The process of fermentation is the action that creates wine out of grape juice. Fermentation is when yeast interacts with the sugars in grape juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide. When making wine the temperature, speed of fermentation, and level of oxygen present is important. The primary stage of fermentation is very important because several wine faults can occur at this stage which lasts from 5-14 days. Secondary fermentation can last for another 5-10 days after the primary fermentation.
Fermentation can take place in stainless steel tanks, open wooden vats, inside wooden barrels, or inside the wine bottle itself. Most Riesling wines are fermented in stainless steel tanks while many red wines are fermented in wooden barrels or open wooden vats. Sparkling wines are made by fermenting the wine in the bottle itself.
When fermenting there are a few factors that wine-makers must take into consideration. The internal temperature of the “must” (grape juice that contains skins, seeds, and stems). The actual process of fermentation itself creates quite a bit of heat which can make the must get too hot. White wine is usually fermented between 64-68* F although some wine-makers may use a higher temperature to bring out the complexity of the wine. Red wine is typically fermented at temperatures of up to 85* F. Fermentation at a higher temperature can have negative effects on wine making by making the yeast inactive. High temperatures can also boil off some of the flavors in the wine. To bring out more fruity flavors some wine-makers like to ferment their red wines at lower temperatures similar to those of white wines.
Wine-makers need to be aware of the risk of chemical residue and spoilage that comes from fermentation. This can be corrected with the addition of sulfur dioxide. Too much sulfur dioxide can cause a wine fault though so the perfect balance is key. To make a wine with higher levels of residual sugar(like a dessert wine) you would need to stop the fermentation process early by either dropping the temperature of the must by adding a high level of alcohol to kill off the yeast. The total fermentation time can range anywhere from 1 week to 6 weeks depending on what kind of wine and characteristics you are looking for in your wine.