound called acetaldehyde, which is critical for flavor profiles.

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, which is the process of disallowing oxygen to come into contact with the wine. This will help to preserve the wines flavors and tannins and keep the wine fresh, as contact with oxygen will cause the wine to age.

Temperature Control

Temperature control is important during fermentation because yeast can only survive between ceratin degrees. If the fermentation temperature is too low (<53 Fahrenheit) or too high (>89 Fahrenheit) the yeast will die. Therefore, winemakers prefer to use steel tanks (inert vessels) in order to properly control temperatures.

Anaerobic Winemaking

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.