and crushers are normally made of stainless steel (food-grade stainless steel for those parts which come into physical contact with the grapes). Additions at reception The preservative sulfur dioxide is commonly added when grapes arrive at the winery. The addition rate varies from zero, for perfectly healthy grapes, to up to 70 mg/litre, for grapes with a high percentage of rot. The purpose is to prevent oxidation and sometimes to delay the onset of fermentation. Macerating enzymes (for instance glucanases) may also be added at this stage, to aid extraction of color and fruit flavours from the skins and to facilitate pressing. Tannin may be added now, later in the winemaking process, or not at all. Tannin can be added to help stabilize colour, to prevent oxidation, and to help combat the effects of rot. Cooling of the must Some winemakers prefer to chill the must to around 10°C (50°F), to allow a period of pre-fermentation maceration ("cold soaking"), of between one and four days. The idea is that color and fruit flavours are extracted into the aqueous solution, without extraction of tannins which takes place in post-fermentation maceration when alcohol is present. This practice is by no means universal and is perhaps more common in New World winemaking countries. Inoculation and fermentation Once the must is in a fermentation vessel, yeast naturally present on the skins of the grapes, or in the environment, will sooner or later start the alcoholic fermentation, in which sugars present in the must are converted into alcohol with carbon dioxide and heat as by-products. Many winemakers, however, prefer to control the fermentation process more closely by adding specially selected yeasts usually of the species Saccharomyces ellipsoideous. Several hundred different strains of wine yeast are available commercially, and many winemakers believe that particular strains are more or less suitable for the vinification of different grape varieties and different styles of wine. It is also common to add yeast nutrient at this stage, often in the form of diammonium phosphate. Pumping over Soon after the must is placed in the fermentation vessel, a separation of solid and liquid phases occurs. Skins float to the surface, forming a cap. In order to encourage efficient extraction of colour and flavour components, it is important to maximize contact between the cap of skins and the liquid phase. This can be achieved by: pumping over (pumping liquid from the bottom