The single ingredient for the production of wine is good quality grapes. Different varieties of grapes are used in the making of the world's wines. Grapes in their turn are affected by both climate and soil. Grapes are picked carefully by hand to ensure the best results. Efficient mechanical pickers are used where the terrain permits, and the aim is bulk production. Grapes arrive at the winery as soon after picking as possible and are passed through an Archimedean screw to the crusher. At this stage, grapes, which may be either white or red, are destemmed and pressed, and the juice used to make white wine. The white juice flows directly to the fermenting vessel. If red wine is being produced, the skins of the pressed grapes are added to the juice in the fermenting vessel. The skins contain colour tannins and other elements that make the difference between red and white wine. The winemaker may then add specific yeasts to start the fermentation process or may rely on local wild yeast. When the fermentation has ceased and the time varies for different wines, the liquid is run off into oak casks to mature 1-2 or three-year-old casks are used. Depending on how much oakiness is required for the wine, in some cases wine is matured in stainless steel tanks. There is a more complex process when making champagne after the first fermentation has ceased, the still wine is blended with other wines and then bottled along with an added yeast solution to start a secondary fermentation in the bottle. Oak casks are kept in carefully controlled cool environments to enable a steady development of the wine from the casks. The wine is put into bottles where further development can take place.