Prior to crushing, the grapes for the estate's finest wines may go first to a sorting table or conveyer belt so that workers can remove any lesser quality grapes from the batch. They are then tossed in the receiving hopper that evenly feeds the grape clusters into the crusher/destemmer while a small amount of sulfur dioxide is applied. The amount required is less for red wines than for whites because the tannins found in the skins of red grapes help prevent spoilage. Once the bulk of the vegetative material such as leaves and stems have been separated from the grape materials, the must is pumped into a fermentation tank and, if desired, specially selected yeasts are added to catalyze the alcoholic fermentation. Many winemakers rely on naturally occurring yeasts found on the skin of the grapes or in the winery to begin fermentation, however some choose to introduce commercial yeasts for their specific properties and flavors.