How Does Wine Fermentation Work?
Hi there - I’m Kerith Overstreet, chief winemaker over here at Bruliam Wines. Wine fermentation is the conversion of grape sugars into alcohol by yeast. It’s a fun and complex topic that deserves a deep dive, and I’m here to walk you through the details.
Wine fermentation is the heart of winemaking and informs style, texture, aromatics, and flavor. It is data driven and science intensive. Happily, though, yeast consume grape sugars in a predictable way, which offers winemakers some stability and foresight. Still, innumerable variables make every wine fermentation unique, from fermentation tank to fermentation temperature to yeast.
This chart displays a lot of information about wine fermentation and fermentation temperature. Let’s slowly tease it apart until you’re all experts in.
The Basics of Wine Fermentation
The x axis has the date, which is “time,” while the y axis conveniently tracks both degrees brix and temperature. Let’s start at time zero, where the red temperature line and blue sugar (“brix”) line first appear.
This flat-line plateau represents the 5-day cold soak. The fermentation tank is set to a cold fermentation temperature to allow winemakers to extract color before fermentation begins. This is because the color molecules (anthocyanins) extract better in an aqueous medium than an alcoholic one. And since both pinot noir and zinfandel are thin-skinned varietals, I work hard to extract great color so that your wine looks stunning in the glass.
Why Do You Need a Cold Soak?
A cold soak also prevents wine fermentation from beginning on its own (known in wine parlance as a “spontaneous native fermentation”) or before we winemakers want fermentation to begin. The cold basically paralyzes the yeast and prevents them from consuming sugar. Cold temperatures also paralyze any spoilage bacteria that piggy-backed in on our fruit from the vineyard.
Once fermentation begins in earnest, our yeast can out-compete those bad bugs. But until we are ready to commence fermentation, we hold the fermentation temperature cold in our fermentation tanks.
OK, back to the start of the graph and the cold soak. To reiterate, we like a cold soak to help extract more color from the berries, especially since pinot is a thin skinned grape. Cold soak is also reported to increase fruity aromas and flavors, enhance mouthfeel, and amp up the aromatic intensity. Most studies show color extraction peaks between 3 and 5 days, so a 5 day cold soak is just fine for us. The cold soak also allows grapes to… well… soak. Any raisins will plump up and grape skins will begin to break down and release their sugars into the juice.
Thus, a prolonged cold soak allows us winemakers to fine tune our brix reading so we precisely know how sugary our juice really is. This is important since yeast work harder when there is more sugar. When they give up and die, fermentation stops, and this is not good. Plus, more sugar makes for higher alcohol, which itself is toxic to yeast.
What About Fermentation Temperature?
You can’t just dump the yeast into a cold bin of juice, like drunken co-eds skinny dipping in a near-freezing lake. The temperature shock is just too great to overcome; they’ll seize up and die.
So, we wait and wait until the must warms up a bit. Fermentation temperature is a very important factor. As previously mentioned, yeast don’t like the fermentation tank to be too cold. Before I add any yeast (or before I am ready to begin fermentation), I warm up the tank by increasing the fermentation temperature.
Our tanks can hook up to either glycol to cool the fermentation tank or hot water to warm the fermentation tank , so this is pretty easy. You’ll notice the fermentation temperature increase on 9/19. Even when the fermentation tank is closer to ambient temperature, we still need to acclimate the yeast to their new home.
I’ll mix the yeast with water and grape juice, slowly adding more juice until the yeast slurry measures within ten degrees of the fermentation temperature in the fermentation tank. Then, to better acclimate my single-celled buddies to their new environment, I set the mood just right.
I light some candles, play some sexy Barry White music, and whisper one-lined come-ons. Just kidding, but you want those yeast to get ready for wild, fungal nookie, so they can multiply their way right up to a massive 108 biomass to support a healthy fermentation to dryness.
The Four Phases of Yeast
So, let’s talk about sex.
During wine fermentation, the yeast pass through 4 distinct phases. They are named lag, log, stationary and death. Lag is the yeast adjusting to their new environment and starting foreplay. Although the yeast may begin dividing, you don’t really see the results quite yet.
Yeast multiply on a log scale, so there is a gap between when sex starts and when the fermentation really starts churning away. During log, yeast are actively dividing, and we can detect a steady increase in the cell number. Log phase, of course, is rapid yeast proliferation. Log phase is that crazy, precipitous drop in brix, from 20 down to 5, in just a few days. It is a vigorous time in the fermentation tank so fermentation generates a lot of heat and the fermentation temperature rises.
You can see this on the chart in the beginning of this post. During the stationary phase, there is no growth and no death. It’s a non-proliferative phase of non-dividing cells. Sugar consumption still happens here, although it’s less dramatic. The yeast are working hard to consume sugar and poop out carbon dioxide (for lack of a more delicate term), but cell division is undetectable.
The fermentation tank starts to cool down as fermentation temperature drops. Death is pretty self-explanatory. As fermentation progresses, sugar is depleted (no more food), alcohol concentration rises (which is toxic to yeast and screws up their cell membrane), and the temperature rises (too hot can be lethal too). The fermentation curve mirrors these phases, as sugar goes down and fermentation temperature goes up (then down)!
If one were to plot yeast population against time, lag is a flat line, log is a steep decline as the population grows (and brix starts to drop), followed by a stationary phase (with continued fermentation) with a drop back to zero in cell death. And in each phase, yeast need different “foods” to stay alive. During log, the population is an exploding yeast orgy. Here the yeast in the fermentation tank require mostly sugar and nitrogen for proteins.
During the stationary phase of wine fermentation, yeast require specialized fatty acids and sterols to armor their cell membranes to withstand the rising tide of alcohol. Now check out the chart. Winemakers can match cap management to the phases of wine fermentation, so we can maximize extraction, involve oxygen, and help dissipate heat. Knowing the phases of wine fermentation is exceedingly helpful. As you can imagine, fermentation hiccups can happen anywhere along the curve, and the keenest winemakers know how to spot trouble before fermentation arrests.
The Maximum Fermentation Rate
Let’s examine the most exciting part of wine fermentation in greater detail. We’re going to focus on the maximum fermentation rate - those magical 48 hours where the sugar (°brix) drops from 20-ish to 5-ish. At that moment, the slope of the curve decreases dramatically, and the °brix falls more slowly over the next 24-48 hours.
This point where wine fermentation slows back down again is called the transition point. It’s important to know about it since it has predictive value. For instance, if the transition point hits when the must still measures more than 5° brix, your vat may be at risk for sluggish fermentation and possible arrest.
And check out the temperature, too. While the sugar plummets during the log phase, the temperature of the juice skyrockets. Like Paris Hilton says, “That’s hot.” (Geez, I’m way dating myself, here).
The maximum rate of wine fermentation corresponds to the greatest yeast biomass. In other words, bedroom hanky panky in the early part of fermentation drives the faster stuff later.
Early on, the yeast must reproduce robustly enough to attain the maximum number of critters to complete wine fermentation, which happens to be 108 cells/ml. Imagine all of these single celled organisms consuming sugar and releasing alcohol. It is a big job (that happens to be a biochemically exothermic process) so the yeast release heat. That is why you see the temperature rise so dramatically.
And the heat is good. It increases the yeasts’ metabolic rate and boosts the rate of fermentation overall (remember from high school chemistry that enzymes go faster with heat?). Heat also discourages spoilage bugs by basically frying them to death. Color extraction is maximized, as well.
The only downside is that a really hot fermentation may stick in the later phases, and of course, the yeast could boil themselves to death. It’s a little like Bikram yoga. Wine fermentation starts off steamy, which feels awesome, but after 45 minutes crammed into a room with 37 other sweaty people, sweat begets sweat, and the collective, accumulated heat becomes an oppressive fermentation temperature.
At yoga, you need to take a bathroom break. In the fermentation tank, the yeast enjoy a cooling punch down, two or three a day to be exact. Then after that transition point, fermentation slows down and wraps up. Concomitantly, the alcohol rises, sugar dwindles to zero, and the yeast die. The process is self-limiting and runs its course. But we’ll delve deeper still to grasp the details of wine fermentation perfectly.
The Importance of Yeast in Wine Fermentation
Yeast are surrounded by a plasma membrane. It keeps their insides from mixing with the outside, like our skin but more fluid. In some ways, it’s like one of those red bead-string doorways from a 1970’s Austin Powers love lair.
It can sway, swish, and alter its configuration in different circumstances. On the other hand, other membrane parts are more rigid, like a baby’s shape sorter toy. The plastic cylinder can only slide through the circle hole but not through the triangle or square. These different shaped holes are analogous to the transport proteins spanning the yeast plasma membrane. They are doorways to the yeast innards, and only certain stuff can get through.
During wine fermentation, sugar shoots through an exclusively-shaped tunnel and is dumped inside the yeast for fuel. Only sugar can ride that passageway. Luckily, there is more sugar in the fermenting grape juice than inside the yeast, so sugar shimmies down the natural concentration gradient (from high to low). This is called diffusion, and the yeast doesn’t have to expend any energy at all to allow sugar inside.
Just imagine if you could ride that doughnut conveyor belt in the Krispy Kreme factory, cruising under the cascading shower of sugary glaze, filling your mouth with delectable frosting without expending a single calorie. It must be great to be a yeast…except when acid piggyback rides a sugar molecule and ends up inside the yeast (indigestion anyone?). Remember our grape juice is acidic, way more acidic than the neutral yeast, meaning the acid (in the form of protons, H+) is also working a favorable gradient.
But, if acid slides through the door, the yeast must expend energy to push the proton back outside. This isn’t a big deal if the yeast is floating in a sugar bath, like grape juice, where ready meals abound. But late in wine fermentation, when sugar is sparse, things might get dicey. As the alcohol concentration rises, it messes with those rigidly shaped tunnels and screws them up. Now acid is flying into the yeast faster than they can pump it back out. This acidifies their insides, which they don’t like very much. Often they just die.
Likewise, high alcohol is toxic to yeast, and only the staunchest fighters can survive upwards of 18% alcohol. (Actually, that was a lab study. I have never actually seen 18% alcohol at a winery!). Needless to say, wine fermentation can be dangerous, and yeast may die in the presence of the rising tide of alcohol. But they have ways to combat this, by fortifying their cell membranes, like wearing armor.
Will The Yeast Survive?
Here let’s wrap up our discussion of wine fermentation. In the last paragraph, we’d abandoned our heroic yeast to fight a lonely battle against the rising tide of ethanol toxicity.
But as I’d promised, those plucky yeast rely on some nifty tricks to fortify their body armor and better withstand the alcohol soak. As you know, high alcohol concentrations mess with yeasts’ cell membranes by forcing a tsunami of acid inside their insides faster than they can push it out.
Alcohol also inhibits proteins like enzymes or sugar transporters and screws up their membrane fluidity (that is the swishy Austin Powers beaded door). During wine fermentation, the concept of “membrane fluidity” is paramount. The plasma membrane is like a double layer wall surrounding the yeast, with fixed, unbending parts and fluid, flexible parts. The wall includes certain windows and docking ports that uniquely fit specific things, like sugar or nitrogen.
When a fructose sugar molecule floats through the grape juice towards a hungry yeast, it looks for an open parking spot so it can “park” on the yeast’s surface. Like compact spots for mini-Coopers versus giant spots for Hummers, different hexose transporters (sugar corridors) recognize different substrates (like glucose versus fructose).
This is a critical concept during wine fermentation. A glucose molecule fits into a specially configured parking place like a puzzle piece in a puzzle. After the sugar docks and locks, the transporter becomes a Transformer©, changing its orientation and configuration. It will about-face and flip from facing the grape juice side to facing the inside of the cell, taking the sugar with it. The sugar is dumped inside the yeast and consumed for energy. The Transformer© transporter then undergoes a second shape shift to face back outwards again, ready to usher another sugar through its membrane wall. Obviously the Transformer© transporter is pretty rigid, since its shape is fixed to receive only sugar. However, it sort of floats around in a lipid (i.e. fatty) bilayer.
If the plasma membrane is too stiff, the transporters are straight-jacketed. The proteins can’t shape shift, and the sugar is stuck outside. If the membrane is too fluid, then the transporter gets all wiggly-wobbly, loses its shape and can’t do its job either. High alcohol concentrations disturb this perfect balance. But if the yeast can adapt its membrane composition before the fermentation tank is too ethanol toxic, it will live long enough to take the wine to dryness (i.e. consume all of the sugar in the juice).
During wine fermentation, yeast tolerate a higher alcohol environment by altering their plasma membranes in a number of technical ways. These include amping up the levels of sterols, swapping out saturated fatty acids for unsaturated ones (which means more double bonds), and increasing the relative protein content.
What you need to understand is that:
These membrane adjustments require oxygen and nitrogen (the building blocks of protein) and
The changes must be securely in place before the alcohol level gets too noxious. Adding more nutrients or nitrogen once the yeast are already petering out is like fastening your seat belt after you crashed; you’ve already missed the boat.
In other words, unless the yeast have ready access to a protein source, sterols, and unsaturated fatty acids at the beginning of fermentation, they won’t have the necessary tools they need to bulwark their body armor.
When a brisk fermentation grows sluggish and the rate of sugar consumption slows down, it’s usually an alcohol tolerance problem. You’ve got to understand this fundamental concept of wine fermentation. The more sugary the grapes at harvest, the higher the potential alcohol of the finished wine.
Yeast floating in a high sugar must are going to need a lot of protection and TLC. They’ve got to buckle down and secure their cell membrane or face death by acid-fried innards.
Have any additional questions about wine fermentation? Interested in trying Bruliam Wines for yourself? Click here.