What is fermentation of wine?
Fermentation of wine is a natural biological process in which sugars in the must, or unfinished fermenting grape juice, undergo a transformation, from glucose and fructose to alcohol and carbon dioxide. The driving force behind this intriguing biochemical pathway are wine yeast cells, which are present on the skins of grapes (provided pesticides have not been used). These yeast cells on the grapes play a crucial role: they eat the grape sugars and create carbon dioxide, alcohol, and subtle but important aromas and flavors for the wine.
What makes natural wine production special is that natural winemakers use only yeasts on the grape itself, which spontaneously start the fermentation of the grape juice. Adding unnatural yeasts, also known as cultured yeasts, is out of the question for natural wine producers, as they firmly believe in the idea that fermentation works best when you let nature take its course. The fact that this can make fermentation slow to start is actually a good thing, according to natural winemakers, because different natural yeasts each become active at their own temperature and produce unique aromas and flavors. In this way, each type of yeast can contribute optimally to the complexity of a wine.
How long does fermentation of wine take?
So the fermentation of natural wine is a process that is not rushed. Below 10 degrees the yeasts remain practically inactive, but at about 15, 16 degrees things start to get interesting. Fermentation produces heat, and naturally the temperature climbs to about 32 degrees. It is important, however, that the temperature does not rise too far, as this can stop the fermentation of wine. Therefore, the temperature of the must is closely monitored and, if necessary, cooled using cooling elements.
The sugar density and temperature of the must are carefully measured daily. For the first 3 or 4 days little seems to happen, but then slowly the sugar density begins to drop and the temperature to rise. This is the sign that the first yeasts are doing their work, and from that point on things move quickly. How long wine must ferment depends on several factors, but generally the fermentation process takes about 6 to 14 days. During this time, a hat of skins and pips forms on top of the must, which is essential to the color and flavor of the wine.
In contemporary winemaking, temperature-controlled fermentation tanks have been real game-changers. This has given winemakers control over how long and how intense the fermentation process is, thanks to tightly controlled conditions. For the fermentation tank, size and material are crucial. This is because the relationship between the surface area of the inner wall and the volume of the fermenting wine, for example, affects tannin development, color intensity and extraction.
Fermentation tanks come in all sizes, from modest vessels to gigantic tanks with thousands of gallons of capacity. Winemakers also vary enormously in the use of different materials, such as oak, concrete, stone (think slate), (fiber) glass, clay (amphora), synthetics or stainless steel.