Yeasts Without them there is no fermentation There are thousands of types, all natural, all 'selected', but each with different roles. It is the yeasts, groups of microorganisms which can cause fermentation through the enzymes they produce. Those present on grapes (called wild or ambient yeasts) are capable of starting must fermentation on their own. In many cases, however, it is normal practice to add others with different characteristics, depending on the result the winemaker wishes to obtain; some are particularly resistant to alcohol, others to sulphur dioxide, others again to cold and so on. They serve to complete or assist perfect alcoholic fermentation. They are also fundamental in the making of sparkling wines, in fact they are added both to the bottle (in sparkling wine production by the classic method) and to the tank (for Charmat-Martinotti method sparkling wines) in order to obtain a second fermentation which will develop the carbon dioxide responsible for the bubbles. The yeasts also contribute to the organoleptic qualities of the wine, especially the bouquet. They are usually agreeable odours which remind one of flour and the crust of bread just removed from the oven. It is typical of sparkling wines and wines which are still young, when fermentation has just finished.