Vin jaune is made from late harvest Savagnin grapes, a white variety in the Traminer family which is less aromatic than the better-known Gewürztraminer. The grapes are usually harvested in late October when the sugars have developed enough to have a potential alcohol level of 13–15% for the finished wine. The grapes are fermented slowly and then kept in small old oak casks that hold 228 liters (60 gallons). The casks are not topped up, as they are with most wines, so an air gap appears above the wine due to evaporation. A film of yeast grows over the wine, protecting it partially (but not totally) from oxidation. In Jura, this film is called the "voile" (veil), but it is similar to the more familiar "flor" in Sherry production. The "voile" yeast has many similar properties to "flor" but thrives better in a lower alcohol environment and develops less heavily and thickly than "flor" does.