White wine vinification in Bordeaux is now carried out almost exclusively in stainless steel tanks, which offers the ability to control the temperature of the grape must during fermentation, which has increased the quality of white wines. In Graves, most of the white wine produced is now being ages in new oak barrels. Many other areas are following this trend. Bordeaux also produces some of the world's finest sweet white wines in Sauternes and Barsac. These wines are fermented very slowly over long periods of time to ensure that the yeast absorbs as much sugar as possible. They are then also fermented in new oak barrels.