The Importance of the Secondary Fermentation

Simply put, fermentation is the process of yeast converting sugar into alcohol and CO2 (roughly half-and-half).  Although the ratio isn’t exactly 50/50 there’s typically, only about a 4% swing either way. This is caused by external factors such as the type of yeast used, nutrients present, and the availability of oxygen.
For example, during fermentation, 10 pounds of sugar in 5 gallons of juice will be converted into roughly 5 pounds of alcohol, or about 12% ABV.
Fermentation takes place in two stages: Primary Fermentation and Secondary Fermentation- also known as “Aerobic Fermentation” and “Anaerobic Fermentation”.
The Primary Fermentation stage usually only lasts four to seven days; however, about 70% of the fermentation process takes place during this stage. If the yeast works properly, your must will begin to foam as the sugar is converted and CO2 is released. The reason why this stage is referred to as “Aerobic Fermentation” is because the fermentation vessel should be opened to the air. The yeast feeds off of the oxygen and sugar and is multiplied 100 to 200 times during this process, giving off CO2. Alcohol is being produced as well, but the majority of the yeast’s energy is devoted toward reproducing itself.
The Secondary Fermentation stage usually takes anywhere from a week to a month, depending on how much sugar is left. As the last 30% of the sugar is converted, the fermentation activity will get noticeably slower each day. This is referred to “Anaerobic Fermentation” because the must’s exposure to air should be kept to a minimum. This is typically done by inserting a drilled stopper into the carboy opening with an airlock in it.
By reducing the air exposure during the secondary fermentation, the yeast will stop reproducing and devote all of its energy to converting the remaining sugar into alcohol. Once all of the sugar has been converted the yeast will die and settle to the bottom of the carboy. Because yeast is a silty substance, your wine will remain cloudy until the yeast has completed settled. You can then remove it by reracking your wine.