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Let the Wine Fermentation Magic Begin!

Pentailed treeshrews do it.

Loris love it.

Even bears can’t resist it.

Who would have thought that the simple combination of fruit juice and yeast could be so fun? Animals have long figured out what humans did about 7,000 years ago; fermentation is the bee’s knees.

You may wonder what happens in that vat or cask that makes wine so delicious. You have questions. We have answers.

Sugars Plus Yeast Equals Alcohol

Part of the mystique about wine rests with the fact that there are so many varieties, about 10,000 to be exact. One grape known as the Casanova of the winemaking is Gouais Blanc, a parent to about 80 different varieties. Don’t get us started about Pinot Noir and all its clones.

The one thing that links wines all together is fermentation.

At harvest, ripe grapes contain between 15 and 28 percent sugar. A lot of factors influence the outcome, including the amount of sunlight, weather, and the topography of the vineyards. The essential thing is that everything has an effect on what you’re drinking. 

Sugar’s primary purpose is to feed the yeast. That makes it a critical component of a wine. Without it, you’d just be drinking unsweetened grape juice.

How Does the Yeast Get in the Act?

Yeast is the Ying to sugar’s Yang. It does the heavy lifting in wine production.

Yeast exists everywhere in the vineyard, even on the grapes themselves. However, they’re not all the same. Native varieties often get to the plate first. However, more often than not, cultured yeast takes over fermentation and takes it home.

Using cultured yeasts delivers consistent results so that your glass of Cabernet Sauvignon tastes as you expect it and not like vinegar. A vintner or winemaker uses a particular kind to get the desired outcome.

Each type contains a different slate of enzymes, which are chemicals that cause reactions to occur. About 30 chemical reactions go on during fermentation. The results aren’t always the same. That’s one reason why wines don’t taste or smell the same. Essentially, they begin with varying potential base ingredients.

The Fermentation Process

If we can nerd out a bit, fermentation results in ethyl alcohol, carbon dioxide, and heat. We have to be upfront when we say that it isn’t pretty.

It’s a bubbling mass of juice that is cloudy and,frankly, not very appealing. It’s even messier with red wines because the skins are present and sometimes the seeds and stems, too. Those in the trade refer to the other stuff in the vat as MOG or materials other than grapes.

Don’t ask about what some of those other things are.

The difference between white and red wine fermentation is that the latter needs the skins to color the wine. That’s because most grapes yield clear juice. Skin contact makes all the difference.

Steps in Wine Fermentation

Now that you know the basics, let’s delve into the steps of winemaking.

It all begins with the harvest.

Different grapes ripen at various times. It’s probably the most crucial decision that a vintner makes. Choosing the right varieties for the vineyard is an art of itself. The order of the steps varies whether you’re starting with white or red grapes.

White Wine Fermentation

The next step at the winery after sorting through the grapes is to crush them gently to get the juice flowing. Then, they are pressed to extract more juice and to separate the skins and other stuff from the liquid.

You may think, why bother with all this delicate handling. Let’s just make some wine!

The reason is that the vintner is trying to avoid excess tannins and skin contact. Tannins are the compounds that dry out your mouth when you drink wine. Some white wines contain tannins, but that’s usually because of oak aging, which we’ll discuss later. If the vintner didn’t take this care, the wine might taste bitter.

The vintner’s goal is to get that lovely, light-colored juice on its way to becoming wine.

Yeast Enters the Room

Now comes the time for the magic to happen with the addition of yeast. The vintner can choose to ferment the juice called must at this point in either stainless steel or oak barrels. Each has its pros and cons.

Stainless steel doesn’t react with the must or impart any flavors. You’ll just get the delicious aromas of the wine. The vintner also has the option to control the temperature better if the vats have refrigerated jackets that allow him to change it.

On the other hand, oak can affect the flavor of the wine and add complexity, especially when the vintner uses newer or smaller barrels. Some grapes, such as Chardonnay, are often neutral and less intense flavors and aromas that can benefit from this contact. Others like Riesling may lose that characteristic fruity and floral aromas that define the vine.

The downside of using oak is that you have less control over the temperature unless the fermentation room is at the right temp. Generally, white wine fermentation is slow and long, usually around 50–60 degrees. 

As you can see, there are a lot of decisions along the way. But wait! There’s more!

Malolactic Fermentation

Malolactic fermentation or MLF is something of a misnomer. It isn’t fermentation per se because yeast isn’t involved. Instead, bacteria steps in to take over the reins.

We should start by saying that vintners don’t always opt to allow MLF to happen. They can avoid it with the judicious use of sulfur dioxide to kill the bacteria.

The upshot is that the vintner can inoculate the must with a lactic acid bacteria to get things rolling. The result is the conversion of tart malic acid into softer lactic acid. Malic acid occurs naturally in grapes and other fruits, notably, Granny Smith apples. It gives both their crisp acidity.

Some grapes are acid bombs, such as Riesling and Sangiovese. A vintner may choose to take the acidity down a notch if it’s over the top. You may hear MLF referred to as light or heavy malo, depending on far the vintner took the process. Remember that sulfur dioxide has the final say.

One noticeable product of a heavy malo is the characteristic buttery flavors you might taste in Chardonnay. These wines were all the rage in the 1980s and 1990s.

Lees Contact

A vintner has yet another choice that can affect the flavors and aromas of a wine. The yeast will only live so long before they die. Then, they’ll fall to the bottom of the fermentation vessel as sediment known as lees.

Leaving the must in contact with the lees can add more complexity to the wine with yeasty aromas and a creamier texture. A vintner can enhance these effects by stirring them up periodically. It all depends on what the winemaker wants the final product to taste like.

Red Wine Fermentation

Red vine fermentation starts the process just like with white wines with the gentle crushing of the grapes. Then, it forges its own path. It begins with a cold soak where the grape juice and skins get to know each other better.

The Cold Soak

That does some vital things to the wine that will end up in your glass. First, it extracts the color from the skins so that your red wine is, well, red or a shade of it. Second, a cold soak allows the chemicals called phenols to leach out of the skins and into the must.

These molecules will participate in some of those 30 chemical reactions we mentioned earlier. They also bring complexity and tannins to the mix. 

Decision time occurs when the vintner decides how long this cold soak will last. He may choose to let the must and skins linger together longer for paler wines like Pinot Noir. He may break up the party earlier for heavily pigmented grapes, such as Syrah. The length of time also affects the flavor profile.

Let’s Make Some Wine

Then, it’s time for fermentation. The general rule of thumb for red wines is hot and fast. It’s not unusual for a vintner to ferment the must at 85 degrees or more, depending on the grape variety. The alcohol making process helps on this score since heat is a by-product.

The purpose is to get those gorgeous ruby hues. Alcohol helps the process because it’s an excellent solvent with those skins that are still floating around in the must.

A funny thing happens on the way to becoming ethyl alcohol.

The skins and MOG in the vat or barrel start to make their way to the top because of the carbon dioxide formation that literally pushes up this cake of stuff. Unlike white wine fermentation, the vintner must now use cap management.

If he doesn’t take this action, the must isn’t going to benefit from the yummy things in the cap. There’s also a risk that nasty bacteria will form and spoil the batch. There are four options the vintner has at this point.

He can simply punch it down to break up the solid mass and let it macerate with the must again.

He can also siphon off the must from the bottom and spray it on the cap, which will weigh it down again.

He can drain off the juice into another tank and pour it back into the original one.

He can let a machine do the heavy lifting and stir up the must and cap.

Additional benefits of using any of these methods are aerating the wine and releasing the trapped carbon dioxide. The latter is a vital step before bottling.


Unlike white wines, a vintner will almost always opt for MLF with red wines. Since the skins hang around during fermentation, they present a greater risk of bacteria development. MLF can stop these nasties in their path.

Sometimes, it happens on its own. Other times, the vintner has to inoculate it with lactic acid bacteria to get things going.

Rosé Wine Fermentation

Rosé wines have a little of both white and red winemaking methods. It begins with crushing to release the juice. It’ll hang out with the skins, but the contact time is limited. It just needs a few hours to get those lovely salmon colors we love so much. That’s the most common way to make these wines.

Another way a vintner can make a rosé is to start off making a red wine. Then, he can tap some must out of the tank and let the rest become red wine. This method is called saignée. It’s somewhat controversial in France. Some traditionalists think of it as cheating.

A third method is called direct press, used widely in Provence. The process is just as the name would suggest. It’s out of the harvest and into the press for you, grapes! The limited contact time gives the juice just enough color to earn its name.

The Ingredients of Wine

When all is said and done, wine fermentation results in six components from most to least. They include:

  • Water
  • Alcohol
  • Acid
  • Sugar
  • Phenols

Water makes up at least 80 percent of the wine. That’s not surprising, given the fact that grapes are over 80 percent water. The fun begins with the other things in wine.

Alcohol comes in at 10–15 percent of the wine. The top figure exists because of its effects on the yeast. If it’s over 15 percent in the must, the yeast will die, but what a way to go! It may surprise you to know that the weather plays a vital role in the content in your glass.

Warmer climates provide the ideal settings for grapes to ripen fully and make lots of sugar in the process. The more sugar in the juice, the more food for the yeast to make into alcohol. That’s why wines from places like southern California or Australia have such high ABVs. It also explains the lower ABVs of German wines growing in cooler climes.

Acid makes up a measly 0.5–0.75 percent of the wine. Even though it’s a small amount, it has a profound effect on what’s in your glass. Wine is an acidic beverage, typically falling in at 2.9–3.9 pH. A lemon runs between 2–3 pH.

Grapes contain four kinds of acid that contribute to the final product. Fermentation adds a couple more. Acid is your friend, especially if you like to pair food with wine. It makes it easier to find a match. It also helps the wine age.

Sugar is a bit of a wild card since it depends on the style of the wine. Suffice to say that all wines have a smidge of sugar in them, even if they aren’t a dessert wine. That’s because some sugars don’t ferment.

Finally, we come to the most fascinating component of wine, the phenols or phenolic compounds. Some of these molecules are present in such small amounts, yet they have incredible effects on how your wine tastes and smells.

Some lend color to both white and red wines. Others like tannins add structure and complexity to the wine. They also help wines age and act like natural preservatives. That’s one reason why they can age so well. Storing wines in oak will also add tannins, especially with the more aromatic younger oaks.

If you thought your glass smelled like wood, you’re detecting the oak’s effects.

Another pleasant but perhaps surprising phenol present in wine is vanillin. It can probably explain that sweet smell you’ve detected.

Fermentation and Esters

A wonderful thing happens during fermentation when the alcohol starts to form and meets up with the acids present in the must and produced during the process.

Esters are chemical compounds formed between an alcohol and an acid. We think that they are some of the most interesting and delicious aspects of wine.

It’s essential to understand that anything you eat or drink contains chemicals—a lot of them. For example, an apple contains over 300 compounds. It’s the same with wine.

When you hear someone say they smell lime or peach, they are detecting these chemicals that float into the air above the wine when oxygen gets into the mix.

Those fruity or floral notes you smell are called primary aromas and are the products of fermentation.

Other types of aromas develop as the wine matures and ages. Some may sound unusual, such as coconut from oak barrels or the grass from a Sauvignon Blanc produced in New Zealand.

The fact remains that it’s real. And now you know how they got into your glass.

Final Thoughts About Wine Fermentation

Making wine involves many decisions for the vintner, each with the potential to make profound changes in the final product. Wine itself has a magical quality that brings people together.

Sure, it’s great to enjoy a glass. However, it’s also enjoyable to sniff the elixir in your glass and think about its journey from the vineyard to the bottle. You may never view your Chardonnay or Syrah the same again.

Photo by Cameron Venti on Unsplash

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