In order to turn grape juice into wine, you need to ferment the juice or grapes. Fermentation is a biochemical process which will turn sugar into alcohol and will release carbon dioxide and energy in the form of heat in the process.
Fermentation can be carried out by the wild or indigenous yeasts found in the winery and on the grape skins – spontaneous fermentation – or winemakers can choose to innoculate with a chosen yeast strain in order to provide certain flavours or to ensure a successful fermentation.
Many commerical winemakers opt to use an active dry yeast preparation made from different strains of the yeast Saccharomyces cerevisiae as it gives them increased control over the fermentation, decreases the chance of stuck fermentation, when the fermentation ceases, and lowers the amount of volatile acidity produced.
Proponents of indigenous yeasts argue that it produces a more complex wine as more species of yeast are involved. Many winemakers will make a starter culture, known as a pied de cuve, from their first fermentation to help decrease the risks associated with spontaneous fermentation, such as undesirable flavours, a slow start to fermentation or a stuck fermentaion.