Welcome to the fifth part of Winemaking for beginners: From grape to glass. Until now we have followed the grapes from the vineyard all the way to the press. Now the real magic happens: must becomes wine.
Fermentation is the heart of all winemaking. This is where sweet grape juice is transformed into something entirely new, and where the winemaker truly begins to leave their mark. In this part we look at what actually goes on, what role the yeast plays, and what can go wrong along the way. No chemistry lecture, just the broad strokes.
What you will learn
- What alcoholic fermentation is, explained quite simply
- Why the yeast is the winemaker's most important little helper
- How temperature helps shape the wine in the glass
- What it means when a fermentation stalls, and why that is a problem
What happens during fermentation?
Imagine you have a bucket of sweet grape juice. Let it sit under the right conditions, and something begins to happen on its own: the juice bubbles, fizzes gently and gradually becomes less sweet. That is fermentation in full swing.
Put quite simply, fermentation is the process in which the grape's natural sugar is converted into alcohol. At the same time a gas is produced (that is what makes the must bubble and fizz), and a little heat develops. Think of it as a kitchen where ingredients are transformed into a dish. The sugar is the ingredient, and the result is wine.
The beautiful thing is that the winemaker does not create the flavour from scratch. The quality of the grapes at harvest sets the ceiling for how good the wine can become. A skilled winemaker preserves most of the inherent quality, but can never add more than the grape itself brought along. Fermentation is therefore just as much about looking after a potential as about creating something new.
The role of the yeast: from sugar to alcohol
The one that does all the work is the yeast. It is a microscopic fungus, so small that we cannot see the individual cells at all. But they are there in enormous quantities, and they are hungry for sugar.
Yeast occurs naturally in the vineyard. It sits, among other places, on the skins of the grapes in the fine, waxy layer you can see as a matte veil on a ripe grape. Insects and animals in the vineyard help move it around, so it is ready precisely when the grapes ripen. Some winemakers let this natural yeast start the fermentation entirely on its own, while others add selected yeast to get a more predictable process. Both routes are perfectly legitimate, and the choice depends on style and temperament.
When the yeast gets hold of the sugar, it works incredibly fast. A single little yeast cell can handle thousands of sugar molecules per second when conditions are good. Multiply that by the billions of cells in a fermentation tank, and you understand why a bucket of sweet must can transform itself in just a few days.
The yeast keeps going until one of two things happens: either the sugar runs out, or there is so much alcohol that the yeast can take no more. For most types of yeast this typically happens at around 15 to 16 percent alcohol. That is why ordinary wine is rarely stronger than that.
Temperature and controlling the fermentation
If the yeast is the cook, then temperature is the oven's thermostat. It is one of the winemaker's most important tools, because it helps determine what you later experience in the glass.
With cooler fermentation you preserve the fresh, fruity and floral aromas. That is one of the reasons why many light and aromatic wines are fermented at lower temperatures. With warmer fermentation, on the other hand, you draw out more colour and more power, which often suits fuller-bodied red wines.
But the heat must be managed with care. If it gets too cold, the yeast practically falls asleep and stops working. If it gets too warm, the yeast feels poorly and may stop fermenting altogether. A good winemaker therefore keeps an eye on the temperature all the way through, just as you watch a pot that must not boil over.
Tank or barrel
Fermentation and the subsequent storage can take place in different vessels. Steel tanks typically give fresh wines with a clean, distinct grape character. Oak barrels, on the other hand, often add soft nuances that many associate with vanilla. This is yet another knob the winemaker can turn, but more on that in the next parts of the series.
When fermentation stalls
Now and then fermentation stops before all the sugar has been converted. This is called a stalled fermentation, and it is something winemakers would rather avoid.
Why is it a problem? Because there is then still sugar left that the yeast did not manage to convert. This makes the wine unstable, because other microorganisms may set to work on the remaining sugar at an unfortunate moment. The result can be a wine that does not taste as it should.
There are several reasons it can happen. One of the most common is that the yeast lacks nourishment along the way, a bit like a runner who runs out of energy before the finish line. Too low or too high a temperature can also make the yeast give up. Fortunately, this is something an experienced winemaker can both prevent and correct.
Note, by the way, that the winemaker can also deliberately choose to stop the fermentation before all the sugar is gone, if they want a wine with a little natural sweetness left. That is not a fault, but a conscious choice.
In short
- Fermentation is the process in which the grape's sugar becomes alcohol, while the must bubbles and develops a little heat.
- Yeast is a microscopic fungus that occurs naturally on the grapes and does all the work.
- The yeast stops when the sugar runs out, or when the alcohol becomes too high, typically around 15 to 16 percent.
- Temperature is a key tool: cooler preserves freshness and fruit, warmer gives power and colour.
- If the fermentation stops unintentionally, the wine becomes unstable, but this can be prevented.
Frequently asked questions
Why does the must bubble during fermentation?
When the yeast converts the grape's sugar into alcohol, a gas is released at the same time. That is what you see as bubbles and fizzing in the must. It is a clear sign that fermentation is in full swing.
Can wine ferment entirely on its own?
Yes. The yeast sits naturally on the grape skins, so a spontaneous fermentation can start without help. Many winemakers, however, prefer to control the process to achieve a more predictable result.
Ready for the next step?
Now that you understand how must becomes wine, you are ready to see how the same basic principles are used to make different types of wine. In the next part, Red wine, white wine & rosé: How they are made, we dive into what actually distinguishes the three colours.
Take your time to let it all settle, and feel free to drop by the selection when you feel like tasting the theory in practice. Remember that the best combination is always the wine you yourself like, together with the food you love.