How Does Wine Fermentation Work?
Fermentation is at the heart of wine production.
It is during this process that grape juice is transformed into a complex, aromatic and alcoholic drink.
Although it may seem rather technical, understanding fermentation helps any wine lover to grasp:
- why some wines are ‘stronger’ than others
- why certain aromas develop
- why there are differences between red, white and sparkling wines
What is wine fermentation?
Fermentation is a natural process in which yeasts convert the sugar in the grapes into alcohol. During this process, carbon dioxide and heat are also produced.
Without fermentation:
- there would be no alcohol;
- wine would simply be sweet grape must.
The role of yeasts
Yeasts are microorganisms that are essential for wine production. They feed on the natural sugar present in the grapes and produce alcohol. There are two main types:
Natural yeasts – found naturally on the skins of grapes and in the cellar environment. They are widely used in artisanal and natural wines.
Selected yeasts – added by producers to control aromas, ensure stability and create specific styles.
How fermentation works, step by step:
1. The influence of the harvest
It all begins with the grape harvest. Ripeness influences the wine’s sugar levels, acidity and aromas.
The more sugar the grape contains, the higher the wine’s alcohol content may be.
2. Crushing and Grape Must
After harvesting, the grapes are crushed and the juice is released (grape must).
3. Start of Fermentation
The yeasts come into contact with the must and begin to consume the sugars. At this stage, CO₂ bubbles appear, the temperature rises and the aromas begin to develop.
4. Conversion of Sugar into Alcohol
As the sugar disappears, the alcohol content increases and the wine becomes less sweet. When almost all the sugar has been consumed, a dry wine is produced.
5. End of Fermentation
Fermentation ends when the sugar is used up or the producer decides to stop the process. In sweet wines, some of the sugar remains in the wine.
Fermentation of Red Wines vs White Wines
Fermentation of Red Wines
In red wines, fermentation takes place together with the grape skins, seeds and sometimes the stems. This gives the wine its colour, tannins and structure.
Fermentation of White Wines
In white wines, the skins are usually removed before fermentation, resulting in a fresher profile,
fewer tannins and greater lightness.
Fermentation in Stainless Steel or Barrels?
Stainless steel vats produce fresh, fruity wines.
Wooden barrels add complexity, texture and notes of vanilla and spices.
What is Malolactic Fermentation?
After alcoholic fermentation, some wines undergo a second transformation. Malic acid is converted into lactic acid. This fermentation is not carried out by yeasts, but by different microorganisms: bacteria. The activity of the bacteria reduces the wine’s acidity and can impart buttery flavours. In red wine production, malolactic conversion almost always takes place, but the buttery flavours are not very noticeable. In white wine production, the winemaker can decide what to do.
Lees
In white wine production, there is another choice the winemaker must make. After alcoholic fermentation, the yeasts die and settle to the bottom of the fermentation vessel, forming a layer of dead cells known as ‘lees’. The winemaker may choose to keep the wine in contact with the lees, stirring and mixing these lees (battonage) with the wine during ageing. This can give the wine more body and add extra flavours (biscuit, bread).
Fermentation is a crucial process in the creation of a wine, as it is during this stage that sugar is converted into alcohol, aromas begin to develop, and the wine’s style takes on its own identity. The next time you open a bottle, you’ll realise that every wine is the result of a fascinating blend of chemistry and craftsmanship.
