If you’ve ever done a deep dive into how wine is made, you’ll know that wine fermentation is an essential part of wine production.

You also may know that: 

• The process can last from a few days to a few weeks
• The steps slightly differ between red and white wines
• Fermentation is what creates the alcohol in wine

You might not know what exactly is happening in those big magical fermentation tanks, or that wine fermentation is responsible for a lot more than alcohol when it comes to producing wine.

In this article we take a deep dive into the ingenious process of wine fermentation. We’ll go over the essential byproducts of the process, the integral role yeast plays, and the arduous process of creating perfect fermentation conditions. Your glass of wine just got more fascinating.

What Does Fermentation Do?

Simply put, fermentation in winemaking is what converts grapes into alcohol. 

While white wine is created by just fermenting grape juice, red wine is made using the whole grape, grape skins and all. This is what gives red wine such high tannins.

For the wine to ferment, winemakers add yeast to the grape juice. These yeasts convert the natural sugars of the grapes into ethanol and carbon dioxide (which is a byproduct that gets released into the atmosphere and isn’t important for the wine).

However, fermentation doesn’t just create alcohol. In fact, this incredible process also helps to shape the flavor, aroma, and look of the wine.

When winemakers use the fermentation process to convert their grapes to wine, they also produce a whole host of other complex compounds, which help to make wine, well, wine!

Some of these compounds include:

• Hydrogen sulfite
• Esters
• Fatty acids
• Ketones
• Norisoprenoids

All of these complex compounds help to give your favorite wines their iconic flavors and aromas. Esters, for instance, are known for their ability to make wine taste citrusy and floral, while norisoprenoids help to give Chardonnay some of its more spicy notes.

It’s knowledge like this that helps shine a light on the incredible process of winemaking. It really is quite complicated chemistry. These methods have been developed over centuries, all in order to give you that perfect glass at the end of a long day.

The Importance of Yeast in Wine Fermentation

As we’ve mentioned, yeast is a key component in wine fermentation. However, there are thousands of types of yeasts to choose from, all of which have different strengths and weaknesses. 

Wild yeasts (or native yeasts) are found on the fruit or from around the vineyard. Cultured yeasts are created by winemakers in a lab.

While wild yeasts are praised for their ability to add unique flavors to wine, they are unpredictable and can lead to spoilage. This is why it’s common for cultured yeasts to be used for wine production — so winemakers know exactly what to expect.

There are many different strains of these cultured yeasts and each one can affect the flavor outcome of the wine varietal. Using different yeast strains creates more diversity in the wine varietals produced.

Cultured yeasts are stored in a dried inactive form, but diluted with grape juice before being added to the unfermented grapes. To keep these cultured yeasts active (and hungrily converting sugar into alcohol), winemakers must make sure they have enough vitamins, minerals, carbon, and other essential components. While these do occur naturally in the grapes, they can be controlled and corrected during production in a bid to give the yeast the best chance of doing its job.

Creating the Perfect Condition for Fermentation

It’s not just the yeast that makes for a perfect fermentation. Winemakers must work hard to provide the ideal conditions for their wines.

Sugar content is an essential component for successful fermentation. The more sugar present, the more alcohol can be produced by the yeast. However, if there is too much sugar, the yeast has to work hard to convert all the sugar to ethanol, often resulting in “stressed yeasts.” These can produce off flavors in the wine.

Getting the right fermentation temperature is also an essential part of the winemaking process. Red wine is typically fermented at higher temperatures than your average white wine. However, some choose to ferment white wine slightly warmer to bring out more complex flavors, while some ferment red wine slightly cooler to bring out fruity flavors. Fermenting wine too warm can have adverse effects, often resulting in a dulling of flavors or even killing off the yeast too early.

Prematurely killing off the yeast during fermentation can lead to wines with a higher sugar content, since the yeast can’t convert all the sugars into ethanol in time. But it’s not always a bad thing — some winemakers will purposely kill off the yeast cells to create sweet wines.

The right fermentation vessel also contributes to successful fermentation. While the majority of white wines and rosés are fermented in large stainless steel tanks (giving them clean, sharp flavors) many red wines (and of course Chardonnay!) are fermented in oak barrels.

Fermenting wine in oak barrels allows for a smoother, creamier flavor, often with notes of vanilla and, well, oak. This is due to the gradual oxidation of the wine caused by the porous oak barrels. 

On the other hand, fermenting wine in stainless steel allows for no oxidation and no chemical reaction from the steel, giving the beverage those crisp flavors.

Other Types of Wine Fermentation

Many wines go through more than one fermentation. While the primary fermentation creates alcohol and other flavor-enhancing compounds, a secondary fermentation is needed for certain styles of vino.

Some methods even forgo the traditional fermentation process altogether, relying on slightly more natural processes.

Malolactic Fermentation

For this method, bacteria plays an important role. These clever microbes help to convert malic acid (a compound naturally found in grapes) into lactic acid, which makes the wine taste softer, creamier, and less tart.

Malolactic fermentation often takes place after the primary fermentation and is more common in the production of red wine.

Bottle Fermentation

To make sparkling wines, like Champagne, the wine goes through a secondary fermentation after bottling. Once bottled up, winemakers add a sugar and yeast mixture directly to the wine, this creates carbon dioxide, giving Champagne its famous bubbles.

Carbonic Maceration

Sometimes known as whole grape fermentation, carbonic maceration refers to the process of using the whole grape during the fermentation process. This process requires no added yeast and instead relies on the natural enzymes in the grape to convert the sugars into alcohol.

Without Fermentation There Is No Wine

As you can see, wine fermentation is a complex science, one that can go wrong in a whole host of ways. 

Yeast selection, sugar control, fermentation temperature, and even which vessel the wine ferments in can all have catastrophic effects on the final product if selected poorly. After all, fermentation is about a whole lot more than creating alcohol.

While there are plenty of other key elements to making wine, successful fermentation is essential.

Without wine fermentation, our favorite beverage wouldn’t have its alcohol content, but it would also lack all the incredible flavors, aromas, and textures that set our favorite Pinot Noirs apart from sad old grape juice.