Is Wine Fermented? Wine-Making 101

How does grape juice become wine? The answer is a chemical process called fermentation. You may be familiar with its starring role in the making of beer, but wine fermentation is very different. Fermentation makes the difference between one noted grape juice and complex wine.

Grape juice that hasn’t yet begun the fermentation process is called “must.” Fermentation initiates a sequence of chemical reactions that give way to the wine's aromas, flavors, texture, and color. Is it a magic trick? That depends on how you look at it. We can look to science to explain how it happens, but it certainly does seem to have an air of magic to it.

Let’s look at what happens during wine fermentation.

Yeast: The Leading Lady of Fermentation

After the grapes are harvested and crushed, the must (unfermented grape juice) is moved into a vat. There are a handful of different kinds of vats, and they all make different impressions on the resulting wine. Oak barrels, clay, and concrete tanks allow the wine to interact with a dash of oxygen, imparting complex aromas. Stainless steel doesn't add or detract anything.

Fermentation is carried out by microscopic fungi called yeast. And it happens naturally in fruit that has split open by the skins. Yeast lives on the skins of grapes but also mutates in the winery and can be found in the air, on the walls, and on the equipment. It’s unavoidable. Once you introduce yeast, it lives rent-free in your winery.

However, the yeast that lives in the winery ferments very slowly, which isn’t always practical for the winemaker. For this reason, some winemakers will introduce commercial yeasts to get the job done quicker.

But for natural winemakers, wild yeast is worth the wait. It imparts interesting flavors to the wine, like pear, roses, and toasted almonds.

Wine Fermentation 101

Fermentation can last anywhere from 1 week to a few months. Red wine is fermented at warmer temperatures than white wine. The warmer temperatures of red wine fermentation allow for more color to be extracted from the skins of the grapes, while cooler temperatures preserve the fruity aromas of the more delicate white grapes.

Once the grapes are split open, the yeast can feed on the sugar to create alcohol. By the end of fermentation, the yeast has converted all of the sugar into alcohol, creating a dry wine with little sugar and anywhere from 8%-15% alcohol by volume (ABV).

While the yeast is eating the sugar, it releases heat and CO2, both of which escape out of the vat and into the atmosphere. The chemical compounds that make up the flavor profile of your wine, once bound up in the sugar, are also released.

So it’s really important that by the time the grapes arrive in the fermentation vat, they have enough — but not too much — sugar. By this point, it’s fall, and they’ve spent all summer ripening on the vine, making sugar from a concoction of sunshine, water, and oxygen. If the climate is cool, the grape will develop lower levels of sugar and may need to hang on the vine a little longer to soak up as many sun rays as it can. If it’s warm, the winemaker will be careful not to let it hang too long and develop absurd amounts of sugar.

You can imagine, then, that grapes with low sugar levels make a wine with a lower ABV. With more sugar to work with, the yeast will create more alcohol.

Aromas That Are Released During Fermentation

While the yeast is performing its magic show called fermentation, chemical compounds are released into the wine, creating aromas that you recognize in your glass. This is because these chemical compounds also exist in other foods and organic materials.

Here are some of those compounds and their aromas.

Esters: fruit flavors like pear, banana, red apples, and oranges
Pyrazines: green bell pepper, asparagus
Ketones: violets
Aldehydes: nutty, bruised apple, dried wood, vanilla
Diacetyl: butter
Mercaptans: passion fruit, gooseberry, blackcurrant
Terpenes: citrus peel and flowers

When the Job Is Done, the Yeast Dies

Yeast lives in a funny catch-22: its job is to create alcohol, but it is eventually killed by the very alcohol it creates. Once the yeast ferments the wine to about 15%-17% alcohol, it simply can’t survive — but its job isn’t done yet. Yes, it’s working on the wine even after it dies.

Once it dies, it falls to the bottom of the tank or barrel. At this point, the winemaker has a choice: it can move the wine off of the yeast in a process called “racking,” or it can leave the dead yeast cells in the wine to steep in it like tea leaves in a mug.

The dead yeast cells are called “lees” or “lies.” Leaving them to steep in the wine is called “sur lie aging.” During the aging process, the lees impart new character to the wine. They release mannoproteins that give the wine a fuller mouthfeel. They impress flavors like bread, butter, and nutty aromas.

Then, Another Fermentation Process Takes Place

After alcoholic fermentation is complete, another one naturally begins. But this time, the process is bacteria-driven rather than yeast-driven. This process is called malolactic fermentation, and you can tell if it has taken place in your wine simply by giving it a good sniff.

Here’s what happens: malolactic bacteria convert malic acid, which tastes tart and vegetal, into lactic acid, which tastes creamy and buttery. The wine takes on a soft character rather than a sharp acidity.

You might even taste toffee, butterscotch, or caramel in wines that have gone through “malo.”

A winemaker can choose to stop this natural process from taking place, and they might want to if they’re working with an aromatic grape like Riesling and Sauvignon Blanc, which will be muddled by the flavors that malo impresses. Almost all red wines undergo malolactic fermentation.

Champagne Actually Goes Through a Second Alcoholic Fermentation

Have you ever wondered how Champagne is made? All Champagne is made in the same exact way. The method is very old and is so strict it has a name: the traditional method. In France, it’s known as “méthode traditionelle” or “méthode Champenoise.” Here’s how it works.

A base wine is made by way of alcoholic fermentation (see above.)
The wine is bottled.
A solution of sugar and yeast is added to the bottle and sealed with a crown cap.
The wine is then laid on its side and left to sit for a minimum of twelve months.
During the first 30 days, the yeast does what it does best: eat the sugar and convert it into alcohol. This is referred to as the “second fermentation.”
But this time, the CO2 is trapped by the sealed crown cap and, having nowhere to escape, is forced to integrate into the wine itself. The longer the wine sits in the cellar on its side, the more integrated CO2 is, and the finer the beads in your glass will be.
The yeast dies and settles to the bottom of the bottle, imparting what is famously known as a “Champagne bouquet.” In your glass, you might smell brioche, nuts, or baked bread.
The bottle is slowly, over the course of a week, turned on its head so that the dead yeast cells gather in the neck of the bottle.
In a very dramatic affair, the neck of the bottle is submerged into an icy brine to freeze the mass of yeast. The crown cap is removed, and the pressure in the bottle causes the frozen yeast cells to eject out of the bottle.
Finally, a touch of grape spirit is added, and the bottle is sealed with a cork and cage.

The interesting thing about the méthode traditionnelle is that the people of Champagne were perfecting this formula long before scientists discovered what yeast was, let alone its role in fermentation. They were simply paying attention to the context clues of what was happening in their wine and adjusting their process on hunches alone.

Fermentation: Magic or Science?

Either way, it’s pretty incredible that ancient civilizations all around the world were able to turn grape juice into wine. Today we know the fascinating workings behind the scenes that we can’t see with the naked eye. The aroma compounds that are found all over the world are also bound in the sugar of grapes, and introducing a little yeast pops them right open.

So the next time you pop your bottle open, be sure to raise a glass to the microscopic organisms that worked so hard to get you that delicious wine.

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