The Winemaking Process, Explained By A Sonoma County Winemaker

if 50 is the new 30, then October is the new september

Happy October. Harvest is here. If you’re a lover of California wines, you’ve probably heard that harvest 2023 is “late.” That’s true. 2023 fruit is tracking 4-5 weeks later than 2022 fruit. But an October harvest also harkens back to my early years, 2010/2011 when I routinely punched tanks in October. As such, this seems a good a time as any to review fermentation science, a favorite topic of mine.

The process of picking grapes and making wine is simple…

Wine comes from grapes, which are grown in vineyards. Wine grapes are picked and transported to the winery. Once wine grapes reach the production facility, the process differs for white versus red wines. 

The process of picking grapes and making white wine includes squeezing the white grapes to release the juice. This process is called pressing. After pressing, the juice is separated from the white wine grapes, since only the white grape juice is used to make white wine. 

The juice is placed in vessel, like a barrel or a fermentation tank, where yeast convert the grapes sugars to alcohol. 

That’s it. You’re done. Voila, white wine. 

In contrast, to make red wine, the process of picking grapes and making wine is a little bit different. To make red wine, the red grapes themselves are placed into a vessel, like a fermentation tank (or barrel). Once in the tank, yeast convert the grape sugars to alcohol, just like with white wine. 

But since the juice is fermented in a tank along with the red grapes and skins, the resulting wine is red. Once the sugar is converted to alcohol, the liquid juice, which is now wine, is drained from the tank. 

Voila, red wine. You’re almost done. 

The leftover grape skins can be removed from the tank and pressed, just like white wine grapes, only now you’re pressing after the fermentation is complete. The red wine resulting from the pressed skins is called press wine. It can be combined with the free run red wine or aged separately. 

Both white and red wines are aged before bottling, sometimes in oak barrels, sometimes not. The wine is bottled and transported to a dinner party or table near you. 

Now you know a carefully guarded industry secret: the process of picking grapes and making wine is simpler than you ever imagined!

Here are the 6 steps for making white wine

1) Pick white wine grapes

2) Press the white wine grapes to release the juice

3) Put the juice in a container

4) Yeast convert the grape sugars to wine

5) The wine is aged in tanks or barrels

6) The finished wine is bottled

The process of picking grapes and making wine is called fermentation. It is the conversion of grape sugars, glucose and fructose into alcohol, also known as ethanol, also known as wine. 

The process of fermentation by yeast and the conversion of grape sugars to wine is the same process whether the grapes are white or red. The only difference is that to make white wine, the wine grapes are pressed and only the liquid juice is fermented into wine. 

By contrast, making red wine, whole grapes are placed in the fermentation tank. During the course of fermentation, the red wine grape juice is released and converted into wine in the company of the grape skins. This gives red wine its characteristic red color. Thus the 8 steps for making red wine are slightly more involved.

Here are the 8 steps for making red wine

1) Pick red wine grapes 

1a) Optional sort the red wine grapes to remove leaves & “MOG,” material other than grapes

2) Place red wine grapes in an appropriate fermentation container, like a jug, bin or a tank

3) Allow yeast to convert grape sugars into wine

4) Mix the container so that the red wine grapes are in optimal contact with the juice

5) Remove the wine when the sugar is consumed

6) Press the leftover red grapes to release the press wine

7) Age the red wine as desired

8) Bottle the finished red wine

The winemaking process in more detail

The winemaking process is similar for all my pinots and my zinfandel. First, the grapes are hand-picked in the vineyard, often at 2 or 3 am, the coldest part of the night. If the grapes are picked cold, the berries remain cold when delivered to the winery.

Once picked, the grapes are transported to the winery in a truck. At the winery, the grapes are sorted and destemmed. After 3-6 days of cold soak, in a cold, temperature-controlled tank, I warm the tank to ambient temperature to initiate fermentation.

Sometimes I allow fermentation to begin spontaneously; other times I inoculate the tank with yeast immediately. Now the magic of winemaking begins—cue celestial chorus. 

This is my favorite part of the winemaking process.

"Magic" is cute, unintimidating, Disney-speak for "mind-numbing biochemistry." 

And nobody really wants the responsibility of explaining the winemaking process. It's "science," Yes, the winemaking process is science, and my background is, well, science, but really, nary an autopsy doth an enologist make (Shakespeare, 1603). 

If reading about pinot clones puts you to sleep, then this begs for No-Doz. 

So grab a Starbucks and wear your thinking cap, because here we go.

The Chemistry Behind Winemaking

Most simply, during the winemaking process, yeast consume the grapes' natural sugar and convert it to alcohol. Ta da, done!

Hexose (C6H12O6) + 2 ADP → 2 Ethanol (CH3CH2OH) + 2CO2 + 2 ATP  △H=-67 kJ/mol

One unit of sugar (here known as "glucose," CHO) is converted to 2 units of wine (OK, it's really ethanol, C2H6O, but who's complaining) and carbon dioxide (the same stuff you exhale with each breath). 

Notice there's no oxygen here. The winemaking process is largely anaerobic. The yeast backstroking in their Olympic pool of grape juice quickly consume all of the available oxygen with their first sugar binge, leaving them in an anaerobic (i.e. oxygen-free) environment. 

This is why the winemaking process makes wine. Fermentation is an oxygen-free process that ceases in the presence of oxygen. In contrast to fermentation, when oxygen is around, those pesky yeast "respire" instead and use oxygen to spin sugar into plain old water plus carbon dioxide. 

And that's no fun, no disrespect to Evian.

So, what's with the weird triangle? How is that part of the winemaking process?

It's a delta, my Greek “brose-loving” frat brothers. That negative delta signifies an exothermic reaction, one that releases heat. 

Translation: swimming yeast train hard, and they just get hot. Like a fat guy on a Peloton bike, the temperature in the room rises. 

Each unit of sugar consumed releases 67kJ/mol, and approximately every percent of sugar consumed (1% w/w) will raise the temperature 1.30◦C . This is fine. We like some heat. The heat helps extract the maximum color from those testy, thin-skinned pinot grapes. The heat keeps fermentation brisk. 

But alas, too hot and the yeast will crap out and die before their job is done. So, during the winemaking process we winemakers do things to dissipate the heat.

Top Four Ways to Chill (if you're a microscopic wine fungus)

#1) Spend 4 days at the Four Seasons Hualalai with round the clock Mai Tais

(oops, wrong blog...)

#2) Hit the freezer. I am being blithe, but for the lowest tech, bang for your buck, just move your fermentation bin into a cold room or punch in some dry ice

#3) Punch Downs, Pump Overs, & Cap Management

During the winemaking process, grape skins congeal into a gloopy, floating Pangea at the top of the fermentation tank. Called a "cap," this amalgamation makes it hard for heat to escape. 

Winemakers push the cap to the bottom of the tank with a giant, metal, rake-like contraption. This is called a "punch down." (see above.)

Juice can also be pumped from the bottom of the tank back over the top with a hose, as in a "pump over." Both promote even skin/juice contact and dissipate heat.

#4) Be like Meb. In Athens 2004, Olympic marathoner Meb Keflezighi donned a Nike-invented pre-race cooling vest to lower his core temperature. 

Fermentation tanks are also equipped with cooling jackets on their surface. These so-called refrigerator jackets are terrific since we winemakers can dial in our exact temperature choices during the winemaking process. 

At my winery, both the smaller 2-3 ton tanks and the larger 5-6 ton tanks are covered with dimpled glycol jackets that run colling glycol through their pipes and act heat exchange devices.

My Favorite Part?

My favorite part of the winemaking process is fermentation chemistry. Of course, fermentation science is only the beginning of the journey from grape to glass. 

But since it’s my happy place, we’ll delve deeper into the details of fermentation and the winemaking process. Here’s the spoiler alert - maybe you’re already wondering how this ends. 

Well, the yeast stop automatically. The yeast binge and chomp their way through the RHOBH’s only-in-a-dream, 100% carb dinner, until all of the sugar is consumed. 

When the sugar is gone, we pronounce "the wine is dry."  To winemakers, “dry” means no residual, natural sugar is present in the finished product. 

Without an energy source, the yeast die, and their carcasses drop to the bottom of the bin. Rather than "road kill," we call them "lees" and preach they add complexity and flavor to the wine (which they do). 

Vegetarians, fear not; the dead critters are filtered out by the time your pristine and crystal-clear wine is ready for bottling.

And there you have it – an overview of the winemaking process.

The winemaking process involves an infinite number of details and choices, which vary with winemaking style and varietal. 

My good friend, who also happens to be a famous winemaker, like to joke that “pinot noir wants to be wine.  Zinfandel is more like f-you.” And it’s true. 

During the winemaking process, pinot noir can sprint from start to finish in a whirlwind 5-6 days. But in my early days as a winemaker, this undermined my confidence and my textbook knowledge base. Let me tell you, this is not the way the winemaking process happened in my University of California at Davis syllabus. 

In fact, I'd anticipated a more sluggish endeavor. Always neurotic, I even fretted whether fermentation would begin at all. Well, I was wrong. The first time I watched the brix (i.e. sugar) drop from 24 to 10 to 4 in three days, I was flummoxed. 

Now I understand how the winemaking process is dependent on the many factors influencing fermentation kinetics. Let’s discuss this aspect of the winemaking process.

Getting Into FERMENTATION Kinetics

Disclaimer: fermentation kinetics is a beefy, complex topic, usually resplendent with logarithmic scales and fancy, math-heavy graphs.   

Instead of that, we'll just keep it straightforward - my apologies to UCD-decorated enologists everywhere. 

Starting with the most basic premise of the winemaking process, we know that the yeast consume grape juice sugar to make energy, grow, and multiply. The pinot noir in your glass is simply a pleasant and tasty by-product of natural yeast metabolism. 

And wine science has shown that this happens predictably. In the presence of sugar, first, the yeast multiply a lot (a log phase of 2-5 days), then they just hang out and maintain a stable population (a stationary phase- about 8 days long), then sated and post-asexual-coital, they slowly die (a death phase). 

Since they don't all die simultaneously, the death phase may be prolonged (sometimes weeks), as some yeast circle the drain and others scrape by, converting the last bits of sugar into wine. Fermentation is directly related to yeast growth, meaning fermentation is most vigorous when yeast multiply. 

So, during the winemaking process, anything that promotes yeast reproduction will accelerate fermentation. Since yeast are asexual, porn is of no use here. Using an armamentarium of yeast-specific titillations, we aim to achieve a vigorous, complete fermentation and a "dry wine," which is a wine with no residual sugar. 

About “dry” wines: this is not our taste preference but necessity. The fancy term is “mircobial stability.” If sugar remains during aging and into bottling, covert, escapee yeast or bacteria can surreptitiously sneak into our wine, happily romping about and feasting on any residual sugar. This can cause spoilage. This is bad.

Especially after bottling, rogue microbes trapped in a bottle can cause a cloudy and perceptibly fizzy beverage - OK for cola, not for pinot.  Even worse, non-yeast spoilage critters might devour any leftover sugar, resulting in spoiled and undrinkable wine.  Since winemakers understand what can accelerate or decelerate the winemaking process, let’s explore those factors here.

Factors known to accelerate fermentation- either in the winery or via controlled laboratory experiments

1. The initial sugar concentration of the pre-fermentation juice ("must concentration")

2. Nitrogen concentration

3. Growth factors like biotin and thiamine

4. Fatty sterols

5. Oxygen/aeration

6. Yeast hulls

7. Elevated fermentation temperatures

So down the checklist we go. Start with sugar. Let’s consider a pinot noir that arrives at the winery with a sugar level that measures 24 brix, which is just about perfect. 

If the initial sugar is too low, yeast growth may be limited by inadequate nutrients. Conversely, and counterintuitive but true, if sugar is too high, fermentation actually slows down in the end, and may even cease altogether (“stuck fermentation”). 

This is a great topic for another post about the winemaking process. But we won’t be discussing stuck fermentations here. 

OK - so next is nitrogen. Like bodybuilders, yeast require ample nitrogen, the building block of protein, in order to grow. The nitrogen in the grapes is measured in the laboratory as "YAN," the yeast available nitrogen. 

The value is highly variable across grape varietal and vineyard. Often low nitrogen is more pervasive in white wine making, when skins and seeds are absent from the fermentation potion. If nitrogen is deemed low, it can be added to the fermenting brew. 

This is a normal part of the winemaking process. Luckily all my pinot noir lots tend to have high nitrogen content, so I rarely have to add nutrients. But I do add nitrogen when needed, since I want a healthy fermentation and painless winemaking process. Next up is the “growth factor” category, akin to Centrum Silver- A to Z. 

Yeast need vitamins and minerals too, and in lab experiments the addition of thiamine and biotin accelerates fermentation kinetics. But again, our pinot skin and seeds provided enough of both. This is not something I routinely measure during the winemaking process. Fermenting on skins provides ample nutrition.

Here’s Where We Get Tricky in Winemaking

Obviously, yeast "eat" grape sugar, but clearly yeast lack the hands and mouths necessary to gorge themselves at will. So instead, the sugar "diffuses" across its yeast skin ("cell membrane"), like a queue of Nordstrom shoppers floating through the revolving doors at the Anniversary Sale, back when people shopped at stores and Nordstrom anchored the San Francisco mall complex.

Extrapolating back to our yeast… when the doors lock, sugar can't diffuse inside, and fermentation stops.  As it turns out, their yeast membrane “doors” are comprised of fatty stuff called "sterols," and the amount and type of sterol in door either helps or hinders sugar from getting inside. 

Some specific fatty things (like "C18") accelerate fermentation by opening the door even wider; others (short chain fatty acids like "C6, C8, C10") preclude membrane permeability and slam the door shut. 

Yeast hulls, essentially dried, dead yeast carcasses, mop up the bad sterols, essentially restoring the open-door policy and re-establishing permeability. And remember when I told you that in the presence of oxygen yeast respire instead of ferment sugar? 

Well, I lied a little bit…

Since yeast actually need oxygen to make sterols. And so now you know that limited oxygen exposure, particularly during the active growth phase, accelerates fermentation considerably. 

During the winemaking process, I ferment my red wines in open topped tanks. While this seems like limitless oxygen exposure, in reality, the carbon dioxide by-product of vigorous fermentation condenses into a heavy, invisible cloud over the tank, which limits ambient oxygen exposure. 

Our aeration is provided by cap management, either in the form of 2-3 times daily punch downs and/or pump overs. 

If you want to be extra, extra during the winemaking process, you can amp the oxygen intake with delestage. Check out the IG Reel to learn more about this cool technique. 

Let’s Talk Temperature

Finally, let’s explore temperature, the last big factor to influence fermentation kinetics during the winemaking process. 

As you might guess, temperature requirements are fluid - too hot and yeast die but not hot enough, and they can't boogie at all.  It’s also style dependent. A darkly hued and richly extracted wine style might favor a hotter fermentation to optimize color extraction. 

And elevated temperature, up to a limit, accelerates fermentation. Yeast temperature tolerance dictates the upper reaches of fermentation temperature. Heat accelerates fermentation and all enzymes, so hotter ferments move faster. 

And remember too, effects of different manipulations on the winemaking process are additive. So, the addition of nitrogen or aeration will speed fermentation but not without an accompanying increase in temperature, as that exothermic process accelerates and blows off more heat. 

In fact, the layered effects of ample nitrogen, sugar and elevated temperatures all contribute to a vigorous and rapid fermentation. In general, the winemaking process is fairly simple. 

Remember, pinot noir wants to become wine. Once yeast get started eating the grape sugars, it’s like dropping a rowdy kid at college for the first time. With ample nitrogen and sugar and warmth, the yeast just go nuts. 

And if the yeast are happy, then we're happy, too.

Want to talk winemaking in more detail?

Swing by Bruliam Wines in Sonoma County and book a tasting. We can talk about yeast, fermentation, white wine vs. red wine, and more…