A micro-organism which converts sugar into alcohol in a process known as alcoholic fermentation. Present naturally in the vineyard, harvested grapes will begin to ferment naturally, especially if they are crushed to break the skins and expose the sugar-rich juice inside to the yeasts which reside on the grape skins. Some winemakers prefer to add cultured yeasts rather than rely on the action of wild yeasts. This gives greater control over the fermentation, but some argue it may intrinsically alter the style or quality of the wine, as a single strain might not produce the same flavours as the multiple strains present in the vineyard.