Primary Fermentation

• The most common yeast used in the fermentation of juice is Saccharomyces cerevisiae. The addition of this yeast usually takes place during the first phase of fermentation -- primary fermentation. Because the fermentation container is open to the air, the yeast grows rapidly, quickly converting the sugar in the juice to ethanol and carbon dioxide. This part of the fermentation process lasts between three and five days, accounting for approximately 70 percent of the total amount of alcohol produced, according to Winemaker’s Academy.

Secondary Fermentation

• During the next phase -- secondary fermentation -- the process begins to slow. This is because oxygen to the container is cut off, and the alcohol content of the juice is starting to outweigh the yeast population. Unable to survive, yeast cells begin to die and collect at the bottom of the container. Thirty percent of the alcohol is produced during this phase, which lasts approximately two weeks.

Fruit Yeast

• Although the addition of S. cerevisiae is common to fermentation, adding yeast to the fermentation container is not necessary to the process, according to the University of Hawaii System. Yeast is naturally contained in the fruits' skins. The presence of this yeast adds to the unpredictability of the fermentation process. Because fruit grows under differing conditions, the amount of yeast varies from fruit to fruit. Because the outcome is difficult to determine, fermentation without the addition of S. cerevisiae often results in wine of poor quality.

Vinegar

• If the juice alcohol remains in a sealed container after secondary fermentation, it will remain a wine. If, however, the fermented juice is exposed to air, then just like an open bottle of wine, it will turn to vinegar. In other words, when yeast finishes up the job, bacteria take over. The vinegar-making process is actually a second fermentation -- not to be confused with secondary fermentation. During a second fermentation, the container is open and oxygen once again flows into the container. Once this happens, bacteria cells in the container oxidize the alcohol, turning it into vinegar – a process that usually takes about six months or more.