During the primary fermentation (H), the fermentable sugars, mainly maltose and glucose are converted to ethanol and carbon dioxide. This action is performed by the brewing yeast, which during the brewing process also produces many of the characteristic aroma compounds found in beer. At the end of the primary fermentation, the yeast cells flocculate and sediment at the bottom of the fermenter and can be cropped and used for a new fermentation. Not all yeast cells sediment; some will remain in suspension, and these cells are responsible for maturation of the beer. During this process the off-flavour, diacetyl is degraded to below the taste threshold. The fermentation characteristics of brewer's yeast are strain-dependent and are genetically inherited. Much of the genetics of Saccharomyces yeasts has been elucidated, and the knowledge gained, forms the basis for breeding of brewing yeast. Thus, new types of beer with altered aromas can be produced with yeast strains selected through breeding.