Yeasts have perhaps the most important role in the winemaking process. These are responsible for converting the grape must into wine and more specifically the sugars contained in the must into ethanol. Yeasts range in size from 5 to 8 μ and are a group of unicellular organisms and are referred to by their Latin genus name and species name. The term yeasts refer to a group of unicellular organisms that are neither fully defined nor homogeneous. However, they share many characteristics, the most important of which is that they cause various fermentations. By fermentation we mean the breaking down of complex organic components into other simpler components.