Wine is a byproduct of the fermented juice of grapes. This is the process whereby the grape or fruit juice turns into alcohol. Understanding the process of fermentation will give you the basic concept of how grapes and yeast make wine. There are two parts of fermentation, primary and secondary. Here we focus on primary fermentation.
Primary Fermentation is centered on the addition of the yeast to the crushed grapes. Because of natural forces, the two grape sugars (glucose and fructose) are converted to alcohol by the action of yeast. Yeast converts sugar into ethanol and carbon dioxide. The carbon dioxide is vented out into the air, while the alcohol is retained in the mix. This conversion to alcohol process is influenced by yeast strain, quantity of yeast population, must temperature, and cellar conditions.
Experience has proven there are ways to expedite and improve the primary fermentation phase. Yes, a few very selective extra ingredients are added. Care is also taken to the correct proportions and event timing, as all of this affects the aroma and taste of wine.
Once fermentation starts, it is important for the winemaker to mix the must periodically. This adds air in, helping the yeast to continue to do its work. This also prevents bad bacteria or mold from colonizing on the cap that forms on top.
Temperature is an important factor here, and commercial wineries keep the temperature specifically controlled. However, this can be more difficult for the home winemaker. Nonetheless, the rule-of-thumb for white wine is the 55 to 65 degree range. Red grapes ferment best much warmer, 80 degrees Fahrenheit. If too warm, the yeast can become inactive and put the brakes on the fermentation process.
For the home wine brewer, you will see the must start to bubble within 8 to 20 hours. By 24 to 48 hours look for lots of fizzing and foam. In general, the overall time for this phase depends on the wine style.
Overtime the process will begin to slow down. Ultimately, the primary fermentation ends and the wine is siphoned out to carboy, relatively oxygen free, but with lots of alcohol!
Contributor: WineDefintions Staff Writer