What is alcoholic fermentation?
There is a precise moment when must stops being simply grape juice and begins to become wine. That moment is called alcoholic fermentation — and it is one of the oldest and most fascinating processes known to man.
This transformation is driven by yeasts: single-celled fungal microorganisms, invisible to the naked eye, naturally present in the air, on the soil, on leaves, and on grape skins. They are the true protagonists of winemaking — even before the winemaker, even before the cellar.
Yeasts: indigenous or selected?
The yeasts that bring fermentation to life can be of two types, and the choice between them is already a statement of style.
Indigenous yeasts — also called autochthonous or wild — are those naturally present in the vineyard and on the grape skins. They belong to different families: on unripe grapes, genera such as Torulopsis, Cryptococcus, Rhodotorula, and Candida predominate; with maturation, apiculate yeasts like Hanseniaspora and Metschnikowia arrive. The main agent of alcoholic fermentation, Saccharomyces cerevisiae, is only present in small quantities on fresh grapes — but it is this yeast that, in contact with the sugars in the must, completes the process.
Selected yeasts are strains of Saccharomyces cerevisiae isolated and reproduced in the laboratory to ensure more predictable and controlled fermentations. They offer stability, but sacrifice some of the complexity that only a living terroir can express.
At Colline del Vento, we believe that the vineyard already has much to say. That's why we work in close dialogue with nature — and with the yeasts it offers us.
How fermentation works: from respiration to glycolysis
It all begins with the grape sugars — glucose and fructose — and the yeasts' ability to transform them.
In the initial phase, with oxygen still available, yeasts perform aerobic respiration: they convert sugars into CO₂ and water, accumulating energy to grow and multiply. But the oxygen in the must is quickly depleted, and at that point, the yeasts switch to anaerobic fermentation proper, transforming sugars into ethyl alcohol and carbon dioxide.
This set of reactions — over thirty in sequence — is called glycolysis, and it occurs within the yeast cells themselves. The indicative distribution of products is:
- ~50% of sugars → ethyl alcohol
- ~45% → carbon dioxide
- ~3% → glycerol (which contributes to the wine's smoothness)
- ~2% → other substances: acetaldehyde, acetic acid, ethyl acetate, and polyvalent alcohols that define the wine's aromatic and gustatory character
Duration and temperature: the two balances not to break
Alcoholic fermentation generally lasts between 5 and 15 days, depending on the type of must and cellar conditions. Speed matters: a fermentation that is too fast disperses aromatic compounds; one that is too slow risks generating unwanted volatile acidity.
The most critical factor is temperature:
- Below 15°C: the process might not start
- Within the right range: everything happens naturally, slowly, and precisely
- Too high: yeasts die and fermentation stops
Controlling the temperature is not a technical detail — it is an act of respect towards the wine being created.
White fermentation: preserving aroma
In white winemaking, the skins are separated immediately after crushing. The primary objective is just one: to preserve aromatic complexity. For this reason, the fermentation temperature must not exceed 20°C.
However, there is also an older and more fascinating approach: fermentation in small wooden barriques, followed by aging sur lies — on the lees — for a shorter or longer period. In this case, batonnage comes into play: the periodic stirring of the lees which, by partially decomposing, release colloidal substances into the wine, increasing the sensation of body and roundness on the palate.
This is exactly what we do with our Zankitai — our Orange Wine refermented in the bottle, born from the sun and granite of Villasimius.
Red fermentation: extracting color and structure
In red winemaking, the skins remain to macerate in the must during fermentation: it is from them that the wine gets its color, tannins, and structure.
Temperature here is an active tool. Fermentation should not exceed 30°C:
- Below 25°C: difficult extraction, light and "insipid" wine
- Between 25° and 30°C: optimal extraction for structured wines
- Above 30°C: risk of herbaceous aromas and yeast death
Racking: the moment of separation
When the must's density drops below 1 — a sign that the sugars are almost depleted — fermentation is complete. It is time for racking: the separation of the wine from solid residues — exhausted yeasts, skins, tartrates — that settle at the bottom.
The right moment depends on the type of wine desired:
- Young red wines: early racking, after 4–5 days of maceration
- Quality red wines: at the end of alcoholic fermentation
- Structured and aging wines: a few days after the end of fermentation, to further extract structure from the skins
Racking occurs in contact with air: oxygenation eliminates unpleasant compounds such as hydrogen sulfide and reactivates residual yeasts to complete the slow fermentation of remaining sugars.
Fermentation is nature, not technique
What we have described is a chemical process. But those who experience it in the cellar know that it is something more: it is the grapes transforming, the territory revealing itself, the winemaker who — with patience and respect — guides without forcing.
Every fermentation is different. Every vintage carries a story that does not repeat itself.
At Colline del Vento, we know this every time we enter the cellar.
