The action of yeast upon sugar results in its conversion to ethyl alcohol, with carbon dioxide as a by-product. Fermentation will often start naturally with yeasts on the grapes, but cultured yeasts may be added. The process generates much heat, and temperature control during alcoholic fermentation can have a significant effect on the style of wine produced. The process will cease either when all the sugar has been consumed, or more likely when the increasing alcohol content of the fermenting solution kills the yeast, or when the external temperature drops too low. It may also be arrested by adding sulphur or by fortification with spirit.