Fermentation in winemaking is a core step in the entire bio-process chain, playing a decisive
role in the quality, flavor and chemical properties of the finished wine. This is a biological
transformation process that takes place thanks to the activity of microorganisms - mainly
yeast species, most notably Saccharomyces cerevisiae. These yeast cells will use the natural
sugar source in grapes (mainly glucose and fructose) as a substrate, and through anaerobic
fermentation, they convert sugar into ethanol (ethyl alcohol), carbon dioxide (CO₂) and a
series of other by-products such as glycerol, organic acids, esters, aldehydes, aromatic
compounds, polyphenols, ... contributing to the characteristic flavor and rich sensory of
wine.
The process begins after the grapes are crushed and pressed to collect the juice. For red
wines, the skins and flesh are retained during fermentation to extract tannins and
anthocyanins (red pigments), while for white wines, the juice is separated from the skins
before fermentation. The juice is then pH adjusted and SO₂ may be added to kill saprophytic
bacteria or other undesirable microorganisms. Temperature is an important factor affecting
the speed and quality of fermentation. Typically, fermentation takes place at around 20–
30°C: lower temperatures (10–18°C) are often used for white wines to retain fresh aromas,
while red wines are usually fermented at higher temperatures (25–30°C) to optimize the
extraction of phenolic compounds from the grape skins.
In the sugar-rich juice environment, yeasts begin to grow and multiply rapidly. In the early
stages, they use sugars for energy via aerobic respiration if oxygen is present. However, in
anaerobic conditions (little or no oxygen), yeasts switch to ethanol fermentation: glucose
and fructose are broken down via the Embden-Meyerhof-Parnas (EMP) pathway, forming
pyruvate, which is then decarboxylated to acetaldehyde and further reduced to ethanol. At
the same time, CO₂ is released as gas, creating the characteristic bubbling phenomenon in
the fermentation tank. The whole process lasts about 7–14 days for red wines and can take
longer (up to several weeks) for white or high-alcohol wines.
During fermentation, a number of factors can be controlled to shape the flavor and quality
of the wine. For example, using pure yeast strains will help to homogenize quality and
enhance the production of desirable compounds such as aromatic esters (ethyl acetate,
isoamyl acetate, etc.), while using natural microorganisms (wild yeasts available in grape
skins or the environment) can bring about a characteristic “terroir” flavor but also carries the
risk of bacterial contamination. In addition, adding enzymes such as pectinase to the juice
can help break down plant cell membranes, supporting extraction and clarification.
After most of the sugar is converted to ethanol (about 90–95% of sugar), yeast activity will
gradually decrease due to the high concentration of ethanol causing inhibition. Once
fermentation is complete, the lees are separated from the young wine and can be
transferred to further processing steps such as malolactic fermentation (with the
participation of lactic acid bacteria – Oenococcus oeni) to convert malic acid to lactic acid to
soften the wine, or to aging/aging to develop more complex flavors.
The entire fermentation process is a prime example of the application of microbiology in the
food and beverage industry. The process is not simply about ethanol production, but also a
synthesis of biology, chemistry and art, where the choice of yeast, temperature control,
time, and fermentation conditions all have a profound impact on the character and sensory
value of the final product. It is a complex, multidimensional bio-process, and is the focus of
research in fermentation science, food technology, and applied biology.
